New To Me: Datron 1081 Autocal Standards Multimeter

I’ve passed up a few chances to get a Keithley 2001 7.5-digit multimeter on eBay for ~$500, because while that’s a pretty good deal for a Keithley 2001 in working order, it’s more than I can justify spending on a 7.5 digit multimeter that I want, but don’t need. Somehow though in my twisted psychology, spending $300 on a two-decade older 7.5-digit multimeter with known issues is perfectly acceptable, because I recently did just that.

A couple of weeks ago, I was checking eBay on my phone while waiting for an appointment and came across a new llisting for a Datron 1081 Multimeter that caught my eye. It was listed for parts or repair for $300, or best offer.

Datron 1081

According to the description, it had been damaged in shipping, and gave an error during selftest. From the photos, it looked like the shipping damage was confined to a broken “ear” on the front of the case, and misalignment of the front panel. Definitely interesting…

Some quick googling confirmed that the 1081 was, as I thought, a 7.5-digit capable multimeter with high stability and and the ability to use an external voltage reference. I thought it would be useful for evaluating and calibrating precision voltage references, and 6.5-digit DMMs, like the Keithley 2000 and HP/Agilent/Keysight 34401a. I couldn’t find an operators manual for the 1081, but I hoped the 1071 manual I found was correct that the selftest error was with the AC measurement circuitry. I’m mainly interested in DC, so I wasn’t too concerned.

I reviewed past eBay listings on my phone to confirm that the 1081 typically goes for more than $300, checked my gut, and decided to make an offer of $200.

The seller responded within a few hours that the listing had generated a lot of interest, and counteroffered for $275. At this point, I was back home, with the ability to browse eBay without the limits of a phone. I should have taken advantage of this to investigate past listing a little more thoroughly. If I had I would have realized that some of the higher sales prices weren’t actually sales, they were expired listings that eBay wasn’t filtering properly. I didn’t though, instead I accepted the offer.

It took a few days for the seller to ship the item, and it was shipped by FedEx Ground, so it took over a week to get to me. It arrived last Friday, packed well in a Cisco router box with reused foam endpieces and packing peanuts for extra protection. It was in the physical condition I expected; the case damage was limited to an extremity, and the main enclosure was sound. I opened it up for inspection and to deal with the misaligned front panel.

The front panel is a metal plate covered with a big printed plastic sticker. The sticker holds a smoked plastic protective lens over the display. The sticker was loose at a few spots, including the protective lens, which allowed dust and exposure to further weaken the adhesive. I decided to remove it, clean it up, and reattach it.

I heated the panel with a hair dryer to loosen the adhesive, but that didn’t work all that well. I ended up peeling the outer layer and printed layers of the sticker off its backing. Some adhesive remained on on the printed surface of the label, and the backing remained stuck firmly to the metal plate. I used a plastic scraper to remove most of the backing, but getting the rest off required a razor blade, elbow grease, and solvents (“Goo Gone” worked best). I used isopropyl alcohol to clean the remaining adhesive off the back of the printed sheet. Unfortunately I think the process of peeling off the label led to some of the printed brown background along the left side of the lower edge crazing and flaking off. I considered trying to apply a new background of spray paint, but decided the risk of causing further damage wasn’t worthwhile.

Once I got everything cleaned up, I decided to use some non-corrosive silicone adhesive to stick everything back down again. I smeared a thin layer all over the back of the sticker, and around the edge of the smoked lens before lining everything up and sticking it back down, smoothing it out and wiping off any ooze. I weighted the area over the lens and let it cure for a few hours before reattaching it.

As for the electronics, there sure are a lot of them, and very few of them are electrolytic capacitors – the component most apt to fail on older equipment. I looked everything over very closely.

I was relieved not to spot any physical problems, because while everything is through-hole components, many of them are packed in very closely, and a number of them look like nothing I’ve ever seen before. Repair would be challenging.

Glass Capacitors

My first Glass Capacitors

Actually, there was one problem, but one I expected to find.

The back panel was labeled at manufacture with a battery replacement date of April 1992! Either the battery hadn’t been changed, or whoever did so was too lazy to update the label. Inside I found the truth, the battery had a datecode of 1984, like most of the other components. Fortunately it still had a voltage of 3.7v, but I’ll be changing it soon.

I found a few other interesting things as I looked the device over.


The photos above feature four 1N829a temperature compensated zener diodes. Together, they make up the heart of the voltage reference. They are each numbered with a unique serial number because they were carefully aged for months (or years), then characterized for noise, stability, voltage, and the current at which they have flat temperature sensitivity. My understanding is that the four Zener are connected as two parallel series of two.

I plan to look at these in more detail in a future post, because the unique characteristics of this voltage reference may make it the most notable part of this device. The use of hand-selected temperature compensated Zener was a common practice in a variety of precision instruments at one time, even so, the use of multiple TC Zeners was unusual, as is the stability they obtained. Also by the mid-1980s, when this device was made, use of temperature stabilized burried-Zener voltage references, like the LM199 (introduced in 1976) was commonplace.


This insulated metal strip running the length of the digital board between a row of I/O bufferes, and the ribbon connectors, also caught my eye.


I found evidence of a component level repair on the AC RMS converter board. Most of the components on the AC RMS converter board, and most of the other boards, have date codes no later than mid-1984, but the Fairchild opamp in the hermetically sealed package in the photo above is dated from 1987. The GPIB board seems to date from 1985, and there are some socketed ICs on another board that have 1986 date codes, while other chips on the board are from 1982 or 1983.


Artifacts of repair can also be found on the inside of the case, where some of the melted nubs holding the RF shielding seem to have sheered off and been replaced with some glue.

Functional Tests

After checking it over and fixing the front panel, I reassembled it, and powered it up. On the EEVBlog forum, “dacman” suggested that self-test error could simply be the result of running the tests with the “guard” switch was set to remote. I could see from the photos on the listing that it was, indeed, set to remote, and it still was when I received it, so I set it to local guard and ran the self tests. Everything passed!

After that, I did spot checks on the 100mv, 1v, 10v and 100v DC ranges using the output from my EDC 521 DC voltage calibrator. The EDC hasn’t been calibrated in years, though from my tests, it is accurate to within the combined 1year tolerances of both it and all my 6.5 digit DMMs. The readings are stable over the short term.

I’ve also done some longer term checks of a 10v signal over a period of days and found that for the most part, the peak-to-peak variation is within the 20-40uV range measured by one of my Keithley 2700s (it is a 6.5-digit meter, but in stats mode it collects and calculates 7.5-digit values) in my Seattle home, near an external wall, without any heating. Sometimes  the Datron has reported a wider range than the Keithley, sometimes a narrower range. I’ll need to get logging working over GPIB soon so I can can look more closely at the trends.

Unexpected behavior

In the first few days, of use, I ran into a few instances of unexpected behavior, some of which may have been user error, some of which may have been software bugs, and some of which is as of yet unexplained.

One of the first things to crop up happened while I was checking the 100v range with a 100v output. After the initial readings seemed good, I left it for a while and checked it while I worked. About 15-30 minutes in, I looked over, and it was reporting values of 110v or more, and they were changing quickly. I haven’t been using the EDC much in its 100v range, so I breifly considered the possibility that it was at fault, but a quick look at the Keithley 2700 measuring the same source showed that the voltage was still stable at 100v.

The reading on the Datron was still on the move and soon it was reporting an “Overload.” I tried changing to the 1000v range, but the Overload message remained. I cut the EDCs output and after about 30s, the Datron cleared the overload message and started giving readings again. I applied an input again (I can’t remember if it was 10v or 100v), and it again gave plausible readings. I left it for a while and continued checking it, and after a while, it was again reporting an overload. This time cutting the EDCs output didn’t clear the overload message, and I ended up power cycling it.

Since then, I’ve been focused on the 10v range, and I haven’t seen this behavior again. I have had it with a 100v input for the last 18 hours or so though, and its been solid. I’m beginning to suspect that the problem may have been the result of user error. At some point, I think I’d used a function that “zeros” the meter. I thought this worked like the relative measurement option on my Keithleys, which can give readings relative to any voltage. The Datron 1081’s feature is different. The zero-point is supposed to be set with the inputs shorted, and the value is stored and used until the next time the meter is zeroed. If it is more than a small portion of the full range (1% or so), it will give a overange error. I’m wondering if perhaps the zero-point that I or someone else previously set was near the limit, and perhaps some internal auto-correction ended up pushing things over the limit. This is just a stupid wild ass guess though. All I can be sure of is that since setting the zero point for all the ranges with the input shorted, I haven’t had this happen again.


I spotted the next problem after leaving the DMM on overnight. When I checked it the next morning, I suspected that the display had frozen because the last digit didn’t change once. I pushed a button to change the value displayed, and was treated to the above, after a minute or two, it seemed to reset itself and resume operation.


The next morning, after leaving it overnight, I again found it with a frozen display. The first button I hit produced a similar result to the previous day. I tried hitting another button (I don’t remember which) and the rest of the display segments and all the indicator LEDs on the buttons lit up too. This time, it didn’t reset itself, at least not before I got tired of waiting.

I haven’t seen this behavior since, despite leaving the unit on continuously. A few days ago though, I decided to investigate a hunch. I thought that that when I saw this problem behavior previously, I may have left the unit displaying the delta between minimum and maximum values. So, I again left it in that state, and the next morning, the display was again frozen. This suggested that my memory was correct, and that it was infact a software problem. However, the following morning, after again leaving the unit in min-max display overnight, the problem didn’t present itself. So, it seems that I still don’t have it figured out.

What Next


I’m trying to decide where to go from here.

At the very least, I’m going to power it down, open it up again, and take a close look at power supply voltages and ripple.

When I power it up again, I’ll keep an eye out for a recurrence of any of the bad behavior I observed. If so, it will suggest that some of the problems are the result of bad solder joints that act up when the unit is coming to a new thermal equilibrium.

Beyond that, i’ll do some logging over the GPIB interface to get a better sense of stability and tempco. I also need to investigate the resistance and ACV functions.

After that, I’ll have to consider whether to get it calibrated, or figure out a way to calibrate it myself. I’d also like to investigate its ability to use an external voltage reference to provide high-precision comparisons between different voltage standards. Doing so will require either figuring out a source of the (likely expensive) low-thermal-EMF rear panel connectors, or replacing them with similar performance and lower cost.

I’m also still looking for a Datron 1081/1082 Users Handbook (the 1082 is basically a 1081 with all the options). The Datron 1051/1061/1071 users handbook has been useful, because for the most part, they operate very similarly to the 1081, but there are some important differences with the digital filter, and some of the other aspects.

AVR Transistor Kit and Case Mod

I ordered an AVR Transistor/component tester kit from Bangood. At the beginning of the year I bought AVR transistor tester by Fish8840 off of eBay and found it a bit lacking. This kit is better. It uses the 1.12k version of the open source firmware, and includes a rotary encoder for easier access to the advanced features. I also opted for the case.

The kit makes use of through-hole components for easy home assembly. It includes an external 8MHz crystal, a voltage regulator, and uses narrow 1% and 0.1% tolerance resistors in critical positions for highest accuracy.

The case is designed to use a ceramic socket (provided) for connecting components, and also provides two banana jacks to allow test leads for testing larger components. People have complained that the provided socket is hard to use and doesn’t make good contact, and I wanted all three test connections available via leads, so I decided to modify the case.

My modification is inspired by some of the fully assembled versions of the tester I’ve seen on ebay. I cut a piece of plastic and glued it over the existing holes for the socket and banana jacks on the case and drilled three holes for 2mm banana jacks I ordered on eBay. Then, I cut a piece of protoboard and drilled three holes for some 2mm  banana plugs, which I screwed into the board. Then I soldered in the ZIF socket and connected up the leads to the plugs.

I haven’t tried building the latest firmware from source for this, but my understanding is that “mega328_st7565_kit” is the proper version to build.

Power Designs TP340a Knob Upgrade

My Power Designs TP340a bench power supply has a look and feel of quality stemming from its industrial design roots in the 1960s. Unfortunately, my supply was built in the mid-1990s and the design was compromised by the use of plastic knobs for the voltage controls on each of the three independent power sources.

TP340a with "original" plastic knobs.

TP340a with “original” plastic knobs.

Since getting the supply up and running, I’ve been on the look out for some suitable replacement knobs. Other/earlier models of these Power Designs power supplies appear to have used knurled aluminum knobs supplied by Kilo International. Kilo sells similar knobs to this day, but they apparently long-ago discontinued the model Power Designs used (which is probably the reason for the substitution in the first place).

In the current lineup, Kilo has two lines that might serve as suitable substitutes. The JD series has the right overall profile, with a flat top rounding into a straight side, but it lacks the flared skirt of the originals. The DDS series has the skirt, but the rest of the profile isn’t right, with a flat top dropping to a flat side that then flares slightly for the knurling.

I suspect that the original knobs were a skirted variant of the JD series, probably called JDS. I scoured eBay for old stock or used knobs, without luck, but I did find a seller with used knobs of similar design. In fact, they look like knobs that Power Designs used on a number of supplies made in the late 60’s or early 70’s. The price wasn’t bad either, 5 knobs for about $17, shipped.

They still weren’t quite right though. The fact that the design wasn’t an exact match wasn’t the issue. Rather, I was concerned that they were a little wider than the original knobs, and  that their proportions would seem off (I lack the skill to create a nice, well proportioned design, but I can tell when things are off). Because of this, I dithered on ordering them, but after seeing his stock decline, I finally pulled the trigger.


The knobs were in pretty good condition, other than being a bit gunked up. I gave them a washing and scrubbed them gently with some Barkeepers Friend (a mildly abrasive cleaner for glassware and stainless steel) before replacing the existing knobs.


I think my concern about proportions were warranted. The I think the knobs give the supply a bit of a big-nosed look. Still, I think they look better than the plastic knobs that it had before, and the definitely feel better to use.


The seller also offered some knobs with the same design but a smaller diameter. I would have purchased them instead, but for the fact that they were drilled for small diameter potentiometer shaft. I considered getting them and drilling them out, but I don’t really have the right tools and so I figured it would be a frustrating project that I could do without right now.

Big LED Clock Kit

A couple of months ago I was browsing Banggood and came across a stupid little clock kit that I felt drawn to for some reason. I suppressed the urge, but not before seeing if I could get the kit for cheaper on eBay (I couldn’t).

About a month ago, I was looking at Banggood again, and this time I gave into temptation and ordered the kit and it finally arrived, by way of Sweden!! a week or so ago and I assembled it yesterday.

I’m still not sure why I was so attracted to the kit, even now that I have it assembled and working on my desk, but I have an idea. I think a big part of the appeal is the minimalism of the device. From the front, all you see is the inch high LED digits. The circuitry is all on a board hidden behind the LED modules. The optional clear acrylic case is similarly minimal. It’s just six pieces of clear acrylic, with tabs and notches cut to interlock, holes for the six screws and slots for the six nuts that fasten it all together.



  • 1″ white digits (other colors available)
  • Clock (24hr/military time only, so far as I can tell)
  • Temperature, Centigrade only, alternates with time display
  • Alarm
  • 5v power with cord for powering from a USB connector
  • Battery backup for the clock.

The kit comes with all the thru-hole parts needed to assemble the clock and 8.5×11″ sheet of paper with the instructions printed on one side. The instructions are probably enough to build and operate the clock given some basic familiarity with assembling electronics, which is not to say that they couldn’t be better.


Assembling the kit is quite simple. There are about two dozen resistors of three different types to be added to the board, along with a with a diode. After placing and soldering these low-profile components, I added the six provided solid capacitors and the crystal, then the two IC sockets, six transistors, pezio buzzer, electrolytic capacitor, power socket, switches, and a surface mounted coin-cell holder.

The transistors proved just slightly tricky, because the provided transistors are in a TO-92 package, while the PCB is layed out for something with a wider lead spacing like a TO-126. The solution is simple, bend the leads to fit, but it’s a bit fussy.


Before finishing soldering the components, I partially assembled the case so I could check the distance between the back of the board and the notch cut in the case to provide direct exposure to the thermistor. Photos of the assembled clock on Bangood show the thermistor sticking out high above the case, which strikes me as ridiculous and unnecessary. I decided that ~1cm off lead between the board and the component was enough to position it in the cutout while remaining protected by the case. I bent the leads to be parallel to each other and perpendicular to the axis of the thermistor package, then 1-2mm from that bend, I bent both leads at another right angle, so the leads remained parallel in the same plane, and the thermistor was a elevated slightly above it. I then soldered the thermistor to the board so there was about 7mm from the PCB. I did something similar for the photoresistor, so it could face upward. I bent the leads a couple mm away from the package, and then soldered the leads so they protruded about 5mm from the board.

Once the components were all soldered, and the leads trimmed, I checked over the board and fixed a number of dodgy solder joints before adding the four 7-segment LED modules on the opposite side of the PCB.

The third digit brought unexpected difficulty. It is supposed to be rotated, so that the dot that usually sits at the lower right to serve as a decimal point, will instead sit at the top left, to serve as the top dot of a colon separating hours from minutes formed with analogous dot at the bottom of the previous digit. This rotation presented a problem, because the leads at the top and bottom edge of the LED packages are offset, rather than centered, and the PCB wasn’t laid-out to account for this. All the leads had to be bent to accommodate. Again, a relatively simple task, but fussy in practice, because of the need to maintain the spacing between the 5 leads on each side so they will slip into the holes on the PCB.


At this point, I fit the ICs to their sockets, powered up the clock, and found that it worked. Or should I say, mostly worked? The left-most digit didn’t light up. I tried resetting the device as described in the instructions, but it still didn’t work. I checked all the solder joints for the LED module, and they seemed Ok, but I touched some of them up with another pass of the soldering iron. I also consulted the schematic to figure which transistor is responsible for strobing the power to the module and checked/touched-up its leads. The first digit still wasn’t lighting up. It was time for bed.

After a good nights sleep, and breakfast out with friends, I returned to troubleshooting the clock. I spent 15 minutes or so using the continuity function on one of my multimeters to trace out part of the circuit and better associate the schematic with the actual layout of the PCB. In particular, I was interested in the portion of the circuit responsible for powering the problem LED module. I figured out the center pin on either side of the module provided +5v, and that there seemed to be good continuity between that and the collector lead of the controlling transistor (Q1). I also found that there was proper continuity from the base of the transistor to the associated resistor, and that the other end of the resistor was, indeed, connected back to an output pin on the microcontroller. Finally, the emitter was properly connected to the 5v supply.

Since everything seemed to be connected properly, I needed to look elsewhere to figure out why the clock wasn’t functioning properly. I thought, perhaps, one of the components was defective. Rather than going to the trouble of testing them individually, either in or out of circuit, I decided to power up the device again, and use an oscilloscope to inspect the signals at various points in the circuit, and compare them against the signals at analogous points for the other functioning digits. This would help me pinpoint the component in need of attention.

That was the idea, anyway. Once I powered the clock up again, the problem digit lit up. I’m not sure what happened. Perhaps my probing and shifted a faulty solder joint back into contact? I tried pushing on a few spots, to see if I could reproduce the problem, but nothing happened. In the end, I decided I had better problems to solve than chasing down an intermittent fault in a $12 clock kit, mounted it in the case, and called the project done.

Other Details


The PCB has some unpopulated areas. The documentation doesn’t say much about them, other than that they are “DIY” areas and no components should be installed. The schematic casts some light on what they are for. There are some headers for in-circuit-programming, and another which, I think, may be for resetting the MCU. There is also space for a relay, and an accompanying space for a connector/header for attaching the switched load. Given the design and clearances on the PCB, and the width of the associated traces, I’d guess that it is only suitable for switching low voltages at modest currents (~1A, tops). The firmware also seems to support this relay by allowing you to set an our to turn it on, and and hour to turn it off.

For what it is worth, the device uses a pre-programmed STC15F20EA_28 microcontroller in a DIP package. This MCU comes from STC Micro, in China. It has an implementation of the 80C51 CPU, along with 4K of flash, 256 bytes of SRAM, an 8-channel, 10-bit AD converter and 1K of EEPROM. This chip isn’t particularly hobbyist friendly, other than being inexpensive. Someone did create software for programming these chips called ‘stcdude.’ It is analogous to ‘avrdude’ used for programming Atmel avr microcontrollers. You’ll also need a compiler like SDCC (small device C compiler). That, and experience doing embedded development without much handholding. This isn’t an Arduino, nor is it Arduino compatible.

The MCU is complimented by a DS1302 clock chip (or something that functions like one), which stores the alarm settings, and keeps time on battery power when the external power isn’t connected.


I measured the power consumption using a USB meter, and it read 0.01-0.02a, or 10-20mA. In theory, you could probably run it for ~4 days off of a modest single-cell USB powerbank. In practice though, it probably doesn’t draw enough current to keep most USB power banks switched on.



I’ll give this kit a solid B overall. The kit and instructions are a B- because of the layout problems I mentioned above. The finished product is a B to a B+. It think it is attractive, and useful, but as an American, the fact that it only displays time in 24-hour mode and temperature in Centigrade undermines its utility somewhat.

I should also say that while the temperature seems accurate to within a degree or so, I have no idea how well it will keep time.

New to Me: Power Designs 6010 DC Power Supply

A couple of weeks ago I came across an listing for a used Power Designs 6010 for just $20, shipped. At first glance I thought I’d come across an incredible deal on a 6050, because the 6010 looks very similar. I’ve been keeping an eye out for a deal on a couple of Power Designs 6050 supplies for a while, with an eye to running them in parallel to get 10A for use in calibrating some battery testing equipment. It was disappointing to realize it was a 6010 a 60v/1A supply. I can already cover that voltage and current range in a couple different ways with existing equipment. I bought it anyway, because $20!!


The seller took his sweet time shipping it out, but I finally got a tracking number, and a few days later, the supply arrived, well packed, and in, outwardly, pretty good physical condition. Inside was another story.


One side of the PCB looked great. The other side though…


Absolute crap. The PSU clearly had a thick layer of dust inside before receiving a light sprinkling of water. I set about trying to clean it up. I got two bowls of fresh water and a toothbrush. I wet the toothbrush in one bowl and I scrubbed the PCB with it. When it was too dirty, I rinsed it in the second bowl, then wetted it again from the first. After 5 minutes or so, the board was looking much better. I followed up by soaking it with squirts of 70% Isopropyl Alcohol (IPA), letting it run of onto a paper towel, and then chasing more if off with some compressed air. I repeated the process again with more 70% IPA and then with two rinses of electronics grade 99% IPA.


After cleaning the board looked much better, and I could see that the corrosion that had started was superficial.

While I waited for it to dry out fully, I carefully inspected all the components. All the components on the front-side of the board looked in good shape. The big electrolytic caps on the backside of the board showed signs of leaking small amounts of electrolyte, but they tested out Ok with the capacitance function on my multimeter. I followed up my hooking each one up to one of my other bench power supplies and monitoring the current while I gradually raised the voltage, looking for signs of high leakage. Then I disconnected each cap from the PSU and checked how gradually the voltage dropped. None showed obvious signs of misbehavior. I plan to replace the caps, but I decided they were good enough to use while I did further checks on the supply. Before proceeding to functional checks though, I cleaned up the knobs and the front panel.

The knobs are red “Daka-Ware” resin knobs. Daka-Ware is/was a brand of products made with a thermoset resin, similar to Bakelite. Like Bakelite, there are various fillers mixed with the resin. Over time, the resin and filler age at differential rates from physical ware, and exposure to UV light, oxygen, pollutants, and dirt and grime. The knobs on my power supply had an obvious dull, darkened patina to them. When I removed them, the previously covered areas were still bright and glossy, making the wear even more obvious.

Next I washed the knobs with mild, soapy water. This removed a fair amount of the dark grime, but when they dried, it the knobs seemed lighter, but still dull due to the layer of partially exposed filler. Some people address this problem by painting the knobs, which to me defeats the purpose of making them out of colored resin in the first place. Others buff and polish the surface until they get a smooth surface of fresh resin, but that can end up removing a lot of material. I rubbed the knobs gently with a wet melamine sponge to remove a thin coat of exposed filler and aged resin. Once the knobs dried, I covered them with a generous coat of carnauba wax in order to impregnate and protect the remaining exposed filler, and the resin underneath. I then polished the knobs with a soft cloth.


The results aren’t perfect, but they are pretty good. I’ll probably put another couple coats of wax on before I call it done.

As for the functional tests, well, I’ll save that for another post.


EDC 521 Voltage Source Stability and Accuracy


Last week, I cleaned up the used EDC 521 DC Voltage & Current Source I bought on eBay and ran it through some quick tests. The next day, I connected it up to one of my Keithley 2700 6.5 digit multimeters, started logging readings, and powered it up. Since then, I’ve been collecting voltage readings every 5 seconds so I could get a better sense of the device’s stability and accuracy.

The EDC 521 was set to 10.00000v and powered on.


Once the self-tests ran the output turned on. The initial voltage was only 9.99954v, but climbed quickly. After 15 minutes, it was up to 9.99981v and still rising gradually. At the end of the specified two hour warm-up period, it was 9.99986v, well within the specified tolerance of 0.000258v (0.002% of setting + 0.0005% of range + 3uV), as well as the DMMs uncertainty of 0.000350v 30ppm of reading + 5ppm of range.


After the initial warm-up, the voltage continued rising for another 5 hours, before leveling out at about 9.99989v.


The graph above shows the voltage over ~4 days following the two hour warm-up period. If we look at the stability once the voltage first leveled out ~7 hours after turn-on, we see that it lies between 9.99990v and 9.99994v, which works out to be about 4 parts per million (ppm), which is better than the specified 7.5ppm 8-hour stability and the 10ppm 24 hour stability for the source and ~14ppm 24-hour stability of the DMM.

I’d guess that some of the variation is due to temperature changes. The EDC 521 specifies a temperature coefficient of +- 5ppm/C°, and the DMM has a temperature coefficient of about 2ppm. Unfortunately, I didn’t start collecting temperature readings until yesterday afternoon. Once have accumulated a few days worth of readings, I’ll look at the relationship between temperature than the voltage reading.


The last thing I did was look at short term variation in readings. The graph above looks at a 4 hour period earlier today. The blue line is a moving average of 10 minutes of readings (~120 readings), the dark green shows the standard deviation in readings over the same window, and the light green bounds the average of the minimum and maximum readings over the same period.

It shows that the difference between minimum and maximum readings are ~6uV, which is  less than 1ppm. The DMM is only a 6.5 digit DMM, which means that the smallest reading on the 10v range is 10uV. When readings are taken using the GPIB or RS-232 interfaces, it does report an extra digit, which is useful for statistical purposes. In this case, I think its safe to say that the short term variation in readings is probably mostly down to the DMMs noise floor.

After looking over this data, my conclusion is that I got what I was hoping to get, a stable and accurate precision voltage source. Next step is to test the GPIB interface and use it to collect data across every available setting and range to check for linearity/accuracy and decide whether any adjustments are needed.

New to Me: EDC 521 DC Voltage/Current Source

Last week I came across a miscategorized eBay listing for an Electronic Development Corp (EDC, now owned by Krohn-Hite) 521 DC Voltage/Current Source. It was listed in the network equipment section, with “Juniper” as the manufacturer.

The EDC 521 is a precision DC reference source with high accuracy, precision and stability, for the calibration of meters and sensors. It can output voltage in three ranges, (0-100mv, 0-10v, and 0-100v), and constant current in two ranges, 10mA and 100mA (with compliance voltages up to 100V). In each range, the precision/resolution of adjustment is 1ppm. Overall stability in Voltage mode, within the devices operating temperature range is 7.5ppm over 8 hours, 10ppm over 24 hours, 15ppm over 90 days, and 20ppm over a year. The temperature coefficient (which is included in the above estimates). It is microprocessor controlled and has a GPIB interface to allow remote control.

To achieve its basic stability, it uses an aged and selected 1N829 temperature compensated Zener diode as its primary voltage reference. This diode is driven by a stable precision current source at a current chosen to provide the best combination of temperature stability, long-term drift and low-noise for the individual diode used in each unit. Adjustments are made using a custom, precision 24-bit digital to analog converter.

Voltage divider resistors and 1N829a temperature compensated zener voltage reference.

The DAC works by feeding the reference voltage across a resistor divider to obtain 10 output voltages, tapped at 500mV intervals. If I understand correctly, these voltages are switched to provide analog voltages for each decade, these voltages are buffered, then then weighted and summed using some precision resistors before being fed to the output amplifier.

When the package arrived yesterday, I saw why the listing had been miscategorized — it was packed in a box for a Juniper Networks switch. That, and the sticker noting a failed calibration attempt in 2009 makes me doubt the seller’s assertion that it was “pulled from a working environment.” Not that I expected a pristine, calibrated instrument for $150.


Inside the box, I found things in a bit worse physical shape than I expected. What I thought was shadow/glare in the photo from the ebay listing, was actually a torn red filter over the LED display. And the underside of the case, which wasn’t pictured in the listing, had a huge dent.


On closer inspection, the dent didn’t reach the PCB inside, and I was able to remove the panel and hammer it out. Once inside, I found that everything had a fine coating of persistent dust. Hitting it with canned air shook some of it loose, but most of it remained.

So, I got to work rinsing it with a lot of isopropyl alcohol which I then chased off the edge of the board with canned air. After a few repetitions, the top and bottom side of the board were pretty clean. I then looked over both sides of the board closely, looking for damaged components, and cleaning out little pockets of residue.

I didn’t see any damaged components, but along the way noticed signs that the board had received some major revisions. There was an obvious bodge wire on the bottom of the PCB, but it was also clear that new holes had been drilled to receive additional components. On the top side, I found a cut trace, along with a couple of added resistors and a couple of capacitors. I haven’t traced everything out, but its obvious that the bodge wire connects to one end of the internal reference divider, and the rest of it is on the opposite end, so it would seem likely that its helping isolate the reference divider, and the voltages it produces, from noise sources.

It also appears that a number of power transistors have been replaced. Unfortunately, none of the components in question have obvious date codes, so its hard to guess when the modifications were done, and whether the transistors and the filters were added at the same time. Perhaps one of you knows how to decode the markings?  First line is a Motorola logo followed by “616,” the next line is “JE350,” which is the model/part number. The datecodes on other components pretty much all date to late 1996, and the MPU board has a label with the firmware revision and is dated January 1997.

Before closing it up, I took care of the loose plastic supports for the back-edge of the PCB, which holds heavy electrolytic filter caps for the power supply. I cleaned the old, crusty, failed double-sided foam tape off and replaced it with new tape so I could stick the supports to the back of the chassis again.

I powered it up, and gave it a quick check on all the voltage and current ranges. It seems pretty close to its 1 year tolerances. I was surprised by the amount of time it took to warm up and stabilize, but when I checked the manual, I saw that the warm up time is speced at 2 hours.

IMG_8648 IMG_8647

I powered it down over night. This morning I set up my computer to voltage readings ever few seconds and then powered it back up. I’ll post a graph once I have a days worth of data. After that, I’m going to write a script to run through all the possible settings and log the measurements. So, more to come!

A Trip to the Museum of Communications

This morning, I woke up with an itch, an itch to see a switch.

Version 2

No, not that kind of switch, something bigger!

Battery Reserve Switch

Nah, that’s not a switch…

Panel Switch

That’s a switch! Well, part of one.

Museum of CommunicationsIt is part of a panel telephone switch, one of a number of operational telephone switches at The Herbert H Warrick Jr. Museum of Communications, a little known technological treasure trove in the Georgetown neighborhood of Seattle. The museum fills the top two floors of a Century Link central office building.

The museum’s collection includes all manner of equipment and memorabilia from over a century of telephone history. The presentation of the collection is a bit uneven. There are carefully dated and labeled exhibits of phones and other equipment, along with display cases stuffed with lineman’s tools, but to me, that’s all secondary.

The best part of the museum is that it houses multiple generations of telephone exchange switching equipment, the sort of stuff that used to fill small buildings and connect thousands of homes to the telephone network. Much of it of it is operational, and interconnected, and attended by a staff of volunteers, many of them technicians and engineers retired after long careers with Ma Bell and her successors. They answer questions, give tours, and maintain the equipment.

The automatic switching equipment spans almost a century. The oldest automatic switch is a panel switch that served the Rainier Valley. It was installed in the 1920s and served for over 50 years. Unlike that later automatic switches in the museum, which were made in a factory and then installed, the panel switch was assembled on site at the central office and moving it five decades required removing walls. They also have a #1 Crossbar Switch (#1XB) from the 1930s, and a #5 Crossbar (#5XB) from the 1950s. The panel and crossbar switches are all operational. Calls can be placed between lines serviced by the switches and you can hear the progress of the call set-up and tear-down sound across the racks as the various electromechanical parts do their thing. They also have a number of operational PBX systems, some old-time switchboards, some small Strowger step-by-step switches and a not yet operational #3ESS, a small variant of the first switches using transistorized logic for control.

The collection also includes inside plant, like power distribution equipment, outside plant, like cables, along with a variety of trunking and long-distance equipment, including equipment for carrying national network television broadcasts.

There is also test equipment spanning decades, and a nice cache of HAM radio equipment.

The museum was originally called the Vintage Telephone Equipment Museum when it was created in 1986 by Herbert H. Warrick Jr. Warrick was an engineering director at Pacific Northwest Bell, and started the museum with the company’s support to preserve generations of vintage telephone equipment that was being phased out in the transition to digital switching and transmission. More recently, it became affiliated with the Telecomunications History Group.

I’ve made many visits to the over the years, and each time, I learn something new. I recommend it to anyone with an affinity for technology, particularly communications and computing, but it should also interest anyone curious about industrial and economic history. I hope I’ve whetted your appetite.


Keithley 2000 & 2700 Data Logging in Python

This is a simple, braindead, python script to get measurements from a Keithley 2000 & 2700 DMMs using VXI-11 and store them in a simple timestamped SQLlite database.

It’s not a general tool, but it should be easy enough to tweak it to dump to a text file, take different measurements, use different ranges, different intervals, different communications transports.

It doesn’t have enough error checking, and bug fixing, but it mostly works well enough. It recovers from (some) malformed readings. It doesn’t recover from other errors (mostly communication related)


  • OS X or Linux
    • It may work with Windows, but I haven’t tested it.
  • Python 2.7.x 
    • It may work with others, I haven’t tested it.
  • python-vxi11

How to use

The script takes voltage readings at ~10s intervals, on the 100v range, and stores them in a SQLlite3 DB called readings.sqlite3 in the current working directory. If you want different behavior, look through the source code and make the necessary changes.

You MUST edit the file to configure the name of your VXI-11 gateway and GPIB address(s) of the devices you want to poll.

Finally, run the script, ie:



Since this script doesn’t do much error checking, it occasionally dies. In my experience, when it dies, it is because of a communcations timeout. In such situations, restarting the script is often enough for hours more logging.

I usually run it in a shell loop, so it restart automatically after a delay:

while true; do python; sleep 150; done

If you modify this script to communicate with your Keithley 2000 DMM over RS-232 serial, be aware that a problems with hardware/firmware before ~2007 can result in frequent communication failures.

The simple workaround is to modify the RS-232 cable, or the DMM’s own RS-232 port, to ensure that the RTS pin (#7) is not connected.

Continue reading

HP 6114a Precision Power Supply First Look

I already have more working electronic lab equipment than I need, and more broken equipment than I can fix in the next month or so, but I still check eBay daily, and occasionally, I see a deal that is too good to pass up. This time, it was an Hewlett Packard 6114a power supply on sale for $75 + $19 shipping. The supply had been listed for $150, and I figured that if the seller was willing to cut the price in half, they might be willing to accept even less. I offered $55.

I’m really not sure what I was thinking. Part of me felt like getting the supply for $75 would be a great deal. Part of me thought that not getting it at all would be smart, since I didn’t need it, and had resolved not to take on other projects. Part of me wanted to see if I could get it for even less. In the end, it looks like the compromise I made was to try and serve all three, because I picked a price that was low enough that it might not be accepted, but high enough that I might end up with another piece of equipment. And so I did.

The HP 6114a was introduced in in the early 1970s and produced until at least the early 1990s. They typically go for something over $100 + shipping, so getting one for just under $75 would rate as a good but not great deal. From the photos in the listing, I thought I could make out enough of the serial number to tell that this example was made in 1981, a fact I confirmed once it arrived. It had the base single-turn potentiometer for the current control, rather than a 10-turn pot with a turn-counting dial, but I thought I could upgrade it myself for $10-20 in parts.

Physically it seemed in fair shape. There were some bent fins on the heatsink, and the front panel trim wasn’t seated properly, and might need to be bent back into shape. Most of the finish seemed to be in good shape. The condition of the front panel was harder to judge. It was hard to tell what was sticker residue and what was scratches in the finish.

The unit arrived from Nevada about three business days after I ordered it. It was packed in a stout cardboard box, and heavier than I expected. Inside I found it wrapped in a few sheets of thin foam, nestled in a reasonable amount of packing peanuts. I think there were enough peanuts to protect the instrument as shifted in the box transport, but I’m not 100% sure, because there was some damage to the front panel and its hard to tell if it was pre-existing, or it occurred during shifting.

HP 6144a Front Panel

There weren’t any big surprises after I got it unwrapped. Next step was to start taking it apart so I could figure why the top trim on the front panel wasn’t seated properly, and what I could do about it.

Along the way, I also performed an initial inspection of the electronics to look for damaged components, PCBs and interconnect wires.

I quickly spotted some damage to some of the pvc tubing used for cable management (above left). It looked like it had been scorched by an errant soldering iron, suggesting a previous repair. It took my a while to figure out that the site of the repair was (probably) right there in front of me. That a big resistor (above right) is not like all the other big resistors. It’s epoxy packaged. The rest are ceramic.


My deeper inspection showed that displaced top trim wasn’t bent, as I feared. The aluminum extrusion was in good shape, other some chips and gouges in the finish, some of which had cut into the underlying aluminum. With the top trim removed, I could take a closer look at the other components of the front panel.


The meter was in pretty good shape, but its grey plastic bezel, which also served to help retain the clear plastic lens piece, was broken in the lower left hand corner. I removed the bezel and glued the crack with some superglue. Once it was dry, and sanded it down and polished it with some nail files/polishers I got surplus from my wife. The crack is still visible if you look closely due to glue filling in fit is repaired and the profile and finish of the plastic is pretty close to what it originally was.


During initial inspection, I noticed a rattle as I turned the instrument over. As opened the chassis up, I was attentive to the source of the noise, but couldn’t pinpoint it. In the process of removing the meter bezel, I realized the loose part was in the meter. I desoldered the meter leads so I could inspect the meter more easily. As I did, the source of the rattle quickly revealed itself to be small screw and copper lock washer. Once I had the meter free of its leads, I removed a clear plastic clip holding the meter lens to the back of its housing and worked the lens loose. With it free, I could see the source of the loose parts, they were one of the pair fastening the printed meter dial to the frame of the brass meter mechanism. They’d somehow worked themselves loose.

The copper washer dropped out as soon as I opened the meter housing, but I lost track of the screw. It wasn’t on my workbench, and gentle tapping of the meter housing didn’t shake it loose. I went in search of a suitable tool for removing the recessed nuts at the back of the housing that hold the mechanism to the meter. The socket wrenches I had were too thick-walled, but a fine set of needle nose pliers ended up doing the job. I still couldn’t find the screw though. I pulled the internal leads free of the pins that passed through the housing so I could take a closer look. Perhaps the screw had found a spot inside the works of the mechanism? After 15-20 minutes, I concluded that it must have dropped out as I carried it in search of a wrench. After searching around on the floor around my chair for 10 minutes, I broadened my search area and almost immediately spotted it camouflaged in a dust bunny on the other side of the table, along the path I took to the basement to look for tools.

HP 6144 Voltage Control Sub-panel

After reassembling the meter, I decided to remove the one screw holding the voltage control subpanel to the chassis and take a closer look at it. That doesn’t look right, does it? That bow isn’t lens distortion, that’s bent aluminum. I can’t tell from the photos in the listing if the damage was already done, or if it happened in transit. One way or another, it looks like the result of sliding or being pushed face-first against something.

In preparation for repair, I desoldered the two leads connecting the voltage control to the main PCB. One connected into a PCB on the sub assembly and was easy to remove. The other connected directly to a lead on the potentiometer shown in the photo. It was a bit fussy. The leads from the pot are basically ~22 gauge wire with a hairpin bend at the end. To get the wire free, I ended up removing most of the solder with some desoldering wick, and then alternated between teasing the hairpin open and working the wire free. All in all, it was fussier than I would have liked, and I ended up scorching some of the plastic potentiometer housing when I accidentally touched it wit the side of the soldering iron. In retrospect, i should have removed the knob and unscrewed the pot from the front panel before trying to desolder the lead.

With the assembly free, I removed the knob and retaining nut on the potentiometer, then I heated the aluminum carefully with a hair dryer so it was easier to pull the thinner front sheet with the labeling free of the the thicker extrusion so that I could get to the screws holding the decade switches.

HP 6114a Voltage Control Subpanel Repair

With the bent extrusion isolated, I used a hammer and a few hardwood blocks to carefully beat it back into some semblance of flat. I think I did a pretty good job.


I also spent some time cleaning the stickers and adhesive off the remainder of the front panel. I started by peeling off what I could, which revealed some writing with permanent marker. Isopropyl Alcohol on some cotton balls and a little elbow-grease took care of all the adhesive and faded the permanent marker.

I used an old “drafting” eraser to rub out the last remnants of the magic marker and a few persistent spots. I think its looking pretty good. There are a few tiny scrapes in the white panel, and a few more in the grey strip at the bottom that I think I’ll leave be. Some of the lettering on the white panel is a little worn. I might try touching that up.


The most glaring problem is the missing HP logo badge that is supposed to fit over those two holes in the upper left of the panel. Given the vintage of this supply, I think the original logo badge had charcoal and gray with a chrome border. Anyone happen to have any spares?

Next step, I think is to put it back together and check to see how it works.