A few months ago I was at the hardware store looking for cheap enclosures for electronics projects. Some aluminum junction boxes for electrical conduit caught my eye, so I bought a couple. In parallel, I was interested in building a small, portable amp that could operate off 12v, which led me to buy some little, mono, TPA3110 modules for a few bucks each.
Surveying my box of parts a couple weeks ago, I noticed that the TPA3110 modules would fit nicely in the smaller of the two junction boxes I purchased, and I started tinkering with ways to assemble them into a finished product.
The first idea was to join the boards together with standoffs and slip them inside. I ordered some small terminal blocks for the electrical connections. When they came, though, and I tried assembling things as I planned, I realized I’d need to cut the standoffs down in order to fit a board to hold a potentiometer. I was feeling lazy, and didn’t want to deal with metal filings, so I looked for another way.
I decided to use pin-headers to mate the amp modules with small motherboard made from perfboard. For added mechanical strength, I cut the headers with more pins than needed, soldered the pin positions with through-holes on the amp board, and then glued the rest and trimmed them to the same length as the active pins.
Routing the power input and speaker outputs was kind of a nightmare. Rather than trying to plan it all out, I ended up working a couple of connections at once, trying to leave room for the other connections. It took quite a while. I was concerned about some of the routing and figured I’d probably end up doing a second version, so I soldiered on soldering my prototype.
Once I was done, I used my multimeter to check to make sure that there weren’t any short circuits on the motherboard. Fortunately, everything checked out.
Next I had to finish up the input connections and passive volume control. Rather than routing the audio input on the protoboard, I decided to take advantage of the shielding on the input to help keep the signal clean while passing the high-current power and output connections, and the inductors on the amp board. I connected it to the board with the volume-control board at the far end of the case.
After assembling the components on the volume control board I checked everything with a multimeter. Again, I was fortunate that I hadn’t ended up with any shorts. The volume control board was connected to the the “motherboard” with an excess of soldered pin headers for mechanical stability.
I covered the solder pads on the backs of the amp boards with kapton tape to keep them from shorting on the case. Then I made up a power cable and some speaker cables, screwed them in to the terminals on the motherboard and fed them out of the opening of the case.
Maneuvering the amp board into the case with all the cables attached took a bit more force than I was hoping for, a situation not helped by the fact that the position I chose for for the audio input connector interfered with part of the case casting, depriving me of a few extra mm at the opposite side of the case, and putting the volume-pot a bit off center.
In the end though, I got it to fit.
My plan is to power it off a Quick Charge 2 USB power bank set for 12v output. I have the powerbank, but I still need to make something to negotiate the 12v output, so I powered it off a 12v power brick to test it out.
It works! Even better, it sounds good! So, a second version is a luxury, rather than a necessity. At full volume, the output level with my phone as the audio source is maybe a little lower than I’d like for something intended for use outdoors. I’m not sure yet if that’s a limitation of the 12v supply voltage, or if I need to bump up the gain of the amplifier.
Now that I know it works, I still need to finish it up. I need to fit an extension to the volume potentiometer shaft and pick out a knob that looks good. I also need to put some thermal pads on the bottom of the amp modules to transfer heat to the case. Also I’ll probably find some thinner gauge speaker cable.
I’m building a few custom audio amplifiers for my personal use out of some inexpensive modules based on the Texas Instruments TPA3118 and TPA3116 Class D amplifier chips.
Class D amplifiers are basically variable switching regulated power supplies (SMPS), which makes them compact and power efficient when compared to traditional amplifiers, which are much more like linear regulated power supplies. The switching frequencies are designed to be well outside of the range of human hearing (and the circuits and transducers we use to reproduce sound) so that it can be filtered without interfering with the audio, and so any remaining signal at switching frequencies is inaudible. In the case of the modules I’m using, the switching frequency is ~400KHz.
The modules I am using have LC (inductor + capacitor) filters on their outputs in order to filter the switching frequency before sending it to the speakers, but I was concerned that the modules would also inject noise onto the power supply rails. TI has provisions for slaving multiple chips together so that they use the same switching clock and can avoid distortion due to interactions on the power supply bus, but the modules I’m using don’t expose the necessary pins, and the small component sizes and spacing between the PCB traces didn’t make me eager to hack them.
Some people have addressed the issue by dedicating a separate power supply to each module, but all things being equal, using a single supply should be cheaper and more compact. I asked for advice on the DIYAudio.com forum, and got some general suggestions, but nothing specific enough to be actionable. I was going to need to do more of the legwork myself.
So, first step, was to see if there were any existing solutions. I started reading datasheets and application notes, but I didn’t find what I thought I needed. I did learn though that, in addition to the switching frequency, Class D amps draw current at either 1x the audio input frequencies, when configured as a half-bridge, or 2x the audio input frequency, when configured as a full bridge.
Next step was to measure magnitude of these potential PSU noise sources in a naive implementation, with no added filtering beyond the PSU’s output filters and the modules PSU input filters.
I used the 24v DC PSU I had handy, an ~10-12 year old 65W Apple 24V PSU intended for a Macintosh PowerBook. I haven’t seen a teardown of the PSU, but in general, Apple seems to put a lot of effort into making their PSUs electrically robust and low noise.
I paired this with a Sanwu “Blue” TPA3118 PBTL mono module. which has ~1320uF (2x 2x 330uF) of electrolytic decoupling caps on the power rails. I then hooked up my oscilloscope and an Analog Discovery to do the measurements.
With no signal input to the amp, this is what the power supply rail (yellow) and the amp output (blue) look like. You can see that there is a sinuous ~400KHz, 1.2v peak-to-peak signal on the output from idle switching of the output bridge. You can also see evidence of this switching on the power rail, at the same fundamental frequency, but with a lot of ringing. The 400KHz component is about 50mV peak-to-peak, while the higher-frequency ringing and transients are about 190mV p-p.
I also took measurements using my Analog Discovery as a spectrum analyzer.
First with everything connected, but no AC power applied to the PSU.
Next, with AC power, and various resistive loads. I was sloppy and didn’t record the exact condutions, but it gives some sense of what the PSU noise is and how it changes with load. Blue is with no load, magenta is with ~30 Ohms, Green is with ~15 Ohms. Also, note, this is a relatively wide spectrum range, from ~0 to 2MHz.
With the amp as a load (driving a 7.5Ohm power resistor). There is a 100Hz input to the amp. Each trace is a spectrum measurement of the PSU with a different input amplitude, from signal-off to 1v peak-to-peak. These readings were DC coupled, so the measurement reflects the DC voltage component. As a result, the average level for each trace is closer to the baseline as the DC voltage of the PSU rails sags at higher output powers.
There are peaks at the 400KHz switching frequency, and its harmonic at 800KHz. I’m not sure I understand all the little peaks that group around on each side of the switching frequency.
In the time domain, we can see the amplifier-induced PSU ripple & noise in the yellow trace, along with the amplifier output signal from a 500mv p-p, 100Hz input signal. There is a 200Hz ripple on the power supply rail as the amplifier bridge switches on 2x for each input cycle, once to drive the output high, and then again, in reverse polarity, to drive the output low. This ripple is ~500-600mV p-p and its phase is offset a bit from the output waveform. On top of that, you can see the switching noise in the high-frequency “fuzz” on the power supply rail. This “fuzz” is about 1.3v P2P. The two components together add up to ~1.96v of swing.
Pushing the input higher quickly runs into problems. With 1v input p-p, things look rather nasty as the peaks get brutally clipped off against the limits of the PSU voltage.
0-500mv 50Hz input signal
0-550mv 100Hz input signal
0-500mv 10KHz input signal
I also took PSU rail frequency measurements with input signals of 50Hz and 10KHz.
I decided to wrap things up by looking more closely at the impact on PSU voltage at different input audio frequencies. Since the human hearing falls off sharply above 20KHz, and the amp current draw varies at 2x the input frequency, I limited the top of the measurement range to 42KHz. The input signal was 300mv peak-to-peak, and the input frequency was stepped starting at 40Hz and ending at 1,280Hz.
I’m really not sure what the high-frequency peaks are starting at 18KHz or so. I probably should have looked into them more closely.
Now that I have the measurements, I have to make sure I actually understand them before I figure out what to do, and how to do it. I note that in PBTL mode, the chip has a power supply ripple rejection ratio of 75-80dB up to 1KHz, rising to ~55dB at 8KHz and continuing at that level for the rest of the audio range.
Earlier this week I fired up WaveForms in order to do some spectrum analysis of power supply rails hooked up to a Class D amplifier board using my Analog Discovery. Before I got the Analog Discovery plugged in though, my computer froze, the screen went dark, and then a kernel panic error displayed.
This was new, I’ve never had a problem before, whether or not the device was hooked up before firing up the software. I tried again, this time making sure the Analog Discovery was plugged in before launching Waveforms. Another kernel panic. I tried again, this time plugging it directly into my laptop, rather than the USB3 hub I use for convenience. Another kernel panic. I muttered and cursed and speculated that the cause was the OS X 10.11.6 update I’d recently installed, and ended up using a Windows machine for the task instead.
A few days later, I received a cheap RTL2832 USB DVB receiver I’d purchased in order to (finally) experiment with Software Defined Radio (SDR) using RTL-SDR software. My first explorations were tacked from base camps on my patio and bed. Everything worked without issue. The third time though, I was at my desk, and as soon as I launched CubicSDR my computer froze, went black, and then complained of a kernel panic.
I was peeved. I thought, perhaps that my hub was an issue, so I plugged the RTL2832 in directly. It worked. I tried a different hub. It worked again. I tried something else, it crashed.
Eventually, I figured out that the problem only occurs with a USB3 to Gigabit Ethernet adapter connected (with or without a hub involved). The adapter in question is based on the RTL8153 chip, which is used in USB3 to GigE adapters sold under many different names.
This chip is complaint with standard USB Ethernet protocols, and works with generic drivers provided by Apple in OS X 10.11 (El Capitan), and perhaps earlier versions. I’d installed the Realtek provided driver in order to take advantage of some features of the chip which might improve performance and power consumption, but not to a significant degree.
By removing the Realtek RTL8153 driver using a script provided in the installer package, I was able to revert to using the generic apple USB Ethernet driver. Now, I can use my Analog Discovery with Waveforms, my RTL-SDR with CubicSDR, and my USB3 Ethernet adapter at the same time without Kernel panics.
I’m asking, but I think I already know the answer: No, no one knows anything about the SMSL HV-50 audio amplifier, or if they do, they aren’t talking (not in english, anyway). I found a recent reddit thread asking about it, but no one knew much, and I haven’t found much else.
The TDA7492 chip is used in a lot of compact, inexpensive audio amps made by various Chinese manufacturers and sold under various names on Amazon, Ebay, AliExpress, etc.
It is also used in the older, almost identically specced 50 Watt/channel SMSL SA-50 amplifier, pictured above. The chip and specs aren’t the only thing the two have in common. As you can see, the case is the same. The only outward difference is slightly different markings and detailing.
The insides are a different story. Both use the same TDA7492 amplifier chip, but as you can see, the PCB and components are quite different.
Last night I noticed that one seller was offering the HV-50 for just $35 with free shipping, so I ordered one. At that price, it was a no-brainer. I’d have easily spent $25 and several hours getting one of the cheap TPA3116 boards I’ve ordered mounted in a suitable case.
I’ll follow up with a new post in a few weeks, once I have the HV-50 in-hand.
I recently discovered and purchased an inexpensive, unofficial WiFi-enabled AirPlay and DNLA audio receiver called the AirMobi iReceiver. I couldn’t find much information on the device, but for $12, I thought it was worth buying and trying.
It works reasonably well, but that’s not really why I bought it. I bought it with the intention of taking it apart and seeing what makes it tick. And now, having done that, I plan to hack it to run OpenWRT so I can secure it, customize it, and update the software.
It is based on a Ralink RT5305T WiFi SoC which suggests to me that it is running linux, and probably has a serial console exposed via some test points on the mother board. I only found handful of candidates during my teardown. My guess was that the Tx and Rx lines were available on the unpopulated 4-pin header at the edge of the circuit board. From visual inspection I could tell that the second pin from the left was a ground pin. A little continuity probing with a multimeter suggested the first pin provided power, a fact confirmed when I check its voltage when I powered up the device.
I hooked a logic analyzer up to the other two pins to see which one toggled on and off at boot, but that was really overkill. I could have done just as well figuring out which one was pulled high when I powered up the device.
Once I had the pins worked out, I hooked up a TTL level USB/serial converter to my laptop, connected the ground pins and cross connected the Tx and Rx pins between the adapter and the board. Once I powered everything up, my screen started to fill with garbage. I guessed that 115.2Kbps was too fast, and tried 57.6Kbps instead. Bingo!
After booting up, I hit return and was presented with a login prompt. I tried the password for the webui and was pleased to find that it worked. I poked around the filesystem, looking at various config files, the various files for the web UI, and checking what binaries were installed on the system.
One of them is a telnet daemon (implemented as part of busybox). So, I started it, connected to the WiFi, and was able to log in over the network.
From there, I gathered more information. I was dissapointed that there wasn’t really anything like zip, or tar, or an ftp or ssh server that would make it easy to pull a bunch of files off at once, so I dumped the web UI files to the terminal one at a time and then saved them for further inspection.
Hidden test_ate.asp page
Once I did, I found hidden functions in the firmware update page for uploading the bootloader over the webui. Exposing it required tweaking the page using web developer tools, which is kind of tedious. Then I hit the jackpot, I found an unlinked file called test_ate.asp. When loaded, it has a button to fire up the telnet daemon, making a command line available with just a WiFI connection, no serial console necessary. It also has an option to update the boot loader and a mysterious ATE function. This discovery made it easier to return and poke at the device at my leisure.
From what I learned in my poking and prodding, it appears to be based on the Ralink provided SDK with some modifications. With any luck, the modifications will be minor, and it will be easy to load an OpenWRT firmware over the webUI.
Before I do that though, I’ll need to take special care since this device doesn’t have an ethernet port, and so recovering from non-working firmware will be more difficult.
I’ve ended up with five small, inexpensive ($7-15 each) routers, running OpenWrt and only really need two of them, so I’ve been thinking of ways to use the others. One of my ideas was to get an external USB DAC, install Shairport-Sync, and use it as an AirPlay receiver for my car stereo, eliminating the need to connect an audio cable to my phone, and avoiding the mediocre sound quality of Bluetooth audio. It hasn’t quite worked out that way though…
While looking for an inexpensive (>$20), compact USB DAC with reasonable quality, I discovered there were integrated commercial products that already do what I planned to do. I already knew there were Apple-approved MFi-certified devices, but they tended to be expensive. I discovered there were cheaper devices using Shairport, but they tended to start at $30+.
Damaged while trying to open the case.
With a little more digging though, I found a device called the iReceiver, from AirMobi that sells for as little as $12!!!. According to the scant marketing materials, it has a 24-bit Wolfson DAC. I was surprised I couldn’t find anyone who’d opened one up to see what was inside. I did find an Amazon review from someone complaining that the usb power connector had broken off on theirs, and the included photo showed it had a Ralink RT5350F WiFi SoC, which gave me hope that it would be hackable. So, I bought one.
Before opening it up, I tried it out. It works as promised. It defaults to broadcasting an unsecured WiFi network. Once connected, it shows up as an AirPlay receiver in iTunes, etc. From there, you can connect it to some powered speakers, select it and start playing music. The audio quality doesn’t suck (no obvious noise, clipping, or distortion), and in my limited use, there were fewer dropouts that I’m used to with Bluetooth.
Beyond that, there are various configuration options available through a browser based interface. There are no audio-related settings at all. Most of the settings are networking related. You can rename and secure the WiFi network with a password (good), WPS (bad) and by limited connections to specific devices by MAC address (meh). You can also connect to an existing network (good), and, optionally, extend it (meh). This seems like a good point to mention that it also works as a DNLA “renderer” (DNLA is a more open standard than AirPlay, making this useful to Windows and Linux devices, and Android phones with an appropriate app)
Of course, I didn’t buy it to use it with the stock firmware, so after trying it out, I opened it up to take a look inside. In the process, I managed to tear the translucent plastic that was affixed to the top of the case with adhesive. With the trim removed, it was easy to pry off the top, revealing the single PCB inside.
The other major component is a Wolfson WM8960 CODEC to provide the audio output. This chip debuted in 2006, and includes 24-bit stereo DAC and ADC converters supporting sample rates up to 48Khz, a 40mW headphone driver, and a 1W Class D speaker driver.
Despite being a 24-bit DAC, the specified SnR of 98dBS matches that of the 16-bit TI/Burr Brown PCM2705 DAC used in the original AirportExpress, rather than of a modern, premium 24-bit DAC used in more recent AirportExpress’s. Oh well. Good enough for my purposes. Most of what I’m playing is compressed AAC files derived from 16-bit sources, and, AirPlay only passes 16-bit anyway. Beyond that, the design of the rest of the circuitry matters, and I’m not qualified to analyze it, nor am I equipped or inclined to try and measure it.
Beyond that, I see two inductors on the board (one of which is cracked). My guess is that these are part of some small switch mode power supplies, perhaps one for the digital section, and the other for the analog. There are two small LEDs to indicate device status and two momentary switches, one to reset the device, and the other to trigger WPS. It looks like it uses a single ceramic chip antenna for the WiFi.
There are a few unused pads for components, eight test points (half seemingly to do with power) and four unused holes for pin headers that I suspect provide a serial console.
That’s really it for the hardware. I’ve already started poking more deeply into the software and investigating the suspected serial console, and I hope to have another post soon documenting what I found.
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.
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.
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.
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.
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.
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.
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.
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.
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.