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Power Designs TP340A Repair/Refurb

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I picked up an old Power Designs TP340A bench power supply on eBay. The TP340A is a three channel (or “source) power supply. Source A & B have identical specs, providing up to 1A from 0-32v DC. They can be operated independently, or in tracking mode to provide positive and negative voltages. The third source only covers from 0-15v, but can deliver 5A at up to 6V and 2A at up to 15V. I bought it to power projects as I teach myself more about electronics. Little did I know what I was in for.

It showed up well-packed in great physical shape. There were a few scuff marks on the case where it had probably been pushed up against another piece of equipment, and some stickers and a few scuffs on the face plate, but otherwise, it looked nearly new.

When I took it apart, I found the insides were in similar condition.

On closer inspection though, I noticed something that wasn’t quite right

Bad C104 is Bad

The 100 uF 25V Sprague electrolytic capacitor in postion C104 looked like it had a bad inner seal. I decided not to power up the PSU until I’d replaced this cap.

The other two channels have the same type of capacitor in the same position in the circuit. They looked Ok, but I decided they should be replaced too, and while I was at it, I figured I’d also order replacements for the other big electrolytic caps. This decision proved to be a mixed bag.

The visually intact sibling 100 uF Sprague caps proved dead when I tested them after replacing them. On the other hand, the other electrolytics were still in spec. Which is more than can be said about some of their replacements.

After replacing all the caps, I powered things up and was greeted by a wretched buzzing metallic groan . I quickly switched the power off and gathered my wits, such as they were. Then I turned it on again for long enough to twiddle some of the knobs, things still weren’t right, but I had slightly more information. I switched it off again, thought for a minute, and switched it on again. This time the horible groan was joined by a ffffffssstPOPffffff. I switched it off, but there was another ffffffssstPOPffffff. I’d put two of the capacitors in backwards and they’d vented.

I replaced the vented caps with the originals getting the polarity right this time, so I could see if I’d done the thing permanent harm. Happily, the horrid groaning sound didn’t return the next time I switched the power on. It didn’t work though.

It didn’t take long to find them problem, I’d turned the voltage and current limit knobs the wrong way. After correcting that problem, I found that all the channels of the supply were fully functional, though things didn’t seem quite right. In tracking mode, source B didn’t respond at all until the voltage was up to about 8V and then started dropping off as it was turned up past 16v.

I started going through the troubleshooting steps in a PDF copy I’d found of the operating manual, but that’s going to be the subject of another post.

Tektronix Mainframes

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I’ve been looking into old Tektronix osciloscopes and related gear lately, and I thought I should write-up some of what I learned.

Last year, I posted a few installments in my saga of figuring out what to buy for my first osciloscope. I ended up with a Rigol DS1074Z, and while I haven’t gotten a lot of use out of it, yet, when I have used it, its saved me a lot of troubleshooting time.

Recently though, I’ve been looking for ways to address some of the limitations of my scope. In particular, I’d like to be able to do low-noise differential measurements on one or more channels. In part, this allows more flexibility in using all my scope channels to look at power supply circuits. It can also be useful for looking at power supply output noise and ripple.

One approach is to use the math function of the oscilloscope to calculate a differential between two of the input channels. This has its uses, but suffers from slow-update speeds and the fact that some of the signals I’m looking for are already at the limit of the DS1074z’s resolution.

Another approach is external differential probes. Unfortunately, these are expensive. New they start at $300 or so. Used are a little better, starting at $100, but most seem targeted at high-voltage rather than high-sensitivity use.

This brings me to the Tektronix gear. I’m less interested in the 7000 and 5000 series scopes themselves, than in all the various modular “plug-ins” (particularly high-sensitivity differential amplifiers) Tektronix developed for them. Tektronix also sold a line of stand-alone chassis called the TM500 and TM5000 series, and an accompanying line of plug-in modules.

Now, the first thing you need to know is something I was lucky to figure out before buying anything on ebay, which is that, while the plug-ins for the 7000 series, the 5000 series, and the TM500 and TM5000 series all appear to have superficially similar form-factors, they are incompatible. You can’t use a module intended for a scope in the stand-alone TM500 or TM5000, or vice-versa. Nor can you use a module for a 5000 series scope in a 7000 scope, or vice-versa. There are other important distinctions too.

Within the 5000-series of scopes and modules, there is a distinction between “slow” (~2MHz bandwidth) and “fast” (50MHz bandwidth). You can use slow modules in fast scopes, but you can’t use fast modules in slow scopes.

Within the 7000-series, which cover an even wider range of bandwidths from 25MHz all the way up to 1GHz, most scopes are compatible with most plug-ins, according to Tektronix.

For the stand-alone mainframes, modules for the TM500 will work in the TM5000, but the reverse isn’t always (usually?) true.

My inclination is to get a 4-slot stand-alone chassis like the TM504 to save space and minimize shipping costs. Unfortunately, it seems that the AM502 differential amplifier module is rather rare and relatively expensive. There is just one on eBay at the moment and only a few in the available history of past sales, and the prices seem to start at $100.

Meanwhile, there are multiple examples of the equivalent 7A22 or 5A22N modules for the 7000 and 5000 series scopes, with prices starting below $50. The necessary scope and chassis can be had for as little as $100 or so more, about 2x what a TM500 chassis might go for, with the downside of added shipping costs and the (possible) upside of a second scope. Moreover, there are apparently pass-thru outputs so would still have the option of using any modules I acquire with my existing digital scope. I’m also interested in other modules, like function generators.

The smartest thing, at this point, would be to put this project on hold and finish up the half-dozen Keithley 197A multimeters I’m in the process of restoring and repairing, or the Power Designs TP340A I’m in the middle of fixing (destroying?). If wisdom prevails, I’ll have this post to remind me of what I’ve learned, should I ever come back to the idea of buying some old Tek modules.

To that end, here are some of the resources I found useful in researching this:

USBHV, because why not have a USB-powered 2000V source?

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In the last 5 years or so, USB has emerged as THE standard power source for portable electronics, and a host of other low powered devices.

GNEMCO_05Today, I happened to stumble upon an early example called the USBHV on eBay. The USBHV is a USB powered high-voltage source from EMCO High Voltage, released in 2009. The USBHV was positioned as a compact, USB-programmable (and powered) high-voltage source for research use. From what I’ve been able to tell, there was actually a line of products, differentiated by positive or negative voltage, and maximum voltage, with the option of 200V,  500V, 1000V, 1250V and 2000V at up to 1W of output (USB can deliver 2.5W). My guess is that they had a board with a USB controlled AD/AD converter for setting and reading back voltage, and mounted one of their standard high-voltage power modules.

The present-day EMCO High Voltage website only has one tiny reference to the product, a link to a generic form for information on off-catalog products, so no datasheets, manuals or software.

Keithley 2000 RS-232 Serial WTF

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My Keithley 2000 DMM craps out during serial communications with a PC over RS-232.  When it does, the voltages on the TxD (yellow trace) and RxD (blue trace) lines look like this:

19.2kbps Fail KE

There are a few things wrong here.

  • The idle voltage of the RxD line should be -7v or so, similar to that of the TxD line, rather than the -1.7v it starts at.
  • The obvious decline in signal quality before the RxD locks at the ~1.2v shown at the end of the trace.
  • RxS locks at 1.2v, rather than returning to it’s idle voltage.

I came across someone who had trouble with the RS-232 level shifter IC on the similar Keithley 2015, so I carefully checked it out. It seems to be well under-spec on the output of its internal -10v power supply, which can only deliver a sustained 6mA. The +10v supply, on the other hand, can provide much more. I also diligently checking of voltages and current delivery of all the signal lines both the multimeter and the USB to RS-232 adapter its connected to.

It appears that the PC is trying to drive the RTS line to +7v. At the same time, the multimeter is trying to drive the RTS line down to -9v, and its loosing. As a result, it can’t drive the RxD line below -1.7v, and eventually, while transmitting, it gets stuck at +1.2v.

But why is the the DMM trying to do anything on the RTS line? That’s the job of the “data terminal equipment,” or DTE. I checked the reverse-engineered Keithley 2000 schematic, and it shows that the RTS line is connected to one of the transmit outputs of the level-shifter IC, something I confirmed by doing a continuity test. This makes no sense to me.

If I disconnect the DMM from the RTS line, everything seems to work fine. The DMM drives the RxD line to -9v at idle, and sustains signal quality throughout a transmission.

RxD is in much better shape when RTS isn't connectedI can’t imagine I’m the first person to come across this. I’m surprised though that I can’t find any mention of it online.

Update:

I posted about this in the EEVBlog Forums and a few users provided some details of similar issues they’ve had with RS-232 communications on the Keithley 2000 DMM. None of them have gone as deep as I have, but their descriptions are explained by my hypothesis.

I’m interested in whether later versions of the firmware leave the RST pin floating. So far no one with more recent firmware has checked for me, but one user remembers someone getting a similar problem fixed with a firmware update.

I’ll probably try doing a firmware update on my own. One user reports that he figured out he could use some Flash chips replace the EPROMs, which is attractive because I can re-use the chips and I don’t have to buy a legacy device like an EPROM programmer.

In the meantime, I picked up a straight-through male-to-female DB-9 cable and clipped off the RTS pin (#7). With it in place between the USB RS-232 adapter and the DMM, I ran a test querying the DMM with “*IDN?” every second for an hour or so. The DMM remained responsive for the whole time, and the responses it sent were complete, and uncorrupted. Previously things crapped out within a few minutes and only a few commands.

Update 2015-07-20:

I now have a unit from ~2007. It leaves the level of the RTS pin to the DTE, as it should. Upon closer inspection I found that the board has been revised so the RTS pin is no longer connected to the level shifter IC at all.

Keithley 2000 “Repair” Tip

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I won an auction to buy a Keithley 2000 6 1/2 digit multimeter on Ebay for ~$250, a pretty good price. The seller said it had some scuffs, but was “tested and ready for work.”

When I received it, it was clear that it was a little worse for wear than claimed. It had a cracked rear bracket, and a yellowish/brown tint, rather than the shades of grey of a new machine. At first I thought it might be yellowing due to sun exposure, but the yellowing seemed to afflict the painted metal case as well as the plastics.

It did power up, and when I tested it with a voltage reference I have, its readings came in pretty close to the expected value, so I didn’t worry too much about the physical condition beyond trying to wipe the outside down well with cleaner and isopropyl alcohol.

Once I’d done that, I decided to look inside, to see if I could get an idea of the manufacture date. As soon as I removed the case, it was obvious where the yellowing had come from. It stank of old tobacco smoke on the inside, though fortunately, there wasn’t an obvious film. As I looked around, inside I could see a number of components with date codes for mid 1995, which matched well with the date of the first and only calibration, November 1995. It was almost 20 years old.

In the process of looking at the insides, I noticed that he input wires seemed a little close to some metal projections from the input selection switch, which seemed a little sloppy. Then I realized the board seemed a little slanty, and was out of its mounting tracks, possibly it had been jarred loose in an impact. On the opposite side of the chassis, I saw that some wires to the front panel and the power transformer weren’t routed through a retaining clip. Someone had taken this thing apart, and done a poor job of putting it back together. I loosened some screws so I could slide the board back into position and noticed a gouge in the PCB when it had been forced into place during a previous reassembly. Fortunately, it only damaged some solder mask, and not the trace underneath.

Once I had it back together, I ran the self-tests and was discouraged when it reported a number of faults for tests 100.1, 101.2, 101.3, 200.1, 200.2, 201.1, 201.2, 300.1, 301.1, 301.2, 302.1, 302.2, 303.1, 303.2, 304.1, 400.2, 401.2, 402.2, 403.2, 500.1, 500.2, 600.1, 600.2 and 601.2. That’s most of the self tests.

I tried to work through the troubleshooting in the repair guide, but it was dismal, it didn’t even describe the signals on the half-dozen or so test points on the board. I found a reverse engineered schematic and dove in.

I was working my way around the A/D chip trying to orient myself to the various signals, when I noticed…something. At first I thought it was a stray line on the silkscreen, but on a closer look, it seemed to be a fiber, cat hair? Dog hair? I flicked it away with my gloved hand, and then blew the area clear with some canned air.

I reran the tests and they all passed! One tiny fiber in the wrong place was enough enough to through the A/D converter out of spec cause a cascading failure.

The cheapest/best 2012 Retina Macbook Pro SSD Upgrade

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Long-time Retina MacBook Pro owners finally have some reasonable storage upgrade options. Apple has used proprietary SSD form-factors and connectors for their thin notebooks since the second generation MacBook Air was released, but they’ve always used standard storage protocols, like SATA.

I’ve long hoped that someone would come out with a simple, inexpensive adapter that would allow a commodity mSATA or m.2 SSD in my Retina MacBook Pro, but I could never find any on the market. When I needed more storage, I decided to supplement my original SSD by fill my laptops SDXC slot with a 64GB micro SDXC in a shortened microSD-SD adapter. It wasn’t exactly cheap, or fact but it was affordable, worked well enough. OWC offered an upgrade for the original SSD, but it was relatively expensive, particularly since I replaced, rather than augmented the original storage, and it was slower than the stock option.

I knew I’d outgrow the add-on card someday. I was heartened that some companies offered larger, higher-performing shortened SD cards, and that Trancend released a line of Apple upgrade SSDs to compete with OWC, but I still thought an adapter would be the best approach. I gave more than idle thought to developing and selling my own adapter, since no one else was doing it.

In the meantime, I managed to keep ~40GB free on the original SSD. Then, this past week, all that space filled up almost overnight. I’t seems that a 15 video I was editing in iMovie resulted in the creation of lots of temp files. I tried to make more space, by copying stuff to a server, and I succeeded well enough to have enough room to finish editing the video. I knew though, that it was time to find an upgrade.

The OWC options still weren’t very compelling, and my next step was to see what the market for original equipment SSDs was on eBay. The prices weren’t too bad, if I could put aside concerns about the quality and provenance of the drives. I decided to search for adapters, and didn’t find anything at first, but stumbled across a company called MaxUpgrades, that was selling Samsung EVO 840 SSDs with an rMBP adapter for non-obscene prices (Warning: BAD website).

With more digging, I found similar, if not the same, adapter cards were on Amazon for ~$20 and on eBay for ~$10. So, I ordered the adapter, a 500GB Samung 840 EVO SSD, and another adapter that would let me use the mSATA SSD with a USB3/SATA adapter cable I already had to migrate my data before installing it in my Macbook. It only took a few days for all the parts to arrive, and an hour or two to get the data copied and install the new SSD + adapter in my computer.

Everything was pretty straightforward. I think the one thing that I wasn’t sure about was how to transfer everything to the new SSD. I originally settled on using demo version of Carbon Copy Cloner, but then I realized that I could do what I needed by rebooting in recovery mode, running Disk Utility, and then restoring to the new SSD using the old SSD as the source. In addition to moving the OS install and all my data, DiskUtility automatically creates a new recovery partition.

Once that was done, I rebooted from the new SSD over USB to check to make sure it worked before shutting down and installing the new SSD, with the OS, data and everything already transferred. Once I had everything back together, it booted right up.

I’ve ordered an adapter that will let me use the old Apple SSD in place of a 2.5″ SATA drive. I’ll probably put it in a case with an integrated USB3 adapter and use it for extra speedy storage and backup.

A few things to consider before doing this yourself:

  • The mid-2012 and early-2013 retina MacBook Pros (MacBook Pro 10,1, A1398) use the SATA 6Gbps protocol to connect to SSDs. Later models use PCIe, but I don’t know if the necessary adapters exist.
  • This upgrade might create issues if you are still under the original or extended AppleCare warranty. In theory, “user replaceable” parts don’t invalidate the overall warranty in the US, but Apple might argue that their SSDs are no longer user-replaceable.
  • Opening up an rMBP and removing the SSD requires a special pentalobe screwdriver. I used this inexpensive toolkit, which has the necessary bit, and many others.
  • Cheap Chinese electronics from ebay can be a great deal, but the quality and documentation is inconsistent. Compatibility (and incompatibility) information should be taken with a grain of salt.

Fish8840 AVR Transistor Tester Review

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Today, I’m looking at a neat gadget I got on ebay for about $20 called the “Big 12864 LCD Transistor Tester Capacitance ESR Meter Diode Triode MOS NPN LCR.”

There are hundreds of listing for dozens of variations of these under different names, for prices ranging from ~$12-40.  Most, if not all of them, are made in china. Most, if not all of them, are descended from the AVR Transistor Tester project by Markus Frejek (or google translated), with further improvements by Karl-Heinz Kübbeler (or google translated). Unforunately, none of the Chinese clones honor the projects license and release source-code for their firmware modifications. Fortunately, people are figuring out the hardware differences on some of them, and adding support for to the open source project. The english language documentation for the project is great. It actually includes information on some of the chineese clones. Even better, the design and documentation are a great example for learning how to make good use of the hardware on an AVR MCU.

The Fish8840 version I have, which has a PCB date of 2014-07, has stupid bug in the power-management circuitry which causes it to have excessive current drain when it is supposed to be “off.” This video review by George Thomas of AmateurLogic.tv includes a simple modification that fixes the problem.

I didn’t really love this one. In addition to the flaw described above, some of the graphics are hard to read. Plus, there are rumors that the hardware is locked to block installation of different firmware.

For more information:

Animated Crashplan Icon Causes Window Server to Burn CPU on OSX

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I use Crashplan for backup on OSX. Tonight I happened to notice that the OSX windowserver process was consistently burning ~25% of a CPU core on my Mac for no reason. I also noticed that Crashplan was backing up my computer at the time.

I decided to try switching the preferences for the Crashplan menu bar icon to disable animation, windowserver’s CPU utilization dropped. I turned animation back on it jumped back up.

Someone should change the default.

Quick impressions: iOS 8 on the iPhone 4s and iPad Retina (3rd generation)

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I installed iOS on both my iPad Retina (3rd gen) and my iPhone 4s. I had some misgivings, since they are both at or near the bottom of the barrel in terms of supported hardware for this release. After some research though, it sounded like the iPad would do reasonably well. As for the iPhone, well, I should have a newer model in a week or two if it doesn’t work out.

My report is that both of them seem to work just fine. There may be a few more rough spots and hitches than before, but if there are, they are rare and subtle (for the most part), and they are counterbalanced by new features and refinements.

One issue on the iPhone 4s is that all the other models supported by iOS have taller displays, and so some UI is more suited to larger phones. The only place where this really stands out is when composing an email. Between the predictive text selections above the keyboard (which can be hidden, if desired) and the new UI affordance for easily switching between the email you are drafting and other messages, there isn’t much room to read the text you are writing.

There are definitely signs of memory pressure though. Tabs in Safari need to reload more (too) often and switching between some apps can take a little too long.

My advice for iPhone 4s users is to wait until they release a round of bug fixes, unless you absolutely need to try the latest and greatest.

As for the iPad, I don’t notice many differences in interaction experience from iOS 7. Safari may need to reload tabs a bit more often, but no where near as often as on the iPhone.

My advice for anyone with a 3rd generation iPad, or better is: If you want it, go for it.

If you want to know why you’d want to upgrade in the first place, there are lots of reviews of what’s new in iOS to choose from. Its worth noting though, that some features aren’t available on all devices. It looks like my devices will be left out of some of the seamless handoff as you move from a Mac to your iPad, or vice versa. One thing that just worked though, incoming phone calls show up on my iPad, and I can choose to answer them there, if I want. Not a huge deal, but convenient in some situations.