Power Designs TP340A Repair/Refurb

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

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?

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.