Power Designs TP340A: Repair and facelift

[bitseeker]

A little while ago I acquired a Power Designs TP340A listed in classic fashion as for parts or not working. Following is a little repair diary on it.



When the supply arrived, it looked fine on the outside with all appropriate controls and even its original feet. A quick look inside revealed nothing catastrophic, so I checked the main fuse in preparation to turn it on. Blown. Had to get some new ones.

Meanwhile, the 10-turn pot (blue housing in the photo, below) for setting the voltage on Source A (the top-most channel) was popped open. Weird. I tried reassembling it a few times, but it wasn't reliable. On the low end, the wiper would pop out of alignment. Ordered a new one.



Once the I got the fuses and popped one in, it was time to see if it'd fire up. It did. With a sizzle and then the fuse blew.

Probing around, it seemed that Source A and Source B were fine. No shorts. Diodes and transistors were behaving fine. Below is a shot of the output board (left) and Source A & B board (right). Both sources have their own fast-blow fuses and they were intact.



Continuing onto Source C (board shown below). It also has its own fuse, still intact.



Checking before the fuse there seemed to be a short at the input filter caps. You can see these whoppers on the underside of the Source C board, below.



After getting some new caps to replace these, I ran into what seemed to be the bridge rectifier for Source C, based on the schematic. It's bolted to the rear of the chassis and, as you can see below, was burned to a crisp. Obviously, this is the source of the problem. The filter caps are probably fine, but since I got new ones, changed them anyway.



The schematic shows that the bridge rectifier is a BR-252. Discontinued. http://www.mouser.com/ds/2/345/br2505-2510-11168.pdf

Fortunately, there are replacements by both Rectron and Fairchild. Although Mouser had the old BR-252, I opted for the newer MP-252. http://www.mouser.com/ds/2/345/mp2505-2510-14498.pdf

When the MP-252 arrived, I was surprised how big it was. The fried part was approximately 10mm wide, whereas the new part is 30mm wide. So, I cut open the old module and what did I find? It wasn't even the correct part. Someone made their own bridge rectifier with four puny diodes and covered it in silicone caulk! Well, is it any wonder it didn't survive?

Installed the new bridge rectifier and probed around to verify that everything was soldered back correctly. Looked good. The moment of truth. Since I don't have a variac, it was going to be an all-or-nothing test.

Plug it in. Turn it on. Et voilà! No magic smoke. Voltage and current settings on Source B and Source C work as expected and the analog meters match my bench meter.

So, up next is replacing the 10-turn pot for Source A and probably adjusting the trim pots. Also, I'm not a fan of the black plastic knobs that PD put on the voltage pots. They're a bit small, not aesthetically pleasing, and they leave a chasm between the front plate and the knob's flange so the shaft of the pot is visible. The earlier TP325 with its aluminum knobs looked much better.

Stay tuned...

Jump to part 2: https://www.eevblog.com/forum/repair/power-designs-tp340a-repair-and-facelift/msg770494/#msg770494

[FlyingHacker]

Nice article and pics. Thanks for sharing. Those PD supplies are beautiful.

Get a Variac... I found a General Radio V20 on eBay and Best Offered it at $45. I have used it extensively since then.

[ez24]

Get a Variac... I found a General Radio V20 on eBay and Best Offered it at $45. I have used it extensively since then.

How would a variac help? thanks

[eas]

Get a Variac... I found a General Radio V20 on eBay and Best Offered it at $45. I have used it extensively since then.

How would a variac help? thanks

Gradual ramp up of the AC input to give the chance for caps to reform, but also so that failures aren't as spectacular/damaging.

Also useful for checking line regulation.

I've been keeping an eye on variac listings on eBay, but haven't pulled the trigger yet. Interesting fact, GenRad, or one of its successors, which held the Variac trademark for a long time, sold it, along with their then-current Variac product line to, Technipower Systems, a DBA of Power Designs Inc. That didn't happen until 1996 though.

Nice troubleshooting, bitseeker! Interesting to learn about the internals of your dead bridge rectifier. Interesting coincidence that we both have TP340a with rather poorly considered repairs to Source C. Interesting to see the different sourcing of components between your unit and mine. Also interesting to note differences in analogous components between the three sources. Looks like you've got two different zeners for Vref 101 and 201, and, like mine, differing sources for the quad opamps.

I was going to post a link to those knobs I told you about in case anyone else is interested, but I'll leave it to you to do the reveal in good time...

[FlyingHacker]

In this case it would not help much, since the OP was successful and there were no other problems he was not aware of.

In general, it helps you bring devices up slowly to avoid damage (mainly by watching current usage with an ammeter as you increase the voltage, with the idea being to turn things off quickly before they might be damaged if there is a high current draw and a high voltage.) It also helps with avoiding inrush spikes on old capacitors.

It's kind of like driving your car real slowly at first so if you hit something you do less damage, rather than flooring it right away like turning on a switch with full mains voltage.

A series lightbulb as a current limiter is also useful (with or without a Variac).

They can be less useful with switch mode power supplies that might not work at lower voltages (or some designs may possibly be damaged).

[bitseeker]

Nice article and pics. Thanks for sharing. Those PD supplies are beautiful.

Get a Variac... I found a General Radio V20 on eBay and Best Offered it at $45. I have used it extensively since then.

Thanks, FH. I hope to do it justice with some new (to me) knobs as part of the cleanup and facelift.

A variac is on my shopping list.

[FlyingHacker]

Yeah, I lucked out on this Variac. They were asking $60 for it. I low balled at $45 and it was accepted! Tested it out watching the output on an analog meter and it is very smooth, with no dead or reduced voltage spots.

I have been using it to bring up any used equipment since I got it. It's a lot nicer than the big pop you get when things go badly at mains voltages.

[bitseeker]

A series lightbulb as a current limiter is also useful (with or without a Variac).

Yeah, eas recommended that to me. Great idea. I guess I was getting impatient to see it run.

[bitseeker]

I've been keeping an eye on variac listings on eBay, but haven't pulled the trigger yet. Interesting fact, GenRad, or one of its successors, which held the Variac trademark for a long time, sold it, along with their then-current Variac product line to, Technipower Systems, a DBA of Power Designs Inc. That didn't happen until 1996 though.

Hopefully, we don't end up trying to outbid each other. EEVBlog needs an eBay collision warning system.

Nice troubleshooting, bitseeker! Interesting to learn about the internals of your dead bridge rectifier. Interesting coincidence that we both have TP340a with rather poorly considered repairs to Source C. Interesting to see the different sourcing of components between your unit and mine. Also interesting to note differences in analogous components between the three sources. Looks like you've got two different zeners for Vref 101 and 201, and, like mine, differing sources for the quad opamps.

Thanks for your inputs along the way too! Yeah, and I don't know what the deal was with my dangling 10-turn pot. Makes you wonder what these things have been through.

I was going to post a link to those knobs I told you about in case anyone else is interested, but I'll leave it to you to do the reveal in good time...

Thanks, that'll certainly be part of the facelift coverage.

[ez24]

Hopefully, we don't end up trying to outbid each other. EEVBlog needs an eBay collision warning system.

I am bidding on three

I think you would see me as bidder  e***s

For example currently I am high bidder on this one

http://www.ebay.com/itm/351535824859?_trksid=p2055359.m1431.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I am looking for an old one with no grounds.  I have no problems not bidding against each other

[bitseeker]

Ah, yes, I recognize that one. OK, I won't bid on that one (it looks like it has a ground line, though).

Maybe we should use a different thread for shopping.

[bitseeker]

I'll wrap up the pot replacement and facelift tomorrow and post accordingly.

[crispy_tofu]

Hopefully, we don't end up trying to outbid each other. EEVBlog needs an eBay collision warning system.

Maybe one with Dave's voice going 'wah' repeatedly. 

[bitseeker]

Pulled the busted 10-turn pot out and installed the new one. Works great and didn't even have to mess with the trim pots. The new pot takes a little more force to rotate, but I think it's better that way. The original ones turn much too easily.

Now, with regard to the voltage knobs, I had looked for quite a bit online for aluminum knobs that were similar to those used on the TP325 and TP340 (non-A version). Most things I found were either too big, totally smooth, or cross-hatch knurled instead of straight.

Then, eas, who also owns a TP340A, pointed me to an eBay listing for some used knobs that look a lot like the ones on the Power Designs 3650S. http://www.ebay.com/itm/350957127412 So, I ordered a set and put them on. What a difference!



The flange on these knobs is narrower than on the TP325 and have a satin finish, but I think I like these better. Here's another angle.



And compared to the original, plastic knob.



Finally, here's a comparison of the gap left by the original black knob (right).



Compared with the aluminum ones (behind the black knob).



So much better!

Is that all? For now it is, but some other things I have in mind are to eventually replace the remaining electrolytic capacitors (there are quite a number of them), remove some sticker residue in a few places, maybe add some LED lighting to the meters, and I'd like to have the needles in the meters colored orange like the older PD supplies.

[eas]

Nice! I'd missed this update.

I share your desire to add some lighting to the meters.

For sticker residue, I've found that IPA is often a good solvent. Googone, orange oil, or naptha/spot remover can also be useful. Melamine sponges are great for tasks that need a little mild abrasive, but use rubber gloves or a paper towel to hold them, because otherwise they can really iritate your skin.

I've also used various pencil erasers for targeted abrasion or just friction polishing, depending on the grit of the eraser.

Finally, I've had good luck using "Barkeepers Friend" which is a mild abrasive cleanser, for cleaning the brushed aluminum. I put some of the powder in a bowl and then use a damp Q-tip or similar to pick some up, apply it, and rub it in. I avoid rubbing it in areas with silkscreening or painted engraving, but it doesn't do any harm if it gets applied there.

A question, what is the "grain" direction of the brushed aluminum on your supply's faceplate? In the photos of your supply, it appears vertical. On mine, its horizontal, as it is on my other PD supplies.

[bitseeker]

I share your desire to add some lighting to the meters.

Yeah, not that I plan on working in the dark, but it would look cool in addition to aiding visibility. What color would you use? Warm amber? Cool green or blue? Stark white?

For sticker residue, I've found that IPA is often a good solvent. Googone, orange oil, or naptha/spot remover can also be useful. Melamine sponges are great for tasks that need a little mild abrasive, but use rubber gloves or a paper towel to hold them, because otherwise they can really iritate your skin.

Near the bottom is some really hard adhesive. I put Goo Gone on it and let it soak for the past week. I should check on its progress.

Melamine sponges are great. I use them to get hard water deposits off glass, too.

I've also used various pencil erasers for targeted abrasion or just friction polishing, depending on the grit of the eraser.

They're also good for getting oxidation off electrical contacts.

Finally, I've had good luck using "Barkeepers Friend" which is a mild abrasive cleanser, for cleaning the brushed aluminum. I put some of the powder in a bowl and then use a damp Q-tip or similar to pick some up, apply it, and rub it in. I avoid rubbing it in areas with silkscreening or painted engraving, but it doesn't do any harm if it gets applied there.

Oh. I have the liquid version. I'll have to try it, though it's probably less abrasive in such form as compared with the dry kind.

A question, what is the "grain" direction of the brushed aluminum on your supply's faceplate? In the photos of your supply, it appears vertical. On mine, its horizontal, as it is on my other PD supplies.

Correct. Mine has the grain going vertically. It's interesting that the same model would have different orientations. Different factory or panel material provider?

[FlyingHacker]

Best thing I have found for gum and sticker removal is lighter fluid, like for a Zippo lighter, Ronsonol brand works well. Always worked way better than GooGone for me.

 I would use warm amber for the lighted meters. This will give a more vintage feel, well suited to the PD design. I'd like to add some lights to my Lamda supplies. Good idea!

[bitseeker]

Best thing I have found for gum and sticker removal is lighter fluid, like for a Zippo lighter, Ronsonol brand works well. Always worked way better than GooGone for me.

I semi-accidentally got some contact cleaner on sticker residue. Worked great! However, have to be careful as I'm sure it'll take off lots of other things too.

I would use warm amber for the lighted meters. This will give a more vintage feel, well suited to the PD design. I'd like to add some lights to my Lamda supplies. Good idea!

Agreed. So, what's the best and/or most economical (not always the same) way to get amber? Separate red and green, single bipolar (red/green), or RGB LED? I guess you could also use a white LED with amber filter.

[ruffy91]

Agreed. So, what's the best and/or most economical (not always the same) way to get amber? Separate red and green, single bipolar (red/green), or RGB LED? I guess you could also use a white LED with amber filter.

There are amber LEDs. They have the desired color (592nm wavelenght). When mixing red and green you will get colored shadows which don't look that great.

[bitseeker]

There are amber LEDs. They have the desired color (592nm wavelenght).

Perfect!

[eas]

Correct. Mine has the grain going vertically. It's interesting that the same model would have different orientations. Different factory or panel material provider?

Odd. All of my (10!!) PD supplies, save one, have aluminum faceplates with a brushed finish with a horizontal grain. The exception is a 6010 with an aluminum faceplate with a satin or semi-matte texture.

I'd guess that PD contracted to have the faceplates cut and printed, and I'd think that your faceplate was probably the result of a mistake by the supplier that PD either accidentally or deliberately accepted.

[bitseeker]

I'd guess that PD contracted to have the faceplates cut and printed, and I'd think that your faceplate was probably the result of a mistake by the supplier that PD either accidentally or deliberately accepted.

Cool! That quadruples its resale value!

Hrm, I guess it's not like postage stamps.

[eas]

I'd guess that PD contracted to have the faceplates cut and printed, and I'd think that your faceplate was probably the result of a mistake by the supplier that PD either accidentally or deliberately accepted.

Cool! That quadruples its resale value!

Hrm, I guess it's not like postage stamps.

Nope, not yet You just have to find the right buyer...

[FlyingHacker]

I'd guess that PD contracted to have the faceplates cut and printed, and I'd think that your faceplate was probably the result of a mistake by the supplier that PD either accidentally or deliberately accepted.

Cool! That quadruples its resale value!

Hrm, I guess it's not like postage stamps.

Funny. i saw your first remark and was about to reply, "What is this, a postage stamp!?!" 

[bitseeker]

Funny. i saw your first remark and was about to reply, "What is this, a postage stamp!?!" 

Very good. Splitting the lines had the desired effect.