Power Designs TP340A: Repair and facelift - Page 1
Forum section: electronics / repair

author: bitseeker
author_rank: Super Contributor
timestamp: October 03, 2015, 02:58:48 am
content: 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.pdfFortunately, 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.pdfWhen 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

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 03, 2015, 04:45:26 am
content: 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.

author: ez24
author_rank: Super Contributor
timestamp: October 03, 2015, 05:07:44 am
content: Quote from: FlyingHacker on October 03, 2015, 04:45:26 amHow would a variac help? thanks

author: eas
author_rank: Frequent Contributor
timestamp: October 03, 2015, 06:04:28 am
content: Quote from: ez24 on October 03, 2015, 05:07:44 amGradual 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 andmine. 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...

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 03, 2015, 06:14:26 am
content: 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).

author: bitseeker
author_rank: Super Contributor
timestamp: October 03, 2015, 06:17:41 am
content: Quote from: FlyingHacker on October 03, 2015, 04:45:26 amThanks, 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.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 03, 2015, 06:18:51 am
content: 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.

author: bitseeker
author_rank: Super Contributor
timestamp: October 03, 2015, 06:20:25 am
content: Quote from: FlyingHacker on October 03, 2015, 06:14:26 amYeah, eas recommended that to me. Great idea. I guess I was getting impatient to see it run.

author: bitseeker
author_rank: Super Contributor
timestamp: October 03, 2015, 06:23:53 am
content: Quote from: eas on October 03, 2015, 06:04:28 amHopefully, we don't end up trying to outbid each other. EEVBlog needs an eBay collision warning system.QuoteThanks 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.QuoteThanks, that'll certainly be part of the facelift coverage.

author: ez24
author_rank: Super Contributor
timestamp: October 04, 2015, 04:34:32 am
content: QuoteI am bidding on threeI think you would see me as bidder e***sFor example currently I am high bidder on this onehttp://www.ebay.com/itm/351535824859?_trksid=p2055359.m1431.l2649&ssPageName=STRK%3AMEBIDX%3AITI am looking for an old one with no grounds. I have no problems not bidding against each other

author: bitseeker
author_rank: Super Contributor
timestamp: October 04, 2015, 04:48:35 am
content: 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.

author: bitseeker
author_rank: Super Contributor
timestamp: October 04, 2015, 04:51:25 am
content: I'll wrap up the pot replacement and facelift tomorrow and post accordingly.

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 04, 2015, 12:10:34 pm
content: Quote from: bitseeker on October 03, 2015, 06:23:53 amMaybe one with Dave's voice going 'wah' repeatedly.

author: bitseeker
author_rank: Super Contributor
timestamp: October 05, 2015, 03:23:41 am
content: 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/350957127412So, 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.

author: eas
author_rank: Frequent Contributor
timestamp: October 12, 2015, 06:22:08 pm
content: 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.

author: bitseeker
author_rank: Super Contributor
timestamp: October 13, 2015, 12:58:15 am
content: Quote from: eas on October 12, 2015, 06:22:08 pmYeah, 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?QuoteNear 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.QuoteThey're also good for getting oxidation off electrical contacts.QuoteOh. 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.QuoteCorrect. Mine has the grain going vertically. It's interesting that the same model would have different orientations. Different factory or panel material provider?

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 13, 2015, 04:17:38 am
content: 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!

author: bitseeker
author_rank: Super Contributor
timestamp: October 13, 2015, 04:35:52 am
content: Quote from: FlyingHacker on October 13, 2015, 04:17:38 amI 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.QuoteAgreed. 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.

author: ruffy91
author_rank: Regular Contributor
timestamp: October 13, 2015, 04:50:03 am
content: Quote from: bitseeker on October 13, 2015, 04:35:52 amThere 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.

author: bitseeker
author_rank: Super Contributor
timestamp: October 13, 2015, 05:48:40 am
content: Quote from: ruffy91 on October 13, 2015, 04:50:03 amPerfect!

author: eas
author_rank: Frequent Contributor
timestamp: October 14, 2015, 04:54:09 am
content: Quote from: bitseeker on October 13, 2015, 12:58:15 amOdd. 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.

author: bitseeker
author_rank: Super Contributor
timestamp: October 14, 2015, 05:11:21 pm
content: Quote from: eas on October 14, 2015, 04:54:09 amCool! That quadruples its resale value!Hrm, I guess it's not like postage stamps.

author: eas
author_rank: Frequent Contributor
timestamp: October 15, 2015, 08:25:07 pm
content: Quote from: bitseeker on October 14, 2015, 05:11:21 pmNope, not yetYou just have to find the right buyer...

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 17, 2015, 05:36:02 am
content: Quote from: bitseeker on October 14, 2015, 05:11:21 pmFunny. i saw your first remark and was about to reply, "What is this, a postage stamp!?!"

author: bitseeker
author_rank: Super Contributor
timestamp: October 17, 2015, 07:20:11 am
content: Quote from: FlyingHacker on October 17, 2015, 05:36:02 amVery good. Splitting the lines had the desired effect.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 17, 2015, 07:26:16 am
content: As a flyer I gotta say, this is no Inverted Jenny!

author: bitseeker
author_rank: Super Contributor
timestamp: October 17, 2015, 07:34:45 am
content: Speaking of which, you can get the $2 commemorative reprint of Inverted Jenny.https://store.usps.com/store/browse/productDetailSingleSku.jsp?productId=S_580004&categoryId=buy-stamps

author: Sparky
author_rank: Frequent Contributor
timestamp: August 03, 2018, 06:35:17 am
content: Quote from: bitseeker on October 05, 2015, 03:23:41 amThis thread is 3 years old but still a goodie! I need to replace the Bourns multi-turn pots on a unit I want to restore and I'm glad to read your comment that "the new pot takes a little more force to rotate".I have a TP340A also...and it's perfectly fine...except one aspect that I find annoying --- the pots turnwaaaaaytoo easily --- the control is extremely "light" (takes no effort) with zero static/dynamic friction of the pot shaft rotating. I have to delicately let go of the knob when I reach the set voltage to avoid rotating it otherwise I could change the output by several volts! I also tape down the voltage knobs with painters tape so I don't accidentally knock them when I'm using the power supply...Do the Bourns pots wear out and they become friction-less?On my HP 6227B the pots are very firm and you have to rotate thema lotto change even 1V. I like the precise control without worry of accidental changes. Anyway to have TP340A voltage control have a similar feel?

author: bitseeker
author_rank: Super Contributor
timestamp: August 03, 2018, 09:34:54 pm
content: You can check datasheets for info on how much torque is required to rotate the wiper. It's among the mechanical specifications. I used a Bourns clone, which is why the feel is different (and I suspect the longevity will be, too).I've not heard of these things getting easier to turn with age. I suppose it's possible if the wipers and/or shaft wear a lot. The little HP E361xA power supplies I have all use Bourns pots and, despite being newer than the TP340A, also turn with very little torque.

author: Sparky
author_rank: Frequent Contributor
timestamp: August 04, 2018, 04:53:20 am
content: Quote from: bitseeker on August 03, 2018, 09:34:54 pmThanks for noting that about the torque in the mechanical specs -- I will check it out and probably choose the one with highest torque requirement!