tektronix TDS500 TDS700 power supply models PSU repair - Page 1
Forum section: electronics / repair

author: Tantratron
author_rank: Frequent Contributor
timestamp: November 10, 2023, 09:10:42 am
content: Hello, I've been using for my lab as well as trying to repair, recondition or hack different TDSxxx/C/D models plus purchased of pure interest a TDS510A.Different threads discuss the topic of CRT, the logic board, the acquisition board but I notice two versions for the PSU board (old style and new style). I'm attaching differents pictures of both models where the old design seems to differ only on the EMI filtering, the PFC boost converter and the standby circuitry.Unless I'm wrong, it seems that old models might be fully compatible or inter-changeable with new models where you would just need to use the appropriate flexible ribbon and connector to match the specific acquisition board power rail ?What is your return of experience, which model do you think is more reliable with less failures ?In the past, I did repair one new model by changing two mosfet transistors of the PFC output.Now I do have a failed old model which seems to have faulty standby issue, it starts then stop then starts...Do you confirm to have working old PSU installed in your C or D oscilloscope or do you recommend to not spend time repairing old PSU's ?Similar question, do you confirm new PSU working inside a A or B model ?Thank you in advance, Albert

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 13, 2023, 01:31:29 am
content: The new type PSU can be used in place of the old type PSU no problems. The only real difference in output is the new PSU can supply 4A on the +25V rail whereas the old PSU can only supply 2.5A.So new can be used in place of old, but I wouldn't use old in place of new without verifying current draw on the +25V rail.The benefit of the new design is that it doesn't suffer from capacitor C17 going high ESR and taking out the TVS diodes and BU508 transistor, also no Rifa madness, as all those parts simply don't exist.I have attached your image showing the location of these parts. If the PSU works, the capacitor C17 (47uf, 80V) should be replaced without any hesitation to prevent future damage to the TVS diodes and transistor. Itwillbe bad. Use a good quality LOW! ESR capacitor of equal or greater voltage rating (I personally like the United Chemicon KY* and KZ* series, but any good quality part can be used).If the PSU does not work, you may need to replace the two TVS diodes VR4 (1.5KE220CA) and VR5 (1.5KE250CA) and transistor Q9 (I use a BU508AW) too, so that's the first place to look after replacing the capacitor.The most common failure mode is no power when pressing the front power button on the scope. If this is the issue, check these parts first (after checking the line fuse and mains supply..)An easy check is to listen carefully right when plugging in the mains power cable. If you listen closely, you should hear a faint 'ziiippp!' sound as the standby circuit powers up. This indicates that it isprobablyworking.As for Rifa caps, there are 4x 1000pf 10mm pitch Rifa caps in the old PSU that should also be replaced on sight.

author: Tantratron
author_rank: Frequent Contributor
timestamp: November 13, 2023, 06:05:42 am
content: Quote from: TERRA Operative on November 13, 2023, 01:31:29 amMany thanks TERRA for explanation on which cap, diode and transistor are critical in old style PSU.As for the +25Vdc output rail, not sure of my new PSU being different than the ones you refer to. Please check attached different pictures I've just taken from my three PSUs (two old style, one new) but it seems all source 2.5A output for +25 Vdc.I do have two other new PSU but they're installed in my test lab (TDS794D and TDS784D each with LCD kit) so I'd need to open all (long time to deconstruct) to check if more recent new shifted to 4A output for +25Vdc rail.Do you confirm the 4A output, what year it was made since attached time stamp of my new PSU is 1998 as you can see on my last attached picture ?

author: Tantratron
author_rank: Frequent Contributor
timestamp: November 13, 2023, 08:39:52 am
content: Hello again TERRA, I've checked now my old notes to track different part number, versions wether PSU, the CPU board, acquisition board, CRT board based around nine TDS different C/D models.Are you really sure that newer PSU has specific 4A on +25Vdc rail output because in my notes, I see all these PSU from TDS754D TDS520D TDS784C TDS794D and TDS714L produce 5V/22A -5.1V/12A +5V/12A +15V/1.5A -15V/1.5 and +25V/2.5AAll the AC-grid inputs wether built by ZYTECH and later acquired by ARTESYN mention 400 W/100-250Vac/4.5A different Revision (RE F G or K) all built between 1997 and 1999Thanks to check again, Albert

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 13, 2023, 03:12:06 pm
content: Well, there you go.... I just checked the three spare PSU's I have here, and they are all different!The pictures are below, grouped by part number as per the label.The older one and one of the newer ones I have are both made by Zetec and have exactly the same output specifcations. They have the same Tek part number besides the last two digits (-04 to -05) which indicates a cross compatible upgrade to the higher part number.The other later one made by Artesyn with 4A on the +25V rail has a completely new Tek part number, so I'm not sure if the one was used for a specific scope model or if it was just how it ended up when the PSU manufacturer changed from Zytek to Artesyn..Maybe more power was needed for the D suffix TDS700 series scopes? I don't know.... I can't remember where these PSU's were pulled from.Also, I know that some TDS500 scopes (I.e. TDS520 maybe?) have yet another PSU design, the long black DC output connector that is furthest from the mains input is smaller, so it isn't compatible with scopes that use these PSU's we are talking about. I have a scope that uses one of those PSU's, but it's a pain in the arse to remove for a photo, so.... no photo of that one.

author: Tantratron
author_rank: Frequent Contributor
timestamp: November 14, 2023, 12:53:13 pm
content: Quote from: TERRA Operative on November 13, 2023, 03:12:06 pmMany thanks Terra for taking time to report, pictures and comment.In the meantime, I've looked at my older notes since 2019 where progressively, I've had in hand 9 different models. Just made bit clearer notes, see attached my new hand writing normally I did not make a visual copy mistake.In RED is old PSU design which I've found in one TDS510A and in one failed TDS784D, the later is a complex topic with this old PSU failed, logic board failed, color CRT damaged physically.On the old PSU, there are 3 PCBs (main, vertical 1 and vertical 2) but with my two units, no part number neither version letter as printed with new PSU design (only two PCB's).Not sure what I can do with the two old PSU, one still works but I understand the need to upgrade C17 and RIFA's as you mentioned. The 2nd will not start, only standby click then stop then click then stop.Do you think best to only keep new PSU design for all models (A, B, C and D) due to better design ?As for your last PSU with +25Vdc/4A for sure it is not due to Artesyn purchasing Zythech as my document shows clear evidence with dates, part numbers. Could you check on this unit what are the RLB and TLB part number and revision letter to see if it corresponds to my list ?Maybe they printed a wrong marker because 400W 4.5A grid part as previous model whereas I'd expect say 450W due to higher power from +25Vdc rail.

author: Tantratron
author_rank: Frequent Contributor
timestamp: November 14, 2023, 12:56:38 pm
content: P.S. This could be a cross refernce thread but seems very important preventive repair for those willing to keep working old PSU designhttps://www.eevblog.com/forum/repair/tektronix-tds744a-cannot-boot-completely-flashrom-checksum/msg5166528/#msg5166528

author: inlinebeginner
author_rank: Contributor
timestamp: November 14, 2023, 11:42:57 pm
content: Quote from: Tantratron on November 14, 2023, 12:56:38 pmTantratronI saw your question in above link about replacing C17 or finding a new PSU, it better to move our discussion to this topic.My suggestion is: replace C17 first, then replace C93,C94 and C102 (10u/35V). The PSU design was good, seem to be reliable.I repaired four, two TDS744As, one TDS684A and one TDS754C. The TDS 754C PSU, different form TDS744A, does not contain a C17, because its standby circuit was redesigned. After repair, all work well.Lin

author: Tantratron
author_rank: Frequent Contributor
timestamp: November 15, 2023, 06:07:50 am
content: Quote from: inlinebeginner on November 14, 2023, 11:42:57 pmHello Lin, it seems almost certain due to year of production that all TDS744A make use of old PSU design.Your sentence which I quote againone TDS754C (PSU is different form TDS744A, the C17 was removed in TDS754C PSU)is not clear. Do you confirm this TDS754C-PSU is new design, if yes where is C17 ?Have you ever seen any TDSxxxC/D using an old design PSU or were there all new PSU design ?On a side note, I have only repaired so far one TDS794D-PSU few years ago, it consisted to replace to power mosfet transistor Q7 and Q8 switching the +400 Vdc rail to the down-converter trafo.

author: inlinebeginner
author_rank: Contributor
timestamp: November 15, 2023, 03:17:47 pm
content: Quote from: Tantratron on November 15, 2023, 06:07:50 amHello Tantratron, Sorry for the confusing, I edited my statement: "The TDS754C PSU, different form TDS744A, does not contain a C17, because its standby circuit was redesigned.""Have you ever seen any TDSxxxC/D using an old design PSU or were there all new PSU design ?" I only repair one, TDS754C, its PSU might be a new design around the standby circuit, that is why it does not contain a C17.Lin

author: Tantratron
author_rank: Frequent Contributor
timestamp: November 15, 2023, 05:38:14 pm
content: Thanks@inlinebeginnerfor clarifying the C17 topic where as a reference, my very first post shows pictures of old and new PSUhttps://www.eevblog.com/forum/repair/tektronix-tds500-tds700-power-supply-models-psu-repair/msg5160828/#msg5160828Yes clearly the standby circuit seems really different, same for the PFC-boost converter section when dealing with new design. As you know, there are no schematics for TDSxxxC/D but the low volatge DC output parts seem similar. The differences are really the standby section and the PFC-boost section plus I find new design easier to access the mosfet, the screws...

author: TerrySt
author_rank: Regular Contributor
timestamp: November 15, 2023, 08:02:44 pm
content: Terra,Thanks for the heads up on the capacitors in the old power supply that need changed. I have my old 524A opened up to work on the CRT power supply, so I'm going to change the parts you suggest (C17 and the 4 RIFA caps).I noticed that in the picture you posted of the old supply with the suspect parts circled, that you circled C16 instead of C17. C17 is the cap right next to the one you circled. At least that's how it looks on my supply, which matches your picture. Unfortunately I don't have a parts diagram for that model of supply to be sure, but the part next to the one you circled is 47uF at 80V.Terry

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 16, 2023, 08:08:41 am
content: Ah, I may have made a mistake. I'll correct the image and reupload it tonight.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 17, 2023, 01:35:13 am
content: Corrected image is now uploaded.

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 17, 2024, 08:36:49 am
content: Hello Terra, it took a while to import from eBay-USA to France but finally got your amazing TDSxxx tester.Attached pictures showing its use on a valid PSU (new style) then later, I'll try to use it on my two old style PSU, one being broken.I understand your recommendation to replace some RIFA caps first to not aggravate these two PSU.Respect for your work, Albert

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 17, 2024, 04:38:25 pm
content: Nice!Let us know how your testing goes!

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 18, 2024, 08:54:25 am
content: This morning I've connected the PSU tester into the old style PSU which has been failed for mor ethan 3 years and just sitting here. This time, no LED lights up where from what I see or hear, there is some kind of oscillation or attempt to run the standby.As for the four RIFA capacitors, what new part number do you purchases to be equivalent ?When the PSU tester does not show any LED to be ON, what other means could I use to investigate the power on failure through the PSU tester ?

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 18, 2024, 04:49:01 pm
content: For replacing the Rifa capacitors, I used part number R413F11000000K but any similar equivalent would be fine.As for troubleshooting, check the parts I pointed out in the image I posted above. If there is no standby voltage, then that part of the circuit is probably bad.

author: PwrElectronics
author_rank: Regular Contributor
timestamp: February 18, 2024, 06:19:45 pm
content: I hadn't read this thread when it was originally posted.Don't have any advice on repair of these but....I was a product and test engineer at Zytec from 1994-1996 and worked in the factory in the small town of Redwood Falls, Minnesota where these were made. I remember seeing them in an adjacent production line area to the one I was assigned to.Zytec was a spinoff of Control Data (mainframe computers) power supply division in the 1980s when Control Data was closing down.Zytec merged with some other company whose name escapes me around '97 or '98 and the joint company was called Artesyn. This lasted a few years until bought by Emerson and it was known as Emerson Power Systems or Emerson Network Power. A few years back, the name returned to Artesyn Embedded Power. Now, its been bought by Advanced Energy in 2019.The Redwood Falls factory was closed/sold around 2000 or 2001 and production was moved out of the USA.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 20, 2024, 05:08:03 am
content: Just repaired the PSU from a TDS 520D, found no schematics, it was blowing the breaker.The PFC circuit has two small bjts driving the big fets, the 2n4401 was shorted so it was setting the fet permanently on.Replaced with a MPS2222A, it's back to live, 420V generated in the beefy cap.It's had been touched before (Failure sticker from 2017), the IRF450 fets looked new.Though the guy gave up and put a sticker "Short in transformer".

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 20, 2024, 05:38:44 am
content: I had a PSU with a similar failure a while back.I was lucky enough to find original parts, but if substitutions fits, it sits.Nice work!

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 20, 2024, 06:11:02 am
content: It's just a generic 500mA 50V bjt, if it fits, it fits. Nothing fancy required here!

author: dnowakowski
author_rank: Newbie
timestamp: September 05, 2024, 03:47:40 am
content: I have followed the suggestions on repair of this PSU. However, the disk component at RT1 was broken; probably my fault. The schematic found elsewhere in this blog shows a value of 10, but no units. I am guessing it is a polymeric PTC. The part had no markings. Can anyone tell me what replacement part I can use? I hesitate to just put a wire jumper in there. Thank you for any help you can provide. - Don

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 05, 2024, 05:00:41 am
content: Sorry, can't tell.I'd put a generic one from a computer atx psu or another smps.

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 05, 2024, 03:09:29 pm
content: Post a photo, I'll take a look at one of my spare PSU's and see if I can find a part number.

author: sambor
author_rank: Newbie
timestamp: November 05, 2024, 07:27:42 pm
content: Thanks to all of you for many useful hints in this thread and others that concern Tek TDS LV PSU repair.I am a troubled owner of Tek 784D scope that simply stopped switching on after several months of storage. It has later style power supply with schematics available here:https://download.tek.com/manual/070971003.pdfI was able to pinpoint component U3 on the vertical board which happens to be dual flip flop latch dealing with 'on/off' signal as well as 'one-shot trigger'. After replacing U3 all back to normal but after few days it got damaged again after plugging PSU to the grid and flipping switch on the backplane.After failure U3 has a resistance around 3 ohm between VDD and VSS which causes standby circuit to go into hiccup. Replaced U3 again and after a day or two it's the same story. I went through several U3 components after which I soldered DIP socket which obviously didn't sort my problem.I am able to bypass U3 but don't want to give away front power button and 'one-shot trigger' functionality. Any ideas what could be causing surge that is damaging U3 upon connecting to the grid ?I checked and replaced VR26 and CR17 on the main board (although found no damage to the components) as well as replaced most electrolytics as a matter of precaution. This change nothing regarding repetitive U3 failure.I was thinking of replacing VR24 and CR48 but they measure fine in the circuit using diode test. BTW, my 15V standby voltage is around 12.4V and not 15V which I find odd. Could it be related ?Many thanksSam

author: sambor
author_rank: Newbie
timestamp: November 17, 2024, 06:32:34 pm
content: Update: U3 component changed to TI supplier instead of Fairchild. Unit is operational again although requires NVRAM battery replacement.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 18, 2024, 05:23:59 am
content: Ebay user kellyjo_13 has replacement NVRAM modules that work really well. I have a them in a couple scopes and they work perfectly.https://www.ebay.com/str/kellyjo13https://www.ebay.com/itm/302913384290

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 02, 2025, 08:16:04 am
content: Regarding the old design of these PSU, I do have left two units: one still working and one failed as mentioned in my first post in this thread.Yesterday I've de-solder from both units all their RIFA capacitors (1 nF 250 Vac PME 271 Y) as well as all C17 electrolytic capacitor (47 uF 80 Vdc).I've then check their values and ESR using my Extech LCR meter (DC pulsed frequency 1 KHz). The measurement helps find ESR via the formulae Q (serie quality factor) = 1 / (RCw) with w=2 x PI x FreqBoth C17 read 44 uF instead of 47 uF, the Q factor is 2.4 and 2.9 respectively so this estimates ESR to be 1.25 ohms and 1.51 ohms.All the RIFA caps are estimated to be between 1412 pF and 1583 pF instead of 1 nF but none seemed cracked visually. I'm not sure if estimating Q factor from RIFA tells us anything valuable but I do have values in case. Do you think the LCR estimating around 1.5 nF instead of printed value 1 nF in these old RIFA caps looks for trouble ?Nota. I read thisother postwith suggested values and reinforcement with ceramic caps.What shoud be the maximum ESR value for C17 capacitor in need to be replaced ?

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 03, 2025, 11:53:33 am
content: Hello again, before I place an order to purchase different components for my projects including the reapir or partial recap of these two PSU (old design), I have a question regarding the four RIFA capacitors (C4/C8/C5/C11). All these RIFA which I've de-soldered (2x4 so total 8 of them) are marked 1 nF but when I look for the service manual of TDSxxx or TDSxxxA, they are mentioned on the schematics to be 2.2 nFIs there any explanation or was there different design of these PSU through time ?1 nF or 2.2 nF for the four RIFA's ?Thank you

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 03, 2025, 03:13:42 pm
content: It wouldn't matter much I think, but I'd probably replace like for like. There were a few revisions of the PSU, so best to keep them the same for EMI reasons etc.

author: dakata1337
author_rank: Newbie
timestamp: June 25, 2025, 01:24:21 pm
content: Hello everyone! First I wanna thank all of you for sharing & documenting your experience with these TDS scopes.I have a Tek 744A that had the usual issues: C17, Q9 & TVS diodes. I replaced them and the power supply went from completely dead to hiccuping. The outputs are loaded and are trying to startup, but the power supply resets itself before they reach their nominal voltage. The hiccuping is ~1hz and its audible even when the "standby button" isn't pressed (when pressed it's still hiccups but twice in a "burst"). The standby power supply voltages are present (although they are jumping around a couple mV). Bulk cap is ~420V, but It's also jumping around a little.I would really appreciate if someone has any idea what can cause these issues & how I can track them down. I can record a video of the psu if necessary. Thanks!

author: Rafiki
author_rank: Contributor
timestamp: June 25, 2025, 04:12:21 pm
content: QuoteMaybe over current on one of the rails. In my case it was the -15V (or was it +15V?) supply to the acquisition board. One MLCC. I found it by loading the PSU alone with external loads. Needed a lot of high power resistors for that. After I knew my PSU can deliver lot of power I think I measured all supply voltages during hickup with another scope and found the failing one. After that I made a special flat cable where I can put over a current probe to the supplies and so found the too high current which leads to hickup.In back sight I think you could start the TDS without the flat cable connected to the acquisition board and see what happens. Not sure if other think this is safe but I did also during my testing. But first measure (scope) all supply voltages to see which one does not come up.

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 25, 2025, 04:13:17 pm
content: Check that transistor Q9 just adjacent to the TVS diodes isn't dead too. if the capacitor and TVS diodes died, the TVS diodes would no longer have been protecting that transistor, which may have also died too.I've had a PSU that had this cascade failure before and replacing all these parts had it working again.

author: dakata1337
author_rank: Newbie
timestamp: June 26, 2025, 11:15:58 am
content: Quote from: Rafiki on June 25, 2025, 04:12:21 pmQuote from: TERRA Operative on June 25, 2025, 04:13:17 pmI took out Q9 and tested it with diode function on my dmm and it measures fine, but I replaced it anyway just in case. The TVS diodes were faulty (they de-soldered themselves) and were replaced, C17 was replaced also.The hiccups occur even when testing the PSU on its own, so if it's an over current issue it would be on the psu itself. I looked at the electrolytics on the secondary side and pulled out one of them to check for leaks. I didn't see anything obvious but the one of the pads came off, so I think they might have leaked?For loading the power supply I used even lower values than Jareds on his TDS powersupply test pcb.The odd thing is that it's hiccuping even when is standby mode (audible zip-zip noise).

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 26, 2025, 12:05:57 pm
content: If it is hiccuping even in standby, then take a closer look at the standby power circuit, it would indicate that it's likely the standby power is what is making the rest of the PSU hiccup, essentially the same as the user turning the PSU on and off over and over.If the TVS diodes desoldered themselves, maybe something else is damaged there too.On application of mains power, you should only hear one quiet little 'zip' noise as the standby circuit energizes, then silence.

author: Tantratron
author_rank: Frequent Contributor
timestamp: June 29, 2025, 09:56:30 am
content: Quote from: dakata1337 on June 26, 2025, 11:15:58 amThanks for reporting and adressing this failure, I do havethe very same problem of hiccupingwith one of my PSU (old style design), still not solved on my side.

author: Tantratron
author_rank: Frequent Contributor
timestamp: July 11, 2025, 03:23:00 pm
content: Here is an update from my PSU after I've replaced the four RIFA and C17 capacitor.Good news, there is no more hiccuping when in standby mode but bad news, the PSU still does not work.When connecting Terra's power rail tester, no LED lights up hence no standby 1 VdcI've measured the value from the PFC output big capacitor, it does read 420 Vdc.

author: Tantratron
author_rank: Frequent Contributor
timestamp: July 12, 2025, 04:22:01 am
content: I've total of three spare PSU (first generation), two always worked but per recommendation of @Terra and others, I've changed on each the 4 RIFAs and C17 capacitor.Now to go back on the one which still does not work, I have 2 questions.Q1: what method to test Q9, VR4 and VR5 if they are soldered on the PCB having in mind I can test (compare) side by side a working PSU and failed PSU. So far I use diode tester between BE and BC of the NPN transistor (Q9), I read same voltage valuesQ2: I suddenly remember long ago that someone said the flip-flop or toggle circuit related to ON/OFF can be inverted and memorized in wrong state so wether ON/OFF, the PSU will never work. There might a procedure to reset or de-bounce the flip-flop, does anyone knows how to ?Thanks

author: Tantratron
author_rank: Frequent Contributor
timestamp: July 12, 2025, 06:38:24 am
content: I've notice the relay static position when PSU is OFF are not the same, see attached picture of the DS2Y-SL2-DC12V double realy being controlled by MC14013 chip. I've tested per the schematics pin 4 and pin 6 of the chip versus the relay. On the failed PSU, it is static bonded differently so as my previous poste, do you know how it is possible to force or toggle its mechanical memory ?

author: Tantratron
author_rank: Frequent Contributor
timestamp: July 12, 2025, 07:27:43 am
content: OK I've been able using a 12Vdc source to reset the relay applied between pin 2-15, now its static connection contact (pins 6-4 and pin 11-13) are the same as working PSU.However the PSU still does not power on when pressing the ON/OFF switch

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 13, 2025, 08:21:13 am
content: Quote from: Tantratron on July 12, 2025, 04:22:01 amIf I suspect VR4 and VR5, I just replace them. They are cheap parts and can take a beating.You could pull them out and put them across a current limited supply, then wind up the voltage and see when they conduct. Or just replace them...For Q9, I pull it out and test (I have a Peak Atlas DCA pro tester that works well). Replace if needed.

author: Tantratron
author_rank: Frequent Contributor
timestamp: July 13, 2025, 02:40:13 pm
content: Quote from: TERRA Operative on July 13, 2025, 08:21:13 amI've checked with my DMM different voltage per the schematics, here the PSU not loaded at all:big C12 has +420 Vsmall new C17 has -5.94 VJ2-7 (12 V bias) has 14.9 VJ4-7 (15 V bias) reads 14.6 VJ4-5 (Bulk PK detect) reads -13.5 VMaybe i'm wrong but for me, the standby power supply works fine hence Q9, VR4 and VR5 are OK otherwise there would not be the above values from the output of T3 (standby trafo) outputs.Since I have two working PSU (same model the failed model), I've tested the Q9 forward voltage (Vbe, Vce) while Q9 soldered, same with VR4 and VR5 soldered, I do not notice any change. Same for both 200 K resistor surrounding V4 and VR5There must be something wrong somewhere unless i'm wrong with my stanbby volatge measurements

author: Tantratron
author_rank: Frequent Contributor
timestamp: July 13, 2025, 05:30:12 pm
content: I've noticed past traces of leaking in LV section so I've desoldered 8 capacitors to see more if any cut trace or whatever, see attached pictures. No idea if this could explain the failure to start of this PSU even though its standby circuit seems to work per my previous measurements.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 13, 2025, 11:43:35 pm
content: Would have to check the exact rails that are affected, but could there be a lack of output voltage feedback that is causing the PSU to say nope?In any case, I think the current job is ripping back the soldermask and tinning the copper traces with solder after cleaning out the corrosion... Have fun!

author: Tantratron
author_rank: Frequent Contributor
timestamp: July 14, 2025, 06:35:42 am
content: I'll try to clean bit more but it seems clear now the caps have been replaced some time ago. The repair center saw the leaking, for sure the replaced the big C29-C26-C30 (each 2200 uF). As for the other caps C38-C93 and C73-C33-C94 maybe they were changed, maybe not I cannot tell, I'm only sure they changed C9/C26/C30. However the repair center did a partial cleaning job which needs to be refined now if I have patience or give up.Maybe the initial leaking had spay to other parts or slightly corrected solder through joints, no idea.The 8 caps which i've desoldered corresponds to 3 disctinct parts of the schematics:C29/C26/C30 connected to trafo F3-F4 sectionC38/C93 connected to trafo F9-F10 sectionC73/C33/C94 connected to trafo F11-F12 sectionQuick test with my polarized LC meter concludes the 9 caps are OK in value, ESR and not leaking visuallyIt really sucks... the real problem with this multiple output PSU, as @TERRA said there are feedback control loops and surveillance so maybe one output fails which results of complete shut down