TDS640A Power supply fail and warning - Page 1
Forum section: electronics / repair

author: MattM
author_rank: Contributor
timestamp: March 31, 2024, 02:24:10 pm
content: Hi guys,does anyone have schematic diagrams for TDS640A, mainly for the power supply but a complete set would be appreciated? I found some schematic diagrams online but they are not corresponding to my power supply. So they are probably not for 640A. The sticker on the PS board says "RLB 23042041 REV G".My TDS640A (FW: v3.8.8e) went up in flames, it was running perfectly fine, funnily enough even while it was on fire, spouting smoke. The images suggest a possible elyt cap issue. So if you haven't done it before maybe check your caps, note that they were NOT swollen.Thanks!

author: CaptDon
author_rank: Super Contributor
timestamp: April 02, 2024, 01:33:04 pm
content: Wow, it looks like that flame out occurred in the low voltage section? Very unusual. I wonder if electrolyte leakage was the start of the downfall or maybe the capacitor shorted or had a very low resistance internally bridging the leads? That certainly is a strange failure mode for a TEK supply.

author: BennoG
author_rank: Frequent Contributor
timestamp: April 02, 2024, 01:46:04 pm
content: What was you powering when this happend?Can it be you DUT was on a high voltage reverenced to the earth ground ?Benno

author: MattM
author_rank: Contributor
timestamp: April 03, 2024, 10:02:42 pm
content: Quote from: CaptDon on April 02, 2024, 01:33:04 pmI'm pretty sure it is the low voltage section, not sure which rail, still looking for a schematic. I think it was the electrolyte leakage, shorted the leads and possibly also to the "heatsink", in the second picture the black stuff on the mounting flange is electrolyte goo.

author: MattM
author_rank: Contributor
timestamp: April 03, 2024, 10:10:46 pm
content: Quote from: BennoG on April 02, 2024, 01:46:04 pmNothing to do with the inputs, there was no DUT, not even probes connected. I was in the middle of trying to do a floppy backup of the NVSRAM. Maybe, just maybe floppy drive pushed the PS (cap) over the limit although it was probably just a coincidence. I have been using the scope previously, no problem.I have replaced all the original SMD elyts couple of years ago but I didn't change big THT caps in the power supply. They seemed fine, Nichicons, none swollen. Now obviously really just 'seemed'.

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 04, 2024, 01:08:41 am
content: This document looks to have a schematic that is similar to your power supply, close enough to be usable at least. Pages 354 - 365https://w140.com/tekwiki/images/0/07/070-9710-03.pdf

author: andy2000
author_rank: Frequent Contributor
timestamp: April 04, 2024, 05:26:34 am
content: I have seen that before on TDS744A. Electrolyte from leaking capacitors causes the board to break down, start carbon tracking, and eventually become severely burnt. In that case, I had to cut away some of the board and reconstruct it with jumper wires.You should check, or replace all of that same brand of capacitor. They were from a bad lot.

author: MattM
author_rank: Contributor
timestamp: April 11, 2024, 07:09:44 am
content: Hi Guys, thank you for your replies. Sorry for the delay, I don't seem to be getting notifications of new posts in the thread.TERRA Operative, thank you for the schematic diagram. It is significantly closer than the schematic I was able to find.andy2000, yes, I will change them all now, don't feel like risking not changing them this time around.

author: gjones
author_rank: Newbie
timestamp: August 08, 2025, 03:33:33 pm
content: Hello Forum, I found this topic after my old faithful TDS680B had exactly this failure. Thanks for the info, which helped me successfully fix it. I thought I would share my triumph and some details in case it is helpful to others. I had to cut away and repair the PCB but the only components I needed to replace were the electrolytic caps. The original parts are now obsolete, but I believe the following parts are equivalent and still available:2700uF 10V: Panasonic EEUFC1A272680uF 35V: Panasonic EEUFC1V681Before and after photos are attached.

author: gjones
author_rank: Newbie
timestamp: August 08, 2025, 03:35:12 pm
content: Before repair pic...

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 09, 2025, 03:57:54 pm
content: Wow, that's some carnage!I wonder if it would be worth epoxying in a new chunk of FR4 to close the hole? Although, if it works as is, then it's probably good enough.

author: madao
author_rank: Frequent Contributor
timestamp: August 12, 2025, 09:10:04 am
content: This is very common problem with nichicon capacitor (PL & VZ Series ) in 1990sThis is why i open every Tek TDS PSU and check carefully. If i see "nichicon" capactor -> PSU is pull out to recapping.And here a second warning about Tek TDS500A/600A: PLeas desoldering one SMD capacitor on Acqusistionboard.Some looks nice, but after removing of SMD capacitor is little carnage visible, or it smells like dirty WC.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 12, 2025, 09:55:13 am
content: All TDS500/600/700 with no suffix, and A or B suffix require mandatory replacement SMD capacitors on processor and acquisition boards.And A, B and C types should have capacitors on the front panel board replaced too (maybe late C went to non-electrolytic?)There is one or two 10V rated caps on the front panel that are running overvoltage too, so good to uprate them as much as possible. I think 35V caps are available nowdays.Not to mention the usual replacements to be done on sight on the earlier PSU's too.