TDS640A Power supply fail and warning

[MattM]

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!

[CaptDon]

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.

[BennoG]

What was you powering when this happend?
Can it be you DUT was on a high voltage reverenced to the earth ground ?

Benno

[MattM]

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.

I'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.

[MattM]

What was you powering when this happend?
Can it be you DUT was on a high voltage reverenced to the earth ground ?

Benno

Nothing 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'.

[TERRA Operative]

This document looks to have a schematic that is similar to your power supply, close enough to be usable at least. Pages 354 - 365

https://w140.com/tekwiki/images/0/07/070-9710-03.pdf

[andy2000]

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.

[MattM]

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.