Burned out Cap and Diode on Power Supply Board - Page 1
Forum section: electronics / repair

author: jmkbass
author_rank: Newbie
timestamp: August 21, 2025, 04:50:50 pm
content: New here and a doozy of a first postHave a burned out diode and possibly a burned out capacitor on a power supply board from a legacy, unsupported thermal camera.I have the replacement diode but am at a loss on the capacitor, looks to be a Vishay Tantalum capacitor?Is is advisable to replace the diode without replacing the capacitor, will doing so risk further damage?What methods are there for checking the viability of the capacitor?How to go about finding the values for the capacitor if it is indeed bad or if condition is unverifiable?ETA Image:

author: Seekonk
author_rank: Super Contributor
timestamp: August 21, 2025, 06:37:43 pm
content: With the cap it looks like a nasty flash over on the board. There has to be a lot more damage to other components. If that is in the power supply, almost any cap would do. That appears to be the least of your problems.

author: jmkbass
author_rank: Newbie
timestamp: August 21, 2025, 07:39:40 pm
content: On the positive side, the power supply board is partially functional in that it still is powering the cryocooler. There's some life in it so hoping with some help figuring out the diode and cap issues that it will be functional.

author: indeterminate
author_rank: Regular Contributor
timestamp: August 22, 2025, 07:43:17 am
content: 1 replace both components2 it is stuffed3 find a similar size cap on the board and measure it, if the cap is on the 5v rail then just use a 10v cap

author: jmkbass
author_rank: Newbie
timestamp: August 22, 2025, 08:13:54 am
content: With the bad diode, can the voltage a be measured at the cap itself, just probe it?There is a nearly dimensionally identical cap on other side of the board, do I need to remove/desolder from board to measure?Checked Vishay to try and match off dimensions but the values for the same case are quite varied; would it be fine to go with the greatest capacitance and voltage if guesstimating e.g. better to go higher or lower?

author: Swake
author_rank: Frequent Contributor
timestamp: August 22, 2025, 09:05:29 am
content: You need to compare with a capacitor from the same brand and generation. As there are no markings, that other cap on the same board is likely the best reference you can have. You have to unsolder it, at least one leg, to measure it.These might be tantalum caps, or not. Aren’t the Vishays tantaliums red in colour? Those that are not in a yellow or black epoxy case I mean. Tantalium caps are polarised, the one in the pictures doesn't seem to have a direction.Is D11 (that other D76 in the pics) still ok? Asking because PCB track lead to it from D10.Any idea what has caused this failure? I have the impression that the root cause might be that the capacitor. It somehow shorted out with the ground and then pulled a lot of current (see left leg, it is brown too) which in turn was breaking down the diode that is in series with it. Make sure to check out D11 and what is downstream the 4222 resistor too.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 22, 2025, 11:47:16 am
content: what is U1 is it an dc/dc converter ? check it's application note, it could give you some insight on this diode

author: jmkbass
author_rank: Newbie
timestamp: August 22, 2025, 11:55:20 am
content: Desolder and test the other cap I can do, only won't be sure if they have the same values, so should I go with the same values or go higher in voltage and capacitance?I really can't say for sure if the caps are Vishay tantalum's, though they do appear more red than yellow (see new attached photos). The wire nib leads me to believe it is polarized, the nib points towards the blown diode.As for the D11 diode, looks good but haven't tested, am still learning and haven't figured that out yet?No idea what the failure was caused by, got it for the objective lens but since the cryocooler powered up and sounded good, seemed to be a waste to not attempt a fix. Since this is an out of warranty and legacy .Mil item, figured it was binned as apposed to getting it fixed.

author: Swake
author_rank: Frequent Contributor
timestamp: August 22, 2025, 12:55:13 pm
content: You are probably right about them being Vishay tantalums and that little nib !