Power Designs TP343A Shorted Output

[xavier60]

None of that make any sense at all. If the crowbar is being a nuisance, disable it by removing CR110.
Another approach is to take another set of measurements with U101 removed. 
I wanted the resistance measurement across CR104 while it was in circuit.
 

[vmallet]

Ok I'll test w/o U101 a bit later. Removing CR110 is a bit more difficult access-wise (but when I disconnected the cathode of the crowbar it made no difference).

I wanted the resistance measurement across CR104 while it was in circuit.

5.66K with voltage pot at min, 6.12K with pot at max.

[xavier60]

Ok I'll test w/o U101 a bit later. Removing CR110 is a bit more difficult access-wise (but when I disconnected the cathode of the crowbar it made no difference).

I wanted the resistance measurement across CR104 while it was in circuit.

5.66K with voltage pot at min, 6.12K with pot at max.

The reason for disabling the crowbar is so that it doesn't interrupt you fault finding and to reduce the chance of more damage being caused.
Best is to disable the drive to CR110 for example by disconnecting the OV Trip Pot at 42.

[vmallet]

None of that make any sense at all. If the crowbar is being a nuisance, disable it by removing CR110.

Sooooooooo hmmm where do I begin...



Going with your "Something doesn't add up" I was trying to revisit all the assumptions to see if I could re-measure or re-try something I hadn't done since yesterday at least. And then I looked at that missing C401 cap right at the output. Initially the supply was shorted so the first thing I tried was to take C401 out to test it, it was fine; then I moved on to the CR401 diode and that one was shorted so ok I moved on to try the supply out again thinking "oh, that cap can probably wait till I put everything back together before going back in"...

Now I feel really silly! I just put C401 back in and ta-da, it's fine and happy all the way from 0V to 25V.

I definitely learned a lot of things along the way here but the repair could have been much faster :p

There are aspects of the system I clearly don't understand yet including why removing C401 makes it behave that way; I might write up a couple of questions tomorrow, it seems you have a good picture of that schematic in your mind now.

Thanks again for the help!

[xavier60]

It was likely oscillating. It doesn't explain every thing but I'm not going to argue with it

[exe]

Got into the same trap myself. This is the reason why I want to buy a dmm that simultaneously shows ac and dc. Meanwhile I measure voltages with my oscilloscope. Not precise way, but good-enough in most cases.

[tggzzz]

Frequency response will be the problem in most cases. Easier to use an oscilloscope.

[vmallet]

I was curious to see the oscillations so I took the output cap back out.

DMM measures ~15.75V in DC mode (like I was doing before), here's the output from the scope:



@xavier60 is that what you were expecting?

[xavier60]

The output oscillation being directly coupled to pin 3 of U101A via C111 would get hard clipped by CR104 an CR105 causing an average DC of close to 0V to be measured there rather than the expected -0.6V.