Fault finding wth?

[pucho812]

Puzzle for the day. Got 2 x power supplies, one works one does not. The working one outputs correct vdc, the non working one output  close to 26vdc lower then the good one.  Working and taking measurements from the good one I can pinpoint exactly where there is a fault/short.
However there is no explaination as to why? Pcb's came from the same batch. All trace check out continuity wise. All parts test as good parts on both units, all parts are the same and all polarity on caps, diodes is correct. All transistors are correct. Both supplies measures the same up until the afor mentioned  pinpoint. All solder joints were reckoned as a precaution.
Any ideas what to look for that have not been tried?  There is obviously a fault but can't find it.

 

[radardoug]

You are the one holding the units, we cant do much from cyberspace. There is a fault, so by deduction you either have a faulty component, or a wiring fault. You will have to find it.

 

[Harpo]

Broken/missing insulator/mica at heatsink ?
Missing min.load might give unpredictable result ?

 

[sahib]

Get a magnifier and closely  inspect the solder side.

 

[jensenmann]

Rattle/bend/shake every part for broken legs

 

[ruffrecords]

Transistors may have different beta.

Cheers

Ian

 

[pucho812]

Good call.

 

[JohnRoberts]

Puzzle for the day. Got 2 x power supplies, one works one does not. The working one outputs correct vdc, the non working one output  close to 26vdc lower then the good one.  Working and taking measurements from the good one I can pinpoint exactly where there is a fault/short.

huh? Do you have a schematic? Compare voltages between working one and low voltage one.

However there is no explaination as to why?

there is always a reason.

Pcb's came from the same batch. All trace check out continuity wise. All parts test as good parts on both units, all parts are the same and all polarity on caps, diodes is correct. All transistors are correct. Both supplies measures the same up until the afor mentioned  pinpoint. All solder joints were reckoned as a precaution.
Any ideas what to look for that have not been tried?  There is obviously a fault but can't find it.

You are looking for an obvious mechanical problem. Fault could be more subtle.

What is rest of story.. are these new builds or old soldiers that suddenly stopped working?

JR

 

[pucho812]

Puzzle for the day. Got 2 x power supplies, one works one does not. The working one outputs correct vdc, the non working one output  close to 26vdc lower then the good one.  Working and taking measurements from the good one I can pinpoint exactly where there is a fault/short.

huh? Do you have a schematic? Compare voltages between working one and low voltage one.

However there is no explaination as to why?

there is always a reason.

Pcb's came from the same batch. All trace check out continuity wise. All parts test as good parts on both units, all parts are the same and all polarity on caps, diodes is correct. All transistors are correct. Both supplies measures the same up until the afor mentioned  pinpoint. All solder joints were reckoned as a precaution.
Any ideas what to look for that have not been tried?  There is obviously a fault but can't find it.

You are looking for an obvious mechanical problem. Fault could be more subtle.

What is rest of story.. are these new builds or old soldiers that suddenly stopped working?

JR

Will sir, these are new builds of an already working design, these were hand soldered vs the usual machine soldered. 

 

[pucho812]

Boy is my face full of embarrassment.  :-[

There is a big difference in a 3k32 resistor and a 33k2 resistor.

The moment I replaced the wrong value with the right one. I got the right voltages.

Back to the regulary scheduled questions.

 

[JohnRoberts]

It's always the last thing you checked...  8)

JR

 

[pucho812]

It's always the last thing you checked...  8)

JR

While that is true, we sometimes skip the obvious looking for the more difficult fault.  In this case I should have checked the values vs the bom and schematic.  While I did not build the boards,  I did do he test and check on these. The upside is being all built the same,  once the first one fell into place the other 11 were the same issue and resolution.