Dodgy Display on a Skytronics PSU

[ceedy]

I bought a "as NEW" Skytronics Variable PSU model 650.679
like this..



And not actually used it myself for a month or so since purchase and now it seems to have problem with the Volts LED display.

The whole thing works as it should, fully range of volt and Current limit at the output .. but the LED Volts display is all over the place, almost completely random digits appear , no relation to the actual output, which I can check with my Fluke meter.

couple of videos in here..

http://s277.photobucket.com/user/ceedy44/library/PSU

The Display PCB has just 5 wires into it .. and the correct variable output voltage is found at 2 points on the this cable .

The display is fairly simple 1 block of 3 x 8 segments for the current and the same for the volts. both driven by an ICL7107CPL and the only other semi is a TL431A .

PICs of the board



Volts on the right side..

and





Can't even find who actually makes this PSU .. so little chance of any spares?.

Any clues anyone ??



The only slight plus it was cheap

Chris

[joshnz]

Re: Dodgy Display on a Skytronics PSU

You will have to check if vref hi and lo are stable and input hi and lo are stable

[mariushm]

Re: Dodgy Display on a Skytronics PSU

ICL7107 are multimeter chips with led driver option ... they measure voltage and activate the appropriate led segments by themselves.

I would bet on cold solder joins on the led segments or some issues with the integration capacitors (the brown ones)
Retouch all the solder on the segments and IC and check the capacitors with a multimeter.

[sam_sam_sam]

Re: Dodgy Display on a Skytronics PSU

I would look very close at all of the soldier joints for the volt meter any joint that has allot of flux on it reheat them or looks even a little bit dull if you have doughs redo all of them

I hope this helps

[ceedy]

Re: Dodgy Display on a Skytronics PSU

I've just reflowed the majority of the joints on the Volts side of the board , there a couple that looked OK until I got my digi microscope on them.

And it all seems to be running ok .. gave it a quick trim to the Fluke ( one of them is probably right )

Very many thanks to all you guys for the clues ..

take care all
See you with my next problem

Chris

[joshnz]

Re: Dodgy Display on a Skytronics PSU

good that it is working now

[sam_sam_sam]

Re: Dodgy Display on a Skytronics PSU

I see that you got it work again