I recently built up my Buff 2 with dual bipolar placid supplies. I have had a few issues with the placid supplies and now have unfortunately destroyed them both and have no idea why.

I was missing some 200R trimmers as the kits had 2k ones instead so i ordered them from Farnell and got a couple spare; the parts i got were the exact same parts supplied in the kit. Once fitted i tested the suppies and all was ok but they were very difficult to accurately adjust the current, it woudl vary wildly with very small adjustments of the trimmer, sometimes D5 or D6 would flicker if the current jumped too high. I swapped out one of the trimmers to see if it improved the adjustment control but it didn't. I managed to set them to 150ma and 15 v and had the dac running all of yesterday, it sounded great btw, easy improvement of the 32s. Today i took it back in the workshop to finish the case etc and checked the voltages and current, to my surprise one side of each supply had dropped and i couldnt measure any voltage across R2 on the left supply and R2 on the right supply.

I tried to adjust the trim pots to get the current up but could not get a reading across R1 or R2 then R2 on one supply burnt out complete with smoke and on the other the adjustment did not increase current but blew D6 instead.

I have tried to establish the extent of the damage, i know one led and resistor are toast for sure, i tried repairing one suplly with parts from the other so i could listen to the dac but replacing the toasted led and R2 didnt make any difference, D6 would light but again couldnt get any current across R2 so there is obviously more damage.

Any ideas where to look for the root casuse of the problem and what other parts might need replacing to repair the supplies? i checked the IVY111 board for shorts and it seems fine so no shorts on the load.

I have succesfully built and used placid's before so not sure what happened this time.

Thanks,

Luke

Thanks Russ; i have made all of the checks and cannot find any problems except for two suspect solder pads for the positive legs of D5 and D6 which have lifted from the underside of the board, but this may have happened when i removed the damaged components. I'm sure they had good joints prior to the components being removed as both led's were functioning and the joints looked good. So no obvious cause found.

I am begining to think it may have been casused by my scopemeter, it may have been left in resistance measurement mode rather than mv and could have applied a small voltage across R1 / R2, could this cause the circuit to destroy R1 / R2 D5 / D6 VR1 / VR2?

I have managed to repair one of the placids by replacing R2, D5 and substited both VR1 and VR2 with fixed value resistors. The supply works perfectly now.

I am hoping replacing the same parts on the other supply will fix it. Is it preferable to have fixed value resistors in place of VR1 / VR2?

Being fairly inexperienced with electronics i cant make complete sense of the schematic and not sure how i caused the damage in the first place.


Very pleased with Buf 2,

Cheers,

Luke

It sounds like you have your meter probes in the Current measurement holes rather than the Voltage/Resistance measurement holes. This would mean they act like a short, so it would be like putting a piece of wire across the test points.

I think I have managed to do the same mistake. Fried two BP placid.

After I had measured the output voltage and adjusted this to 15V I tried to measure the voltage across R1 and R2 and I think I also manage to short to ground. When measuring the voltage from the top of the board this is a very easy mistake to do. It would possibly be wise to put a warning about this in the manual. The VR2 on both power supplies are damaged. All the LEDs are still on and the output voltage is still 15V. Is it enough to replace VR2 or is it likely that other components are damaged?

Edited by user Saturday, April 10, 2010 7:50:11 AM(UTC)  | Reason: Not specified

If you have to, it is best to use a destructive removal method: I crush the tops with pliers, leaving just the pins, which I then heat and remove individually.

I had the same issue, and burned out VR2 this way. Got a lot more careful placing my test leads!

Either value is actually just fine in the Placid BP. It would only effect the range of adjustment. You should be just fine.