BK Precision 1627A When voltage hits 5.1 volts it drops voltage updated symptom

[Dkturner10]

Hello,
 
      I bought this power supply about 5 or 6 years ago new.    I have only used it about 10 times and now when I try going past 5 volts it starts to drop the voltage down to 4.6 volts and trips the relay.   It will go right back after the relay clicks again to the 5.1 volts for a second and then restarts the cycle.    It might hit 7 volts if you go fast on the volts but then drops again to around the 4.6 volts.     There is nothing attached to the power supply and the current is either half way or all the way up and does the same thing.   We tried replacing the large capacitor and it did not fix.  We checked the small ones and they seem to be fine.   The relays seem to be working.  When the relay is triggered we get the higher voltage on the meter  shortly before it goes back down.    We pulled out the adjust nob out of the circuit and it works fine checking it with meter.   Seeing if anyone has seen this issue before or suggestions on what to check.   

Thank You!,

David

[bdunham7]

E-mail BK Precision and see if they have a schematic.  If they do, post it and perhaps someone will have an idea. 

[Dkturner10]

Hello,

      Here is the schematic.   We were taking measurements on it and it just started working and cannot get it to fail.   I am thinking it was either a solder joint or the relay worked it out.   Going to check the solder joints.   

Thanks!,

David

[bdunham7]

OK, when the voltage starts dropping what is the current doing?

[Dkturner10]

We had the current turned up almost all the way but nothing attached.    We never took a current measurement. 

[bdunham7]

I meant the front panel current meter, at least to start.  I'd want to know if the voltage is dropping due to something breaking down and shorting the output or if it is a voltage regulation issue.

If there's no significant current, then the next thing I'd check is the AC voltage across C17--it should be jumping up as the relays change taps and should not drop off.  If it started working again, then you might leave an AC voltmeter across C17 and test it for a while in case it acts up again.

[Dkturner10]

Hello,
 
     We thought we had it fixed but symptoms have changed.   When nothing is attached it starts at C.V. vs C.C.   You can turn the voltage all the way up to 30 volts with current all the way off.   When you turn up the current it shows 0.00 and does not change.  I tried the fine and the course adjustment no change when turning them all the way up.   Normally I have to turn up the current to get voltage but now its like its wide open and can go to 30 volts with current all the way down.  Any ideas would be greatly appreciated. 

Thanks!,
David

[bdunham7]

What are you loading it with? 

[Dkturner10]

There is no load.    I am thinking that 0.00 would be correct on the current.  Normally if I had the current all the way down I would not get any voltage.  Now I can go to 30 with current all the way down.   It would also start at CC and go to CV after turning up the current a little bit.   Now its always on C.V.  with current turned all the way down.

[bdunham7]

That could be a very minor issue with the current adjustment.  Put a very small (500R-1K) on it and see what it does.  Also put an ammeter in series with the load so you have an independent reading.  If it goes to 30V even with a load and the current turned down, there's a serious problem.