Need help repairing a B+K 1650 Tri-Output Power Supply

[GraveFurball]

Hi all, I recently came into possession of an old B+K 1650 Tri-Output Power Supply that has seen better days, and I would love some help with getting it back to normal again. There are pictures and a video, as well as links to the schematic, user's manual, and parts list in this Imgur album.

I have found two main issues with it: One of the outputs (Output A) does not scale its voltage linearly, and has a maximum of around 34 volts, when it should top out around 25. The other output (Output B) scales much more nicely, but its maximum is about 25.4 volts. I realize this is within error, but it'd still be nice to get it right on the dot.

In addition, the built in meter does not move smoothly, so you have to flick the glass a couple times to get it to read correctly. It also tends to read a touch lower than the actual output voltage is, even when it has settled.

I've done some preliminary tests on it, and Output A's signal is relatively stable, with little to no noise, so I don't think it's a bad capacitor that's causing the overvoltage. I did note that the voltages at R39 and C16 are at around 38v each, when they're supposed to be at 32 volts according to the schematic. That said, the voltage at R27 that I measured lines up nicely with what it should be (7v), so I'm not quite sure what to make of that. I also noticed that the resistances of the 2 pots that control the voltages of the variable outputs do not seem to be equal at their extents. The one controlling Output A reads around 10kohms, while the one controlling Output B reads about 2.5k. According to the parts list, these should both be the same 20kohm potentiometer, so I'm not sure if that's contributing to the problem.

Any thoughts on the root cause or where to look next would be greatly appreciated!

 

[rjenkinsgb]

The lower channel in the schematic has a "gain" preset, which looks to be for overall output voltage calibration.
It also has a "tracking gain", presumably for matching it to the upper channel when linked.

There does not look to be a gain setting in the upper channel.

That does have a "zero adjust" in series with the main pot, which appears to actually set the output range and not the zero??
See if that will get it to 25V instead of 34?

The pots may benefit from a small amount of 3 in 1 on the tracks if they are erratic.

The meter probably needs replacing; if you find one of the same style you should be able to swap the existing calibrated scale in to it.

 

[GraveFurball]

Fiddling with the zero calibration pot that you mentioned only made a difference of about 1 volt, so unfortunately it doesn’t seem like it will be that easy.

Popping the meter out to look for a part number is less than pleasant, so I will try and take care of that later today.

Kind of a rookie question, but what do you mean by “3 in 1”? I’m admittedly a novice at electronics repair so I’d appreciate if you could tell me what it is.

Thanks!

 

[Mickster]

 

[rjenkinsgb]

OK; one channel at a time:
Turn output A all the way down and adjust the zero pot until it is just zero.


What voltages do you get between output A negative and

IC2 Pin 12?

IC2 Pin 6?

On the positive of C26 or the junction of R12 & R13?

 

[GraveFurball]

In order, the voltages I read were 19.1v on pin 12, 7.02v on pin 6, and 3.17v at the junction between R12 and R13.

 

[rjenkinsgb]

OK, that means the regulated supply to the electronics is OK and the reference voltages are correct.

Does it still go way over 25V if it has a moderate load on it, eg. a couple of hundred milliamps?

And, how stable is the voltage at pin 4 of IC2 as you vary the output over its full range with or without a load on the output?
It should always be near 3.2V (or the 3.17 you measured).

 

[GraveFurball]

Unfortunately I don’t have any way to pull more than about 60mA from the power supply with the resistors I have laying around, and I can’t just go out and buy more because the only local place that sells them is closed due to COVID-19. I can get some online if needed, but it’ll be a few days until they come.

As for your second question, the voltage on pin 4 was stable through the whole range.

 

[rjenkinsgb]

As far as I can work out, with all the voltages correct as they are, it should give somewhere close to 25V at maximum, with the resistor values shown in the schematic.

Have any of the values been changed - specifically the main setting pot, R15, R18 or R21 ??

 

[GraveFurball]

Not sure what you mean by “main setting pot”, but the values for the other resistors you gave all match up with their values in the schematic.

 

[rjenkinsgb]

The control that sets the output voltage?

 

[GraveFurball]

Ahh, gotcha. The one that controls Output A has a resistance of 9.4kohms across the 2 outer terminals, and the one that controls the other output has a resistance of 2.22k across its outer terminals. Interestingly, neither of those are the 20k that the schematic specifies.

 

[rjenkinsgb]

If that is the same even with the connections removed, then that's likely the problem. The overall resistance of the voltage divider chain that has the pot in it will affect the output voltage range.

 

[GraveFurball]

Just to be clear, you’re suggesting to pull the pots out of the board and test them again?

 

[rjenkinsgb]

Ouch.. I did not notice they are soldered direct to the board...

An alternate approach - can you measure the voltage on each end of R13 with the output set to the highest voltage, and the output voltage you get?

I should be able to check the pot value is correct from those readings, I hope..

 

[GraveFurball]

Sorry for all the questions, but did you want the voltage across R13, or for me to touch one multimeter lead to the negative output and the other to each leg of R13 individually?

 

[GraveFurball]

I went ahead and did both ways, the voltage across R13 is 3.17v, the junction between R13 and R12 reads 3.17 as well, and the other leg of R13 reads 0v. The output voltage was 34.9v.

 

[rjenkinsgb]

Doh.. Wrong resistor; half asleep & wrong glasses reading the circuit.. Sorry.

It should have been R15, (which is 13K) - voltage at each end, to the negative output terminal..

 

[GraveFurball]

Apologies for the delay, my computer’s power supply chose today to die, plus it decided to take my GPU with it, so I’ve been working on that most of the day.

As for your question, the voltage on the end of R15 (which is, in fact, 13k) in contact with the pot was 5v exactly, the other end was 6.43v.

 

[rjenkinsgb]

Right..

That makes the current in the resistor chain from IC2 pin 6 through the voltage pot 110uA
110uA through 20K (the pot) should give 2.2V

You have already confirmed that IC2 pin 4 is stable at 3.2V (or 3.17). 5V (on R15 / pot junction) - 3.17V = 1.83V

Something does seem off with the pot value, from that its nearer 16K ??

You could try adding a preset resistor, say 5K, in series with R21 to adjust the maximum o/p voltage.