Vintage Power Supply fluctuating in output

[mikefromsac]

Hi Guys:

I’m a new to electronics and I mostly repairs things around the house and do simple teaching projects in order to learn. I typically use a inexpensive Korad digital power supply or a homemade supply (ATX based). When I set the Korad for 12 volts (our use the 12v outs on the ATX) they're both pretty rock steady (the Korad locks at 11.98 on a bench meter and the ATX at 11.94, and both stay there).

I just picked up BK Model 1601 regulated dc power supply (from the '70s or '80s I believe) because it looks so cool with the dials and two analog meters. It seems to work well other than the voltage fluctuates. If I set it to 12 volts over a period of about 5 minutes it will fluctuate from about 10.4 to 13.6 volts (based on the same bench meter).

Is this typical for analog power meters of this type or is something amiss? I'm guessing it should be more stable. Any ideas where I might look? I’m not capable of a major repair yet but I could probably replace a cap or resistor, clean the level pot, or something simple.

Thanks in advance.

[Brumby]

Analogue meters do not jump around - unless the driving current does.  Their fault modes tend to be either 'stuck' or 'unresponsive'.


When you say 'fluctuates' - how quickly does it do this and how often does it happen?

That is to say -
 - Does it 'jump' between these two figures instantly or does it take a few seconds or more?
 - Does this fluctuation correspond to any mechanical movement/vibration/jarring?
 - etc.

A little more information in describing the behaviour would be useful.

[mikefromsac]

It's kind of jumpy, the analog needle on the PS moves continuously between the voltages (about a 2 volt range) and the bench meter is changing numbers non-stop. It does this if hooked up to a small 14v mini bulb or just plugged directly into the bench meter or both together. There is no mechanical movement to the PS during this

[Vgkid]

Here is a manual.
https://www.manualslib.com/manual/837816/Bk-Precision-1601.html#manual

[Brumby]

It's kind of jumpy, the analog needle on the PS moves continuously between the voltages (about a 2 volt range)

* Does the needle move from low to high
 - instantaneously?
 - in a fraction of a second?
 - over a number of seconds?
* Does it move from high to low at the same speed?
* Is is doing this at a regular frequency - or does it change quickly at some times and slower at others?

If you were to plot needle position against time, would it look like a square wave, triangular, sawtooth or sine?


If you could get a video showing several cycles, it would be helpful.

[mikefromsac]

* Does the needle move from low to high
 - instantaneously?
 - in a fraction of a second? Yes, a fraction of a second.
 - over a number of seconds?

* Does it move from high to low at the same speed? Yes.

* Is is doing this at a regular frequency - or does it change quickly at some times and slower at others? Quickly at some times and slower at others.

I have the manual and read it but didn't see anything that addresses this issue.

[Brumby]

The manual has the circuit diagram.  Your symptoms can be used to look at that diagram and work out what might be wrong - or at least point you to what to check.

Are you comfortable probing around inside the case with power on?

[Brumby]

Actually, before doing anything else, it might be an idea to open up the case and just have a look - without any power applied.

Sometimes problems will start at you with black sooty indicators ... or capacitors with the 4th July in mind.

[mikefromsac]

Actually, before doing anything else, it might be an idea to open up the case and just have a look - without any power applied.

Sometimes problems will start at you with black sooty indicators ... or capacitors with the 4th July in mind.

This will be my next step. I'm not good reading diagrams yet and thought somebody might know where to point me or have a simple diagnosis.

[Brumby]

Certainly, take a look first.

The manual also has a board layout, which should make it a bit easier to point you to places to check.


BTW, I'm running a bit short on time tonight - so someone might have to take over.

[mikefromsac]

... BTW, I'm running a bit short on time tonight - so someone might have to take over.

Thanks, appreciate your help

[james_s]

The output should be rock solid stable, especially under a steady load. If it's jumping around the first thing I'd suspect is dirty pots. This could be either the front panel pots or the internal trimmers. A cold solder joint is another possibility, rule those out before assuming a defective component.

[mikefromsac]

The output should be rock solid stable, especially under a steady load. If it's jumping around the first thing I'd suspect is dirty pots. This could be either the front panel pots or the internal trimmers. A cold solder joint is another possibility, rule those out before assuming a defective component.

That would be a nice easy fix, I'll give it a shot tomorrow. Thanks for the post.

[mikefromsac]

It seems to working much better after a good pot cleaning. It now only fluctuates a few 10ths of a volt. After I let her settle in a day or so I'll clean again. Thanks so much for the advice. - Mike

[Brumby]

Sometimes it is the simple things.

Good work.   

[dave_k]

Most of the time Sometimes it is the simple things.

Good work.