HP 6114A Newbie Repair

[hifiguy99]

Hi all,
I have an old HP 6114A. I have a few things I noticed that are wrong. I am inexperienced with vintage electronics and any input I would appreciate.
I have an imgur album here: https://imgur.com/a/RlH44oH
It seems I can't upload the pictures here directly as they are too big.

It buzzes quite loudly from the transformer. I assume this is typical of equipment this old, correct?.

The PCB is black. I mean not the entire PCB but I have no idea what would cause this exactly. The top underside panel is darkened in two spots. In does release some heat. Is this a major issue? See the picture:

You can see the distinction here:

As for any of the capacitors, should I replace any of them? I know tantalums go bad with age. I assume they are the yellow ones axial ones? Should I just leave it till one goes? None of the caps on the top side or mains side seem budging:

Lastly, if I halfway press the 2nd digit on the decade switch or if I switch voltages around too quickly, it jumps to a random voltage different than what is selected - usually higher. The meter shows it accurately though. Sometimes when normally switching voltages it will spike instantly and go back to normal.
Would a cleaning of the decade switch usually fix this? Example:

Thanks!

[bob91343]

That 10W resistor is clearly too small.  It's probably dissipating several Watts, like around 100 mA through it for nearly 80V drop.  Its paint job has boiled off onto the cover.  Look in the manual to see what the voltage across it should be and that will tell whether it is a bad design or a problem elsewhere.

As for the switch problem, it does appear a cleaning is in order.

The transformer buzz is due either to the overstress implied above, or that the bolts holding the laminations together have loosened.

[wn1fju]

Your board doesn't look too bad to me.  I've attached a picture of my closely related HP 6115A power supply.  I wouldn't start replacing capacitors unless one (or more) is obviously spewing out electrolyte or voltage measurements show something is wrong.

You will need to remove the digit switches, dissect, clean, and reinstall.  See a pictorial in the eevBlog post:

https://www.eevblog.com/forum/testgear/hp-6114a-precision-power-supply-restoration/

[garrettm]

I would use an oscilloscope to check that the programming switches aren't causing large spikes / drops in the output voltage. One unit I had would jump around wildly until I cleaned the switches. Turns out the lubricant had dried up and was causing problems with the rotary contacts mating with the PCB pads when setting the voltage.

Also, you should clean the slide switch contacts, my analog display was acting goofy until those were properly cleaned too.

I've owned three of these power supplies, one of which was silent, the other two had noticeable transformer hum but one was much louder than the other. So the loud hum might just be an unlucky unit rather than indicating something is wrong.

I've made a imgur gallery with photos from my repair. Hopefully they can help you with yours.

https://imgur.com/a/sf8hrbu

[hifiguy99]

That 10W resistor is clearly too small.  It's probably dissipating several Watts, like around 100 mA through it for nearly 80V drop.  Its paint job has boiled off onto the cover.  Look in the manual to see what the voltage across it should be and that will tell whether it is a bad design or a problem elsewhere.

As for the switch problem, it does appear a cleaning is in order.

The transformer buzz is due either to the overstress implied above, or that the bolts holding the laminations together have loosened.

Your are correct in that the resistor is small. I am lazy though and just cleaned off the paint off the surrounding components with some isopropyl alcohol and a Q-tip. The Q-tip could not remove the black paint caked on the motherboard though. I didn't try other solvents. Thanks anyways.
Tightening the bolts that hold the transformer do not help unfortunately.

Your board doesn't look too bad to me.  I've attached a picture of my closely related HP 6115A power supply.  I wouldn't start replacing capacitors unless one (or more) is obviously spewing out electrolyte or voltage measurements show something is wrong.

You will need to remove the digit switches, dissect, clean, and reinstall.  See a pictorial in the eevBlog post:

https://www.eevblog.com/forum/testgear/hp-6114a-precision-power-supply-restoration/

I dissected the mechanical decade switch and used dexoIT gold on the contact plate and some Non-Stick Dry Film lubricant (with teflon) for the innards. It works like a charm now. Thanks for the help. 

I would use an oscilloscope to check that the programming switches aren't causing large spikes / drops in the output voltage. One unit I had would jump around wildly until I cleaned the switches. Turns out the lubricant had dried up and was causing problems with the rotary contacts mating with the PCB pads when setting the voltage.

Also, you should clean the slide switch contacts, my analog display was acting goofy until those were properly cleaned too.

I've owned three of these power supplies, one of which was silent, the other two had noticeable transformer hum but one was much louder than the other. So the loud hum might just be an unlucky unit rather than indicating something is wrong.

I've made a imgur gallery with photos from my repair. Hopefully they can help you with yours.

https://imgur.com/a/sf8hrbu

Good to know the hum is normalish. Thanks for the photos. I did spray down the meter switch with some Deoxit and then again with some of the lubricant as they do tend to get gunked up over the years, although my analog display was working fine.
I checked with a scope and the voltage changes look smooth now. Thanks for the tip. 

[6151kokodef]

I got a 6115A which is pristine internally, but the 1V and 10V thumbwheel switch ranges would jump wildly to 9V (and 90V!!!) when switching between digits 2-3-4. I bit the bullet and dissected the thumbwheel switches like garrettm. It was quite gunked up as well. I cleaned them with alcohol and they work like new again.

As for the transformer hum I guess it's normal. Mains is 220V 50Hz where I am. My 6115A has very little hum but my HP 6228B and relatively modern HP 6632B hum like no tomorrow.

[Slavko]

I have two power supplies hp6114a and hp6115a and both had the same low frequency hum problem. It turned out that the reason was in the flat yellow capacitor 1mkf 250v. After replacing it with WIMA, the sound disappeared completely.

[AlpineJoe]

Resurrecting an old thread, but I now own one of these and need to go through the thumbwheel switches.

From the photos, I see that I would trim the plastic that staked the PCB to the housing, then open and clean the wipers and PCB with gold plated traces.  What I don't see is any mention of what is done to re-stake the switch back together.

Do folks re-heat stake the posts?  Use 5 min epoxy?  Anything else?

Thanks for any tips.

[coppercone2]

I have two power supplies hp6114a and hp6115a and both had the same low frequency hum problem. It turned out that the reason was in the flat yellow capacitor 1mkf 250v. After replacing it with WIMA, the sound disappeared completely.

some ceramics they have buzz too. idk if its the new power grid situation or what, because the SLCC capacitors should not age. I replaced them with equal caps and there was still a buzz. I even added RFI filter to it, and it still buzzed.

for the switches, I wonder if its possible to build a compression plate thing with some standoffs /long bolts around the entire thing, so you don't need to use any fucking plastics technologies there

like really long thin threaded rods

[garrettm]

Resurrecting an old thread, but I now own one of these and need to go through the thumbwheel switches.

From the photos, I see that I would trim the plastic that staked the PCB to the housing, then open and clean the wipers and PCB with gold plated traces.  What I don't see is any mention of what is done to re-stake the switch back together.

Do folks re-heat stake the posts?  Use 5 min epoxy?  Anything else?

Thanks for any tips.

Just wrap the re-assembled switch stack in kapton tape. That's what I did. This way you can clean the switches again--if needed--by simply removing the tape as opposed to possibly destroying the switch housings by trying to pry apart glued together plastic.

[coppercone2]

good idea, maybe give it 1 layer of self amalgamating tape over it because it wont have a bond failure like kapton

or a giant segment of heat shrink.

but what pissed me off about those switches is the spring loaded bits