Vintage HP 6285A repair with a lot of questions.

[rcowboy78]

Hi all,

I just acquired a new to me HP 6285 0-20V / 5A psu. I plan on using it to compliment my 6237B. I have ran into some situations where I needed more current output for DC motors and such and the 6237B wasn't up to the task. I also wanted to make a project out of it, sort of a restore / learning project if you will. I might have bitten off more then I can chew though hehe.

Symptoms:

When I first got it, the volt / amp meter was broken and just kind of flap-en in the breeze. I got that fixed and zeroed. It seems to be working well.
The volt / amp meter selector switch works, but it is really worn out. I will eventually need to replace this, and will at a later date.

Onto the real issues:

When the PSU is first turned on, it seems to work just peachy, but after about 10mins or so the regulation really starts to go bonkers. At first the output voltage starts to vary in the uV range and steadily gets worse over time. After about 15 to 20 mins, all the variations will stop and the output voltage will settle down to about -300mV, where it pretty much stays till after it cools down. This is with the constant current adjustment wide open, and with the voltage adjustment wide open.

My questions:

This PSU really needs to be recapped, because I'm pretty sure that it has all of the original capacitors installed. I don't see a single new capacitor in it. As far as the film capacitors are concerned, can I just replace them with new modern day ceramics, or should I use something else?

With the loss of regulation, I started checking the reference voltages, and noticed that Q600 is getting quite warm compared to all of the others in the reference circuit. I can touch it and hold onto it for a bit, but it gets quite uncomfortable after about 10 secs or so. However, when the PSU becomes unstable, I don't see a noticable change in the reference voltages. So it leads me to believe that Q600 is okay. Q600 is an PNP, and I checked it for shorting, and it seems to check okay.

The reference voltage specs are as follows a long with what I have measured:

Test point (TP 29)  = 9.4V +- 0.4V. I measured 9.07 Under normal operation and under the fault
TP 30: 15.4V + 0.5V = 15.1, which I also measured under both fault and normal operation
TP 29: -6.2V +0.3 = 6.07, which I also measured under both fault and normal operation

Now I just assume that these voltages are within spec, but the service manual of says +0.5 and +0.3 for TP 30 and TP 29? So are these really out of spec, or can it also be +-? I am a little confused in regards to this.

I also noticed that the zener diode VR600, or altleast I think that is what it is has been replaced. Its a more modern day component, then the rest of the diodes that are on board. There is a third component next to VR600 that I not sure what it is. Any suggestions on the part? its not on the lay out, but it also looks like a capacitor. (see included pictures)

Any suggestions on how you think I should proceed would be greatly appreciated.

rcowboy

[TimFox]

On a quite different -hp- supply, I had problems with the current limit going crazy and found that an addendum to the manual had changed the auxiliary reference Zener (not the main reference, but used for the current limit) from a large metal tube unit to a smaller glass unit.  I made that change myself, and the current limit problem cleared.
The probability of one of the plastic-cased film capacitors being bad is low, but if you do replace one use a modern polypropylene film unit, not a ceramic unit.

[schmitt trigger]

Unfortunately, I cannot help you more, other than the fact the earlier plastic transistors would sometimes allow moisture ingress.

But I still would like to comment on the beautiful  PWBA workmanship.

[Tomorokoshi]

The schematics look like the ones from the scan on the Keysight web site. The scan on BAMA is clearer in some places:

http://bama.edebris.com/manuals/hp/6285a/

I've had electrolytic capacitors short out or otherwise go bad in equipment of that vintage.

With the loss of regulation, I started checking the reference voltages, and noticed that Q600 is getting quite warm compared to all of the others in the reference circuit. I can touch it and hold onto it for a bit, but it gets quite uncomfortable after about 10 secs or so. However, when the PSU becomes unstable, I don't see a noticable change in the reference voltages. So it leads me to believe that Q600 is okay. Q600 is an PNP, and I checked it for shorting, and it seems to check okay.

Check C602 on the output of the reference circuit.

Is anything going on in the "DRIVER & ERROR AMP" circuit?

Also check the variable resistors used for calibration, in case there is a thermally-dependent poor connection.

[rcowboy78]

Thanks for all the replies everyone.

I haven't been able to look at it today, but maybe i'll be able to this evening sometime.

Tomorokoshi,

I will check C602 on the output of the reference. Unfortunately, I don't have an ESR meter, and can only check its capacitance value, but I may borrow one from a friend next week.

One thing I failed to mention and forgot about, is that the voltage across C600 is low. It's suppose to be 33 V, but I'm measuring 29.80 V. This voltage is also unstable. Its range varies by a volt. I've seen it as low as 28V. Something is pulling down that voltage, and I suspect Q600 because it is getting quite warm., but I'm not 100% yet.

I will also check the voltages in the driver & error amp section, as it is definitely suspect, along with the output transistors.

I will let ya'll know.

Thanks a lot.

rcowboy

[rcowboy78]

Okay, thought I would give an update.

I replaced all of the elect. capacitors except for 3, the two big main filter caps C803, and C500. Don't really have the means to test C803 and C500, but I dought that they are bad. C600 checked out fine with an ESR test. Most of the others were out of spec except for one. I replaced two film capacitors, but they checked okay out of circuit. Since they checked okay out of circuit, I left all of the other film capacitors alone. I didn't think they would be bad.

After replacing the caps, it has corrected the voltage regulation problem. YEA! 

However, I seem to have a new problem. Although its probably not new, I just haven't been able to see it until now.

I seem to have an over current situation. <-- Two issues here I think, let me try to explain.

The control knob to adjust the current is not a OEM part. It's either a variable resistor, or some kind of encoder. I dought its an encoder being this vintage, but it has been replaced with some aftermarket type. I have not looked into it yet, but if I turn the voltage control knob wide open, and short the positive and negative post together, I don't have any current control. If I just barely turn the knob, the amp meter slams hard wide open, when I just barely crack the current knob. I'm not sure yet if this is in the variable resistor / encoder, or in the current circuit yet.

Another test I did, was to hook up a load. I put 2 power 12 ohms resistors in parallel, at 10W a piece, for a total of 20W @ 6 ohms. I had the current wide open, and slowly adjusted the voltage knob until I was comfortable at 2 amps. I let that run for awhile just to check for voltage regulation. All seemed fine. I went to eat dinner, and came back to give it a run up of current for a short duration, and I had a read out of 7 amps. It would of kept going, but I didn't wanna blow something up, so I didn't push it any further.

Anyways, this is where I'm at now. I will give further updates as things progress.

rcowboy   

[rcowboy78]

Okay, I checked the variable current control knob. It is a variable resistor. Put a meter on it, and it is definitely a problem. With the knob full counter clockwise, it reads 20 ohms, give or take 200 ohms lol. It jumps around a lot. Once I barely turn it, it jumps to 5.8K and stays there. I am able to play with it a bit and can get intermediate values, and hence some current adjustment.

However, I need some help. Can some one verify that R809, 810 says that it is a 1.2K outer, and 20 ohm inner pot? I can't read some of the writing on these scanned sheets.

The pot that is in my rig, is a single knob. It's doesn't have the inner adjustment, just the outer adjustment. It is a 10 turn pot.

[Tomorokoshi]

That's the same resistance I make from the datasheet. Same values are on the "CURRENT CONTROL" section of the schematic. Note that although it isn't listed in the parts list, those wirewound pots tend to be in the 2 Watt range. If the 10-turn is lower wattage that may explain why it's broken.

[rcowboy78]

Thanks Tomorokoshi,

I'll see if I can source a new one.

In the mean time, I'll see what I can do with the one that in there, and get its specs.

[JXL]

I have the HP6286A 20V/10A which is almost the same as HP6285A.  My dual-axis pots were both shot, so I replaced both V and I pots with 10-turn pots.  Also note that you can control V and I through the connectors on the back of the supply.  You can use that to verify that there are no other issues with the supply before replacing the front controls.

[rcowboy78]

Okay, new update again. 

Pulled the face off to check the pot, and what I though was going to be a cheap replacement pot, I guess, turns out to be a pretty decent one from the looks of it. Its a Daystrom pacific 2k. So i'm guessing this is the original factory one that came with it. I didn't think HP would put some colored pot in there like that.

Anyways, I placed an order for a replacement. Although the voltage one seems to be working good, I went a head and ordered a new one for it too. It's a 5k, but different brand.

JXL,

I didn't even think about that, I'll look at it this evening if I have time. I forgot all about that. 

[rcowboy78]

Oops, forgot to add the pot picture.

[rcowboy78]

Okay, have an update.

The pots came in today so I decided to dig into it a bit. All went well and I now have complete current control. I didn't replace the voltage pot as it seems to be working well. If I have too, I will do it later. Went through the meter calibration and the meter is pretty much spot on.

However I do think I have other issues still. Would like ya'lls opinion.

I attached a 1 amp load to the supply, and did the power on and power off check looking for spikes. I don't have any voltage spikes perse, as far as over voltage, but I do have one mean ripple. See the attached images. This ripple is only on power on. The power off looks very clean.

The ripple is about 850mV, but it never exceeds the set voltage. Frequency is about 120Hz. I guess this is probably from the error correction circuit. Thats just my guess.

I guess my question is, should I chase this down, or is it circuit worthy?

[rcowboy78]

Need to make a correction, the ripple is 860mV peak to peak, not over and under for 1.6V p-p.

[rcowboy78]

Oh, one other thing. Not under load the output looks something like the attached.

[schmitt trigger]

That Daystrom pot looks like top-grade stuff, but it will eventually wear out too.

With what did you replace them with? I assume that Daystrom is no longer in business.

[rcowboy78]

I replaced it with a Bourns, 2k, 10 turn pot. Its probably close to the same quality as the Daystrom. It's about as high quality as I wanted to go at $18. Being as I only bough the supply for $25. I tried to source the Daystrom, but as you guessed it, I couldn't find much.

I still haven't chased down this ripple though. I'm thinking its coming from the SCR being its at 120Hz. I tried a few tricks by not floating the supply, and tying it to earth with a 1uF across the load, but the ripple still exist. Albeit not as bad. I don't think its the voltage / current circuits as I can see the phenomenon, on both the voltage and current outputs. My only guess is that its still one of the caps that I didn't replace in there. There are a couple in the gating circuit and the error circuit that are films that I didn't change out. Although, the gating circuit is for the main transient, and not so much for this ripple, kind of leads me to think that its fine. I also didn't change out the paper cap that's on the series regulator. It tested at a little over 2ohms on the ESR. <=== probably is most likely the suspect. However, I just don't see it as being a problem when the ripple is pretty darn good in a normal load configuration. I don't like just changing parts to be changing parts if ya know what I mean.

Anyways, I'll keep pondering this, and see if I can come up with a fix.

rcowboy

[Tomorokoshi]

Did you check C800? The schematic says 18V between testpoints #44 and #43. Seems to be used in the SERIES REG. circuit.

[rcowboy78]

Tomorokoshi,

It could be C800. I measured 16.29 V. All I can do is replace it I guess. I don't have a 1500 uF, 40V on hand. I'll have to order it. It's one of those big metal cased caps on the back side that I kind of considered good.

I'm also measuring 29V across C600 when it's suppose to be 33V.

I'll replace them and let ya know, will be a couple of days.

rcowboy.

[rcowboy78]

Okay, got an update.

I replaced C800, and let me tell ya it was a bear. Its a five terminal cap, where 4 of them are negative pins. I couldn't get it to come out with just desoldering, and I couldn't get under it to snip the legs off. I ended up getting a fine tooth sawzall blade and cutting through them. Once I got it out of the way, the rest went well. I also replaced the 325 uF reference cap and the paper cap C501.

Anyways, all of this work didn't improve the startup ripple. I'm really at odds end on it. It could be just about anything. I've got low voltages in a number of places. Makes me want to think that T800 could be weak. There could be a transistor that's weak somewhere on startup. When the supply is in a steady state / constant load, its output is just beautiful.

I dunno, maybe I'm asking to much from 1976 components. They are 41 years old.

I guess i'll put it into service just like it is. I only got it to power bigger loads and not so much for circuit design.

Thanks for everyones help.