HP 6209B DC Supply

[orion242]

So I picked one of these up recently and its needs some TLC.

Symptom - No regulation, outputs ~530vdc regardless of controls.

http://www.keysight.com/en/pd-1000001527%3Aepsg%3Apro-pn-6209B/single-output-power-supply-0-320-vdc?&cc=US&lc=eng

Troubleshooting.

Checked all the voltage rails, all fine more or less where they should be.  Checked test points 31, 33, 37, 38, 41, and the +16v rail.  All good.
Checked voltages in table 5-2, again all good.
Check ESR/capacitance on all the large electros, no problems.

Looked over the theory of operation and checked the SCR control block, had the signals on test points 68, 67, 66.  All looked more or less fine.
The "turn on control" seemed to be working.  Upon power up the output is 0v for a few fractions of a second and then shoots to the moon, ~530v.

In the process of checking voltages...I could smell some "hot".  With the voltages at hand, was not too keen to be touching things, but it seemed R49 (center of the schematic near the main rectification bridge) was the source.  R81 & R54 are also in contact with R49, so it could have been them.  This makes sense since C14 had 530v across it when powered.

Then as the manual recommends, I pulled CR35 and connected a DC supply across C14.  Powered it back up and supplied C14 with 20vdc, output was 18.5v with no regulation.  Output was ~1.5v minus whatever I fed in C14.

Followed Table 5-4 and shorted Q4 E-B, no change in the output.  Still 1.5v less than what I fed in.  Checked Q6 out of circuit, passed diode checks no shorts.  VR3, same no shorts passes diode check.

I did a quick diode check on all the silicon between the output and CR35, and didn't find any issues.  CR35 tested ok out of circuit, but since the turn on control works...I would thing at least CR35 or Q6 has to be working.

Then I looked around the feedback path and found R12 (center of the schematic by the A7 terminal) & R1 (center of the schematic in the constant voltage comparator) open and R13 measures 151k out of circuit and should be 36K (org, blu, org, gold).  R10, the voltage set pot, also not so linear any more.  Still within 1-100K buts in has some bad spots.  Looks like someone kicked in the shaft, its a bit dicky as Dave would say...

So I have replacements coming for R12, R1, R13, & R10, the only issue I had was finding good replacements for R1 & R12.  Digikey and mouser didn't carry any 2k 3W 5% 20ppm resistors available in small quantities, only 30ppm.  Checked for 3-10W, still the same 20ppm seems impossible to find in small qty.  Ordered the 30ppm anyway as I want to see if this guy comes to life.  I assume this is not going to make a huge difference in the operation of the unit, but would prefer if I could stick with the original specs.
 
Anyone know where I can find a source for small quantities of suitable replacements or is it pointless?
Anything else that may be damaged from what I what I found so far?

[Stray Electron]

  Can you get 1 W 20ppm resistors and parallel them to get 3W? Or some kind of series-parallel combination to get the right resistance and wattage but it would need to be made of 20 ppm resistors.

[orion242]

Ahhh, its the power rating + 20ppm that is making it impossible to find.

Thanks for the tip.

I'll add it to my next order.  Got the 2k 30ppms and 100k pot yesterday.  Put a 30K 1W in for  R13 and decided to see if it came alive.   No dice, still no regulation and it fried my new $15 100K pot. ARGH!

Going back to the manual recommendations of checking Q6 & VR3 for shorts, I figured out VR3 was installed back to front.  Previously I just did a quick short check in circuit on VR34.  No wonder I didn't find shorts on a quick diode test, the damn thing was in backwards!

I hooked Gary's resistor substitution board in for R10, installed VR3 correctly and tried testing it again with an external power supply.  That was it, now at least the voltage regulation works.  Put everything back on its internal power supply and voltage regulation is fixed.  Quick check with 1ohm 100w resistor on the output and the current limiting seems to be good also.  Fixed the burnt out power indicator with a led and everything but a new voltage adjust pot is good to go.

Someone must have been dinking with it at some point and swapped around VR3.

So another few day wait for more parts begins, but it looks like this guy is back in working order.

Time to move on the the HP 6824A which is the next piece of sick gear in line.

[orion242]

Some pictures of this beast after fixing it, adding front output terminals, and a quick cal of the meter.

Not a bad unit for the $10-20 I have into it.