HP/Agilent 6624A 4ch PSU Repair

[rx8pilot]

I recently purchased this 6624A linear PSU. It is a 4ch supply and 3 of the 4 channels work fine - ch 4 reports OVERVOLTAGE and the output when off is about 620mV or so. At the moment, I only need 2 channels and 1 is a backup. Repairing the #4 channel is a skill-builder challenge and a nice bonus if I can sort it out.

So far, I have done the visual inspection and started troubleshooting from the output stage backwards - thinking that maybe someone back-fed it. The first real slow down has been the service manual I have found appears to be for a newer model. While it is close - there are enough differences that it is making a challenge to figure out what is what schematically. The way this unit is configured is that each output channel is it's own self-sufficient PSU fed from a big transformer in the middle. This makes comparison troubleshooting rather straight forward and I have found some disparities between the working and non-working sections. Without the right schematic, it is hard to get down to a specific component though. There are a few custom parts and the output transistor stage is in a module that may not be economically viable to repair. The goal is to isolate the easy parts like voltage references, OP amps, discreet transistors, etc that may be damaged.

Does anyone know a source for older service manuals that may cover the serial number range that I have in front of me?

[VK5RC]

Some while ago I repaired a 6632,  the biggest problem was the manual covered 8 models,  even the 'right'  schematic covered several models and you needed to edit the schematic according to the parts list! FFS! That could well apply here.
Note,  your title has 6642,  should it be 6624?
My fault turned out to be a missing output cap ( probably from manufacturing as no evidence of pcb rework)  great power supplies,  but a bit noisy.
Sorry not much help.

[rx8pilot]

Note,  your title has 6642,  should it be 6624?

Corrected! Thanks.

Some while ago I repaired a 6632,  the biggest problem was the manual covered 8 models,  even the 'right'  schematic covered several models and you needed to edit the schematic according to the parts list! FFS! That could well apply here.

That may be the case. If so, it will add some time to shake down the problem.

I have managed to get through the fundamentals - various rails and voltage references seem to check out ok. Next, I am looking at the op-amps and transistors to see what I find. Most of the parts are rather common so I am hoping to find a failed op-amp or something equally simple.

One of the hardest thing on these old PTH PCBs is that the components cover the silk screen so the part layout is rather important to ID things......continuing to fiddle. Hope I get lucky.

[VK5RC]

I absolutely know what you mean re the part number issue,  also for good measure my psu pcb covered several models,  so there are lots of blanks! 
Watch out some of the heat sinks were at 80V!

[drforbin]

Has anyone found that manual for older 6624a?

[casinada]

I repaired an HP6624a and Agilent 6627a a couple of years ago. The problems were different from yours. I think I downloaded the manuals directly from Keysight:
http://www.keysight.com/en/pd-836785-pn-6624A/system-power-supply-40w-4-outputs?pm=PL&nid=-536902299.384181&cc=US&lc=eng

Both had a bad card and the problem were 2 bad processors ans a couple of those little fuses that look like resistors. I purchased the processors directly from Keysight since the firmware is not available. I think they were around $20 each.
Good luck

[pelule]

I myslf repaired several Agilent 6624A (newer) as well HP 6624A (older). The last HP repair I documented:
https://www.eevblog.com/forum/repair/(fixed)-hpagilent-6624a-psu-(the-tank)-hw-error-ch3/msg989667/#msg989667
I used the Service Manual (Agilent Part No 5957-6379) downloaded from Keysight page as linked by casinada.
It is close enough also to the older HP mode to get all my repairs done.
You did not mention serail number of your "tank", so I am not able to judge.

[rx8pilot]

Maybe someday, I will get to my 6624A with one bad channel. This is encouraging to see others with success and information.