Keithley 155

[maginnovision]

I've got a keithley 155 that can't zero when powered. Found ±6V rails have no output the +6V rail 330K resistor is open, the transistor is blown, and the tantalum is shorted. The -6 seems ok so I'm not sure why I get nothing from it but I'll recheck after repairing +6V. My question is this... What sort of specifications would you suspect required them to specially select the transistors? I bought 10 of each(the ones they use as diodes, and the ones they use to regulate for both rails) and curious if it's really that critical since most parts of the circuit that use ±6V all have adjustments anyway.

[Ordinaryman1971]

Grats on the meter. I don't have this one.. they go for a lot of money.

I would just pull the other one out and see what's there.... and try to match it maybe. Is it that important, don't know, never worked on one....

[maginnovision]

The -6V transistor is harder to get to but that might be a good idea. The only problem would be if the transistor they use as a diode is what it is matched to. Since I'll have everything I need maybe I'll breadboard both supplies and balance them that way. I'll only have 10 of each transistor but maybe that'll be good enough.

[picburner]

D106 and D107 (2N3638) should be standard components.
If there were a request for selected parts there would be a note as in the first attachment.
The important thing is that the voltages of + 6V and -6V are within the tolerance as per the attached second table as specified in the manual.

[maginnovision]

D106 and D107 (2N3638) should be standard components.
If there were a request for selected parts there would be a note as in the first attachment.
The important thing is that the voltages of + 6V and -6V are within the tolerance as per the attached second table as specified in the manual.

I'm not sure if the standard transistor is supposed to be low leakage or if they're specifying low leakage specifically according to parts list. The 6V ±2V is pretty odd though, I suspect any of them will work for that. Perhaps I'll replace the damaged components and check to see if I at least get those rails back tonight.

[maginnovision]

I took a little time today and replaced R169, Q117 and C118. I checked the rails and both were still down. One last thing to check... "D"107. Rather than desolder it since I don't have a replacement right now I snipped the leg attached to R169 and the negative rail came right up, of course the +6V rail hit 17V or so but once I get the replacement I expect those rails will be sorted and I can continue fixing things if they're needed. Thankfully the whole thing is a single sided PCB making it very easy to work on without removing the PCB.

[branadic]

What a pitty that shipping costs a fortune from US to Germany.

-branadic-

[maginnovision]

I do feel bad about that, it's possible we can work something out after I've got it working? A working unit should be worth the shipping cost, plus a few bucks for parts(cost), since it could be resold when you no longer want/need it.

[branadic]

No need to feel bad I'm looking forward for your progress.

-branadic-

[maginnovision]

I replaced "D"107 today and found -6V rail went to 2V.  So I replaced the transistors for that rail and now the +-6V rails are both +-8V, so within spec. The AC amplifier circuit seems to be a problem now. The needle pegs and wiggles at this point. Since this is definitely going to require a little more in depth checking I ordered some very low leakage zeners to replace the transistors they used originally. So at this point I'll work on some other things to clean up the repair bench a bit and continue after, preferably when it's not 40C there.

[Ordinaryman1971]

Have you seen this one from Marco... https://youtu.be/BPnQeYuuAv4
Very timely of him...
Marco... now they will be even more expensive, stop it!!!!

[maginnovision]

After some checking of the AC amp circuit I couldn't really find any issues. I swapped out Q103-Q107 just to see if there was any change but there wasn't. I'll probably swap them back when I get back to it. Moving forward were the input mosfets Q101 and Q102. When removing the gate leads the chopper drives were both still present. I suspect it's only Q101 that's faulty but the manual specifically states to replace BOTH so I'll do that. These are HDIG1030 mosfets but I can only find only place selling them and they're sort of expensive. 3N163 is a drop in replacement and also newer so I ordered a few of them and will try them out when they arrive. My biggest worries is they'll be DOA due to poor handling, if that happens I'll get some HDIG1030's.