Electronics > Repair
[SOLVED] HP 34401a - Error 612, 613, 615, 617, 618, 619, 621
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spicytofu:
I was getting to the relay next after my first post. I already checked it and it was stuck, but before I saw your reply. The problem is fixed now, just banged on the relay when I took it out and it is working again. My guess is that a very large current was pushed through burnt some traces and possible arched the relay. The fuse couldnt save it in time. I repaired two burnt traces and everything seems to be working now.

Another note: it seems that the transformer wiring was wrong from the factory. (this unit was never opened) I noticed a very darkened and hot PCB area at the regulators. Checking the AC volts, I saw 28VAC and DCV unreg was 33V (total was 66V!) No wonder the vregs were burning up. I swapped the blk/red and blk/org xformer secondary wires since they were reversed and now I am getting the correct 27VDC and 23VAC at the regulators.
amx:
Hi all, thank you so much for this thread, I purchased on eBay 34401a, with an error list: 612,613,615,617,618,619,621

Most of errors was fixed by replacing OpAmp u201 (AD706).
But two errors keep remains: 612,618.
Today I was found that resistor R201 (400k) is open.

The problem that I can't find such of resistor on US market, I did check Mouser, DigiKey, Arrow and can't find in stock any of resistors with 400k 1% 2PPM
The closest one is 25/50PPM in stock.
The original one: is Vishay UXB02075H4003FR100

Is anyone know replacement for this bad boy?

Thank & Regards,
Maksym.
coromonadalix:
the only one at 25ppm are theses Vishay RN55E4003BB14   Mouser etc ...    witch is 10x time the possible drift ??



2x of theses 200k ohms in serial ??
https://www.ebay.ca/itm/225772449330?

PLTU  Vishay Dale Thin Film series and add wires ?
https://www.vishay.com/docs/60131/pltu.pdf

this one ?    
RES 400K OHM 1W 0.1% AXIAL   or lower wattage
https://www.digikey.in/en/products/detail/vishay-dale/CMF60400K00BHEK/3617345
https://www.vishay.com/docs/31018/cmfind.pdf
Kleinstein:
It is rather odd for this resistor to fail. A first test would be just any 400 K resistor (could be 2 resistors combinted), just to see if the errors go away and it the resistor is indeed to problem. One could consider a combination of 2 resistors for the replacement. E.g. 2 x 200 K or even 4 x 100 K, that may be easier to get. 200 K resistors are avilable with 5 ppm/K.
amx:
Hi coromonadalix, Kleinstein,

Thanks for advising, definitely today after work I will test with out of the shelf resistors.

I am going to order two 200k 5 PPM.

https://www.mouser.com/ProductDetail/Vishay-Dale/PTF56200K00BZEK?qs=Cpc6SZ3%252B4uN9nIDG6awleA%3D%3D

https://www.vishay.com/doc?31019

Thanks,
Max
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