Data Precision 8200 - 100V range out of pot range during adjustment

[Rax]

Hi all,
I am adjusting a DP8200 and everything is going alright, except the 100V range is not adjusting enough to get into calibration. I am barely at 69.9038V and I've spun the pot to much I am certain I'll run out of range before I get it within 100uV of 70V as it's supposed to be.

I enclose the schematic of the corresponding circuitry.

Any input and thoughts welcome.

[Kleinstein]

There are relay contacts in series with the 10 K +90 K (incl. trimmer) gain setting resistors. Especially K2 in series to the 10 K may be an issue with relay contacts that were not used for a long time. A few cycles and whatching the change may be a good idea.

[Rax]

My other thing to investigate is whether the 130V rails are in good shape. It's been a while since I last looked at this, but I think the Q3-Q7 is the stage doing the 10x and if the rails are too low maybe that robs the stage of enough headroom to go as high as it needs to. 

[RandallMcRee]

My other thing to investigate is whether the 130V rails are in good shape. It's been a while since I last looked at this, but I think the Q3-Q7 is the stage doing the 10x and if the rails are too low maybe that robs the stage of enough headroom to go as high as it needs to.

Your description makes it sound like that trim pot is busted. Easily happens.  Why don't you confirm that hunch, first?

The pot can only trim a small amount, though, so perhaps look at your working unit to confirm correct voltages around the trimmer and its associated resistor.

[doktor pyta]

Dirty relay contacts are common issue of DP8200.
Fortunately they are easy to open an clean with paper soaked with IPA.

[Testtech]

There is a 68K resistor, R51, near the rear of the board that often burns up or changes value. Also the hv amp power transistors and their drivers can fail.