BK Precision 875B LCR meter repair attempt

[henas]

I am looking for schematic for BK Precision 875B LCR meter or perhaps someone has some experience repairing them.
By mistake I connected nearly twice the voltage to its external power socket. Now, after turning the unit on, plus power rail is slowly floating down. In a minute or so the display starts showing low battery status. Minus power rail is OK. Steady -6 Volts. Without the schematic I can't see what regulates plus power rail. I can't see any regulator??
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[antimatter503]

You will probably be out of luck with finding a full schematic.
Have you taken the unit apart yet?
If you post good HQ pictures of the pcb, people will be much more likely to help.

EDIT:
You can trace out the board to find what regulates the positive rail by going backwards from input. It wont be far away in fact it really should be obvious. I highly suggest posting pictures. Also, probably not a good idea to continue powering the board when you know regulation is fried.

[henas]

Thank you for your time. Yes I agree it will be hard to get schematic but I searched archives and someone claimed on another blog he got one but it was more then 10 years ago.
I opened the unit. Nothing is visually fried. The unit has two boards and after disconnecting the second board the symptoms stayed the same so I suspect the problem is on the first board. It is the mixture off CMOS logic and a few TL061/062/27L2C op-amps. So far what I can see -6V is derived directly from the battery through -6V regulator and serves as negative rail for op-amps only. Battery plus appears to be connected directly to plus op-amps rail and serves as Vdd for CMOS I suspect, there is some circuitry somewhere which creates phantom  GND and possibly serves as Vss for CMOS logic. I checked some transistors and ICs for shorts. So far no success.
I will get some pictures as you suggested.

[henas]

Missing Ic is TL061 which I tested off circuit as inverting amplifier and nothing connected to offset pins 1 @ 5. It appears to be OK. Q2 is 3906 and Q3 3904 both tested OK on curve tracer.

[HG]

BK 875B schematic

[floobydust]

Same PCB "4070-3C" as an American Reliance Inc. (AMREL) 470D LCR Meter I have.

Many trimpots, calibration would be a lot of work, but it hasn't drifted enough to bother me.
Strange NTC+JFET on the "UP BOARD". Using a CD4046 PLL with qudrature outputs. Interesting design.

[HG]

Based on my own BK875B, the MIC-4070D schematic and some engineering I have made my own BK875B schematic.

It has values for nearly all resistors, capacitors, and integrated circuits.

Here is the result as pdf.

I am still working on some improvements of my own BK875B where I have problems with clipped sine waves because of overdriven / saturated operational amplifiers.

[HG]

For those still looking for a 875B Calibration Procedure: I upload a very old one. Written on a typewriter.
But be careful! There are some serious mistakes: Point 1 and point 4 - VR17 is adjusted two times and the second adjust readjusts the first one. The same in points 11 and 21 with VR6.

I've got this "calibration procedure" from the USA BK support after the BK support for Europe, Sefram had told me "unfortunately we cannot provide that type of document. Calibration is performed exclusively at our service center." lousy support!