Fluke 8842a Resistance Incorrect in all ranges

[Ultron81]

Recently picked up a 8842a. DCV seems to read OK, but the 2-wire ohms readings are completely out of whack.

Went through troubleshooting with the service manual, and narrowed it down to the Ohms Current Source section of the schematics.
- Supplies are OK, no ripple and voltages are within range.
- -7.0V Precision Voltage Reference is on the mark.

I then tested the first stage of the Ohms Current Source. The manual says to connect a 2k resistor between Reference Low and U402-19. I checked the voltage across R401 and it is correct (7.0V). Checking the output of the op-amp (U401-6), I only read -1.0V (should be -4.0V). So I noticed that U401 is an OP07, easy to find, but the JFET Q401 I cannot figure out the manufacturer's part from the markings. I attached a photo of the circuit and the markings on the JFET. Do you think I can use any n-channel JFET in place of it (with the proper voltage specs)? I plan on removing the JFET and testing it.

Also, another question, what's the reason for some of the JFETs in the circuit (Q405-Q407) to have the drain and source shorted? I don't think I've ever seen that before.

Thanks for your help!!

[Kleinstein]

Chances are good that one can use a different JFET for Q401. This one is not extremely critical. A low current one like 1N4117/8 or  BF245 should be OK. If at all gate leakage matters.

The JFETs with D-S connected are used as low leakage diodes. For some reason (likely demand) it can be cheaper to find a JFET with guarantied low leakage than a corresponding diode.

However I would not really expect Q401 to go bad - its not that much exposed. The gate still reads negative - so it could be still OK. There is quite some scattering it threshold voltage and that is what you see at the gate here.
I would suspect the protection circuit, e.g. Q402-Q407 and RV402-404.

[Ultron81]

Chances are good that one can use a different JFET for Q401. This one is not extremely critical. A low current one like 1N4117/8 or  BF245 should be OK. If at all gate leakage matters.

The JFETs with D-S connected are used as low leakage diodes. For some reason (likely demand) it can be cheaper to find a JFET with guarantied low leakage than a corresponding diode.

However I would not really expect Q401 to go bad - its not that much exposed. The gate still reads negative - so it could be still OK. There is quite some scattering it threshold voltage and that is what you see at the gate here.
I would suspect the protection circuit, e.g. Q402-Q407 and RV402-404.

Thanks for the help! Yes, you are right, Q401 is good. I removed it and tested it, seems OK. I didn't think it was bad but it was easy enough to check it.
I bypassed the protection circuitry like it said in the manual, and the readings were still no good. For example, with the protection circuitry bypassed, I get around 12.55k reading on a 10k resistor. Without it bypassed, I get 5.4k on 10k resistor (these are in 20k range 2-wire).

I think some of the analog switches are no good. I know at least one is, I've only checked U402 so far, and found that switch C (200k and 2000k) is always closed. I still have to check U403.