Keithley 2420 repair

[martinr33]

Just finished fixing up a nice Keithley 2420 Sourcemeter.

Symptoms
On this machine, the output was always on at about 23V, and a 0.5 ohm resistor on the main board was cooked.
Replacing the resistor changed nothing.
A 4-wire short with the output OFF caused the resistor to burn up.

In circuit testing of transistors is not reliable on this machine.

I removed the main bipolar output pair TIP142 / Q158 and TIP147 / Q521. Both tested as shorted.
I also removed Q514 and Q516 (IRF240) because they also looked odd on on my meter.

Gallery   https://mrtn.smugmug.com/Other/Keithley-2420/n-VzNstb


Once I had done this, the meter started "working", delivering the correct voltages from just the driver amp, which is an AD847/U500

Replacing the TIPs with the positive FETs mostly unpopulated also worked.
I replaced the missing FETs with IRF540s as a temporary measure.

The meter now works, and I am waiting for the correct substitute parts. IRF240 (p-channel is IRF9140)

Pictures as I get around to it! I just wanted to get this in.

Bottom line is, if he meter has a permanent output voltage - pull Q514 and Q516, to make sure that the analog driver is delivering the correct voltage. Then, you can work on fixing up the drive transistors.

[HighVoltage]

Nice of you to confirm this.
It is also my experience.
Usually if something goes wrong with these Keithley 24xx source meters it is the output stage and it is easily repaired.
Pictures would be nice.

 

[goaty]

I´m currently repairing a 2420 with several components blown up.
From the service manual, I can get the component types, but they do not fully match the board I have.
Does anyone know what Q415, Q416 are ? They list as MTP23P06, but have different Case.

Thomas