I want share my experience fixing a problem on a Fluke 8506A bench DMM. The unit started to develop a fault that caused the full-scale voltage on the 20 V range to drift down over time. When switching to the next range it will be correct. I pinned this down to the DC conditioner board where the DC range switching happens.
According to the service manual, drifting readings could be due to range switching JFET’s going leaky. I replaced all the JFET switches. Problem remained. Started to look at the transistor clamps end basically tested everything I thought could affect it. Even lifted up the one side of the smaller value caps, which measure open circuit on the ohm meter. As another precaution I changed all the tantalum caps as well. No luck. Since the card is a pain to work on without the correct extender, I decided to power it up on the bench with external supplies. I isolated the input range switching relays as well as the other range switching JFET’s to eliminate them. So, the unit was hard strapped for X1 range. And the test voltage fed direct into the unit via 150k input resistor, similar to that in the circuit.
I was really surprised that there was no sagging with the test voltage up at 20 V. I left it on for some time and it was rock steady. Checked a couple of suspect old solder joints and tried it back in the meter. Started sagging in voltage after several seconds! I could not figure out why this was happening. I unplugged the output filter board in case it was causing a load in the DC output. Still nothing.
Then I accidentally found something that led me to finding the culprit. I measured for resistance between various test points while the unit was plugged in, and saw a short between TP1 and TP8. These two points (common ground and analog ground) are only connected with the card plugged in!
Back on the bench, I connected these two points and observed the sagging. Great! I was confident now that there is some capacitor leaking, SO again I started lifting up one side of the silver mica caps with the board power on. The moment I lifted C15 near U3, the voltage came back up. Took C15 out. Checked it once more with ohm meter and it still read open circuit. So, I used my Keithley 237 source meter to apply voltage and measure the current. No leakage current till about 9 V. Then as soon as I increased the voltage, the leakage current will be around 4 uA at 20 V. What is even more surprising is this 47 pF cap is rated at 500 VDC. In the circuit it never sees more than 22 V over it. The lesson is: beware of old silver mica capacitors, it can cause very confusing faults.
People fixing old tube radios often talk about silver-mica disease, but it goes short most of the time.
All the modules in this instrument uses lots of SM caps, so it may not be the end of my headaches. Time will tell.
