Keithley 181 Nanovoltmeter OFLO and Fault code(?) on power up.

[Paceguy]

I purchased a Keithley 181 Nanovoltmeter at a Ham Fest years ago. It was known to be defective, and if I remember correctly I paid $15 for it. On power up it displays F60 67 and then OFLO. The talk only led is lit all of the time. I believe that the F60 67 are error codes but I don't have the service manual. All the LED's respond correctly when the switches are activated, except for the Zero switch that just has the LED flash for a split second. Has anyone experienced this problem and does anyone know what it could be? Does anyone know where I can obtain a copy of the service manual for this model?

[picburner]

Some documentation can be found here.

[The Soulman]

Sounds like normal behavior, have you tried shorting the input?

[Paceguy]

Thanks picburner for the pdf manuals!

[doktor pyta]

Mine K181 displays F50 69 immediately after turning on.
So it is rather some firmware revision.
Check the power supply section first.

[Paceguy]

Yes Soulman, I have tried shorting the inputs, but no change I managed to get the zeroing ind. to stay lit a couple of times with multiple presses. I also noticed that once the unit has warmed up after a couple of hours that the OFLO changes to flash random numbers and then returns to read OFLO.

[Paceguy]

I'll open it up this evening and measure the pwr supply voltages and check the filter caps for esr.. I'll get back to you later on for an update.

[Paceguy]

OK, I opened the K181, gave it a good visual inspection, Visually, all looked good, nothing burnt or broken, soldering looked ok and showed no signs of rework, although a previous owner has been inside, either to troubleshoot or calibrate ( front panel has some plastic pieces broken off and a few screws appear to be non original )

Next I measured the supply voltages: The +15 volt supply measured +14.85 and the -15 volt supply measured -15.30. I then used my 88A esr meter and checked all the electrolytic caps. I also checked the titanium caps on the low voltage input board for shorts / opens and all is well there too. I used my IR thermometer to check if anything is overheating, nothing over 90 deg. F.

I then hooked up a 15 volt variable power supply to see if the K181 would measure anything or give any reading other than displying OFLO using only the V input binding post front panel input. I applied various voltages and only had OFLO displayed on all 4 ,2V, 20V, 200V, and 1000V ranges.

Now for something interesting: While applying voltage while on the 2 volt range, I noticed that if I exceeded 2 volts, the display started going back and forth between OFLO and 000000. It would not display the input voltage being applied , it displayed 00000 ! and OFLO . As I mentioned, the LED on the Zeroing switch will not stay on, but sometimes does after 20 or more attempts.

I am thinking that the problem is related to an input board related problem. A previous owner most likely blew something on that board. I will doing some further checks to see if I can find a problem there. Another observation is that input board 2 screws that hold it down are slotted non originals which I believe should be philips type screws. Looks like a previous owner also removed this board at some time.

Any ideas , comments and /or suggestions would be greatly appreciated, Thanks to everyone !

[Kosmic]

Check the relays. I fixed 3 Keithley 181 and had to clean the relays on all of them. Was making the V input flaky.

I used a piece of paper with Deoxit D5 to clean them. Open the relay, insert the piece of paper (soaked with Deoxit), close the relay and take the piece of paper out slowly. Repeat as needed.

Is the mv input working ?

[Paceguy]

I'll give cleaning the relay contacts a try. The problem with the -OFLO is on all the scales including the 3 low voltage ones.