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DM501 Digital Multimeter flashing display, frozen display - posting completed repair


FAZ
 

My DM501 that came with a TM506 mainframe had a flashing display that would come on as different numbers each time it was powered on, 2014, 0004, 2004, 1638, etc.

The DM501 uses an op amp U170 that integrates the input signal on capacitor C170. It's supposed to then discharge that cap through current source Q180 or sink Q200, but mine wouldn't discharge. Well, I found a way for it to integrate in the opposite direction. If you select Temp the integrator goes negative, and if you select from the resistance ranges the integrator goes negative. You can use this to see if the comparator U175 is functioning. When The integrator crosses zero that comparator should change polarity. Okay, so my integrator and comparator were fine, but no sinky sourcy action. Those are controlled by the flip flop U235A. I confirmed that the zero crossing pulse was getting sent to pin 1 of the flip flop, and thought that the set pulse on pin 4 was okay.

Much head scratching later and checking pin 4 again I found that there was no set pulse. I checked the collector and emitter of Q250, which is supposed to amplify that pulse. The base wasn't getting much signal, so I went to the signal source, pin 9 of U310B. It was perfect, so I looked at C310 and it was fine. I bridged a capacitor across it to give the reset pulse more oomph and followed it back up to the base of Q250. Even with now +2V pulses going to its base it wasn't able to bring its collector down. I removed Q250 and it tested as a diode. It's a 2N3904 from 1974 and was probably tired. With a modern replacement the DM501 was fixed. I removed that extra troubleshooting capacitor and calibrated the zero, positive integrator, and negative integrator.

I hope that tip about using the Temp and Resistance ranges will come in handy to someone else someday. You can get the integrator to cross zero "manually," even if the logic controlling the source and sink won't do it for you.


Stephen
 

I don’t have DM501, but I do have a DC503 that arrived yesterday with a TTM506 from eBay. And I have almost the same issue. The display doesn’t come on, and when it does, after powering the unit on and off several times, it’s most always on a different position and either with a 0 or an 8.
I can’t select Temp though, LOL.