Re: DM501A vs DM502A

Kevin Oconnor
 

@Szabolcs /others

Fixing a DM501A:
So cleaning the switch matrix did the 1st order trick. I can calibrate the 200mv & 2v ranges. But when I switch to the higher ranges, there is a clear offset that cannot be removed. (3-7v on 20v range with 1v input). I lifted the top of the Caddock divider and can measure ~9.9Mohm. Top to common. All the AC caps seem ok. When I measure the the pot voltages on the attenuator PCB it is exactly the voltage on the display, (appropriately scaled) but wrong. It seems that a small current is being injected into the high resistance divider which raises the divider output voltage. This wouldn't effect the 200mv/2v range since the impedance is low on those ranges.

Thoughts?
Kjo
Sent from my iPad

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