Re: DM501A vs DM502A


It is possible that what happened is that you calibrated out the zero
error caused by leakage at the high impedance input to the LD120
(U1601) using the zero adjustment. R1613 (560k) is in series with the
input and allows leakage to cause an offset error even when using the
200mV or 2V ranges. Then the change in Thevenin equivalent resistance
when the ranges are changed would show up as offset.

You can test this by adding some series resistance while using the
200mV or 2V ranges and seeing if the measurement changes. The leakage
current is then the change in voltage divided by the series resistance
and should be significantly below 1 nanoamp. Leakage should be in the
10s of picoamps.

The leakage does not necessary have to come from the LD120 (U1601) but
it is the most likely source. Since they are difficult to replace, I
would rule out other sources first.

My DM501A displays similar problems but I have not gotten into it yet
to figure out what is going on. I think I am going to go the whole
refurbishment route including cleaning the boards and changing the
aluminum electrolytic capacitors before

On Thu, 3 Nov 2016 19:58:33 -0700, you wrote:

@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.

Sent from my iPad

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