[Test-Equipment] "possessed'" VOM(s)

[David]

I have seen a similar but not the same problem when repairing a Tektronix DM502.

The DM502 uses the same terminal connection for volt/ohms and amps so it is easy
to accidentally short the output when changing ranges.  The switch contacts for
the 2A range where welded together which shunted large back to back protection
diodes across the output causing odd behavior in ohms mode.

In your case it sounds like the X1 switched output from the ohms converter
current source is almost completely open so the current is low on that range
making any resistor look like a short.

On Sat, 8 Mar 2014 18:00:11 -0500 (EST), you wrote:

>OK, I'm almost embarrassed to post this, but it's driving me crazy....
>I have a LOT of VTVMs, FET VOMs, VOMs, etc. At one of my test benches, I use an el-cheapo Chinese VOM that gets a lot of use. Although for serious work, I have a dedicated VTVM there, the VOM gets a lot of use.
>Recently, I noticed that the X1 ohms function didn't work correctly. (all other functions/ranges are OK). When selecting X1, no matter what resistor is being measured, it indicates ZERO Ohms. Of course, when not connected, the meter reads infinity.
>ie: putting a 1K resistor across the leads, it reads 0 Ohms, but when switching to X10, X100, X1K, the readings are OK.
>So, I pull it apart and other than some sm resistors and a couple of diodes for AC/Audio, there's nothing in there. I have looked at the contacts, the board, etc, and can see nothing wrong.
>
>The thing that bothers me, is that this problem is exactly the opposite of what would seem logical.......If a resistor in the circuit were open, there wouldn't be ANY movement on X1. If a resistor were shorted, wouldn't there be an indication as soon as X1 was selected?
>
>Tomorrow, I'll try to check the sm resistors but they have some strange markings.
>
>What was REALLY crazy was this... I pulled out one of my Triplett 310 VOMs and it did the SAME thing. OK, the 310s are beat up and have been around the world with me, but at this point, it was time to quit.
>
>Is there ANYTHING in the circuit (other than the fuse) on X1? Sure doesn't look like it.
>All my other meters check the resistor OK.
>
>ron
>N4UE