Found a Micronta VTVM
 

Picked this little gem up for $5.00 at a Thrift Store.
It had something banging around inside that I thought might be a battery and it was, a "C" Cell Duracell that expired in 2000, it's not leaking though, not even dead for that matter, my meter shows 1.8 volts coming out of it!
That doesn't sound quite right but it makes a speaker pop so it has something in it.

The VTVM doesn't seem to work, unless the battery needs to be in place to do so? It'll Zero but I get nothing measuring DC volts.
Maybe it needs recapped.

It uses Telefunken tubes, one of which is a 12AU7, have you priced Telefunken 12AU7's on eBay lately? :D

No the battery is only for the resistance ranges. Check the tubes, if they are properly plugged in and light up. There is a power supply of course so maybe something has gone wrong there.

As for recapping, probably the only part would be the filter capacitor. Measure the B+ and see if it's around 150V.

The 12AU7 is the meter driver so it is okay. What are you using for a probe? The DC probe almost certainly would have a resistor inside; maybe yours is broken.

I didn't get a probe with it. I have one from another Sencore meter, it's a Solid State unit and has a BNC type connector where this one has one of the old funky threaded ones.
Is the probe specific to the model of meter or are they interchangeable?

It is specific but there are only a few types. The differences are, generally, the connector and the value of the internal resistor.

Best to find a manual the unit or, failing that, open it and draw out a diagram of the input circuit. That circuit will come from the probe connector to the range switch. You can write down the values of the various resistors, compare to the voltage range, and reverse engineer the value of the probe resistor.

Probe
 

Try a RG-58 or similar shielded cable with a 1 meg resistor in series with the center wire. You want the resistor out at the business end of the test lead. The resistors purpose is to isolate the cables stray capacitance from affecting the circuit being measured. BTW a BNC panel connector will drop right in the same mounting hole as the funky mike jack.

Just to confuse matters, a few VTVM probes also included a switch to short out the resistor for AC and Ohms measurements. That's a very nice meter you found for five bucks.

Just to confuse matters, many VTVMs used other value resistors. For instance, 22 Megohm and 100 Megohm values have been used.

Lacking a manual, you need to trace the circuit and see what resistors are in the range switch. You can apply a known voltage and add resistance until the ranges switch as they should. In other words, the resistor is part of the voltage divider and needs to be the correct value in order for the ranges to work properly, one to the next.

The first thing is to see if the meter responds to a voltage at the DC input. If so, you are in business; if not, some troubleshooting is in order.

That's very interesting. I can't imagine what the purpose would be of putting that high of value isolation resistor out in the prob, unless it's a specific very high voltage probe? :scratch2: 1 meg does seem to be the most common.

The reason is simple. It's money. The higher the resistance, the higher the input impedance to be able to list in the specifications. There is no specific limit on it so they push it.

I thought the 1M resistor was part of a voltage divider with a 10M resistance inside the meter.

That is true. But the values used vary with the model. For instance, some units have 100 Megohms inside and 22 Megohms in the probe for an input resistance of 122 Megohms.

The value of the probe resistor depends on the instrument input resistance on DC volts scale. A 10 meg input uses a one meg resistor. A 15 meg uses a 7 meg resistor and so on. Without the schematic, measure the DC resistance of the device while switched to DC volts.

Wouldn't that be a 1.7 meg- 1/10 the internal resistance? :scratch2: If not then I guess I don't see the common ratio.

There is no rule. All depends on the design of the input attenuator and the specific ranges desired as well as the amplifier sensitivity.

Basically, one must discover the correct value by whatever means, either research or test or reverse engineering.

Kevin:

I do not imply that there is a constant ratio of the probe resistor and the divider. Since the divider must also work on AC, that must be considered. The probe resistor both isolates the rest of the probe assembly and completes the divider for DC.

Actually 15 meg dividers use 7 meg probe resistors for a 22 meg input resistance. That doubles the DC input resistance of 11 meg meters.

For a start, either measure the input resistance or add up all the values in the divider network.