[daveone23]

I think I found a schematic that may work for my 1939 666. The resistors are stamped and match the schematic but the meter operates differently. Its looks like on the 10v scale the 9900 ohm resistor is in place but to get the meter to operate properly, it needs a 2.3k resistor, not the 9900. I really dont want to re-resistor the whole meter. What could be the issue

[morzh]

Why 2.3K?

[daveone23]

Thats the mystery question. I used a variable pot to calibrate it for a correct reading on the 10v scale and thats what I came up with

[morzh]

Do you have the same exact galvanometer head? 400uA full deflection current? If it is the same, the head is out of order, if it is not, you need to know the parameters.

[daveone23]

It appears to be the original meter but it does not say on the face plate. How can I apply 400 micro volts to see the full deflection?

[morzh]

400 micro-Amps, not micro-Volts.

Full deflection current will move the needle to the far right when put through the spindle coil, to which tthe needle is attached.
Then it is simple Ohms law: your full resistance (including the one of the meter itself, though it is rather small) will go into I=V/R formula, where the V is your voltage.
You could simply measure the resistance between the two contacts of your resistor (with the resistor itself disconnected) to see if it is 250 Ohms, or it is more than that. If it is more than that, you need to trace all the resistors that are in series with that resistor in question, and then your resistor will be

R-needed = (10V/0.0004A) - R-measured, or rather R-needed = 25kOhm - R measured.

If your R-measured is more than 250 Ohm, you need to find out what it is and why it is a part of your circuit, which (your colored part) shows straight wires.

[daveone23]

I am lost. Where did you come up with 250 ohms?

[morzh]

This is your schematic. It says under your galvanometer: 400uA, 250 Ohm.
It means that

1) 400uA will fully deflect your spindle.
2) 400uA will develop 0.1V dropout across that spindle coil.
3) this resistance is in series with any other resistance that is a part of your switched divider. Simply speaking, if there is nothing else in parallel or in series with this galvanometer, you will need 25kOhm resistor in series to fully deflect the needle at 10V.

[daveone23]

ok, thanks, I will check it out.

[daveone23]

Looks like 1.8kohms @ 10v is full scale

[morzh]

Have you measured the resistance of the circuit where that 9900 ohm resistor connects to (or where you connect your 1800 ohms one)? With your resistor disconnected?

And, the last thing: is there a shunt of some sort connected across the leads of the galvanometer?

[daveone23]

I think the metering wiring is messed up. I disconnected the wires to the movement. No visable shunt. Measured from the probes to the meter connections. Same readings on both. Neg= continuity. Pos=12k. I dont understand it.

[morzh]

Well, you have to trace it.
But that would explain why your resistor is much lower than what it should be according to the sch.

Does the soldering and all look factory-made, or are there signs of tampering?

[morzh]

https://bama.edebris.com/manuals/triplett/666h

https://elektrotanya.com/triplett_model-...ad.html#dl
(pass Captcha, wait a few sec and download)

These sch are a bit different

[daveone23]

I dont have the R model, its much newer. Mine is the first one made in 1939, 666 but the H model looks similar