Heath kit IM-2558 VTVM

[W5drs1]

Requesting help.. I’m in the process of restoration of a Heath kit IM-2558 VTVM. I thought everything was good to go, but I noticed that the meter needle is slow to return to zero when I do a DC calibrate or any other calibration for ac and resistance.

There is no static buildup at the meter housing, but again the meter drops normally until it gets around zero and then slows and finally zeros out. I changed the capacitors and the diode and the 2 tubes.

Any suggestions or comments on possible problem and solution would be greatly appreciated.

Thanks,
Arnold
W5drs1

 

[danadak]

Can you post a schematic ?

Regards, Dana.

 

[W5drs1]

Yes, I will as soon as I can. I appreciate the reply.

Thanks,
Arnold

 

[W5drs1]

Schematic for the Heathkit IM-2558 Vtvm.

 

[danadak]

Nothing obvious.

But a long time constant, a connection that has gone high resistance, like a tube to socket pin
connection. Or a R, old carbon comp R's notorious for resistance rising over long periods of time.
A grid connection could do that.

Maybe look at dual triode with another meter to see if grid circuit is doing the same thing.

Also adjustment pots in old gear lose contact of carbon track and wiper. Clean them.


Regards, Dana.

 

[W5drs1]

Dana,

I appreciate your time, yeah the zero adjust and the ohm adjust pots seem a little rough in spots.

I will check all that you mentioned.

Thanks,
Don

 

[KeepItSimpleStupid]

My fisrst suggestions are:
a) Static
b) Movement issues like dust, spring etc.

See if you can cleanly move the movement by itself. if you know the sensitivity and resitance then it's easy.

if you have to find it:

Lets assume around 50 uA or lower.

You want a power supply (battery will so) and a variable resistor in series that will deflect the meter 1/2 way.

Set the pot to Maximum.
Adjust the pot (reducing it's value), so the meter reads 1/2 scale.
Measure the voltage across the variable resistor to get current which will be 1/2 full scale.
The resistance of the removed POT is the internal meter resistance.

You just have to be careful.

Dropping, static and overloads can damage the movement.

 

[W5drs1]

Great idea, I’ve taken a static dryer sheet and tried it just in case there was static buildup. I will try your suggestions.

Thanks,
Don