Repair Heathkit IM-11 VTVM

[Johnny10]

I am so surprised at the accuracy of this old VTVM I am restoring..  after 20 min warm-up just look how it compares to my Fluke 87!

Changed main caps, new 12AU7 and 6AL5 Tubes.
It is still missing a case part which holds cord and top battery connection which I imagine will have to fabricate.
$10.00 at my local surplus store.

[med6753]

No surprise. VTVM's can be very accurate as long as they are properly calibrated and you select a range which results in a reading as close to mid-scale as possible, as you have done. But since you changed both vacuum tubes you'll need to do a complete recal. Which begs the question. Why did you change the tubes? Unless there were signs of abuse I would have left the originals in place. Chances are they are OK. But if there was abuse you'll also need to check all the precision resistors on the range switch.

Assuming there was no abuse set up and recal is pretty easy. First is the DC balance. Remove the probe. Range switch to 1.5V. Voltage to +DC. Zero the meter. Switch between +DC and -DC and adjust the DC balance for least amount of meter movement when switching back and forth.

DCV and ACV calibration is pretty straight forward. Select a voltage that results in a reading as close to mid scale as possible. You can use your Fluke 87 as a reference standard. Adjust for the correct reading.

And finally, did the VTVM come with the correct probe? It should look similar to this....



It should be switchable between DC and AC/ohms. In DC mode there is a 1M isolating resistor between the probe tip and the VTVM input giving it an input impedance of 10M VTVM plus 1M probe which equals 11M input impedance. In AC/ohms mode there is no isolation and a direct connection between probe tip and VTVM.     

[Johnny10]

Thanks for the reply.
When I first got the unit  the main power cord was missing, after downloading the schematic and determining the correct line power connections I soldered on the new wires.
The 40uF caps were leaking and smelled something terrible.
Replaced them and looked for other loose connections. Re-soldered a few spots.
Turned the unit on and found the needle wouldn't settle.
If I tapped the side of the case the needle would pin itself to one side or the other.
I tested the 12AU7 tube in my B&K 500 and it tested strong.
However it was very microphonic since each time I tapped it after replacing it in the unit the needle would go crazy.
As soon as I switched it for NOS 12AU7 everything settled down.
The 6AL5 also  tested fine but I switched it with a NOS I had. Will probably put the original back in later.
Soldered a 1 meg ohm resister to the end of the shielded cable to do the DC+ calibration. The unit didn't come with probe.
Also I have to make a new battery holder since the U shaped holder is missing.
Unit was in pretty rough shape after sitting in a barn for 20 years. I cleaned unit with alcohol and sprayed switches with Caig Deoxit.
Happy with results so far on DC scale but this week want to work on AC and Ohms circuits.

[Johnny10]

Old Capacitors

[med6753]

Good deal. Sounds like you are on the right path. Getting a microphonic 12AU7 in a VTVM is very unusual. I would expect that in audio circuits where sound vibrations can cause the grids to loosen and the result is microphonics when you tap the tube. It seems apparent that the VTVM was subject to some physical shock. Any damage to the case?

You might want to do a quick check of the precision resistors before final calibration. Also check, if I recall correctly, the 9.1 ohm resistor in the ohm's circuit. If resistance measurements were attempted with power applied that resistor will fry. Other than that not much more can go wrong with these Heath VTVM's even if abused.   

[jalind]

Good deal. Sounds like you are on the right path. Getting a microphonic 12AU7 in a VTVM is very unusual. I would expect that in audio circuits where sound vibrations can cause the grids to loosen and the result is microphonics when you tap the tube. It seems apparent that the VTVM was subject to some physical shock. Any damage to the case?

You might want to do a quick check of the precision resistors before final calibration. Also check, if I recall correctly, the 9.1 ohm resistor in the ohm's circuit. If resistance measurements were attempted with power applied that resistor will fry. Other than that not much more can go wrong with these Heath VTVM's even if abused.   

Kinda new here, and this thread is about five months old, but saw these remarks about the microphonic 12AU7 dual triode. Been there, done that, with microphonic tubes, but it's been decades (don't ask my age please). You're correct that either mechanical sound vibration can loosen them, particularly with high power audio amplifiers. Internal electrical oscillation that changes bias on the grids can also eventually loosen them. The third, and I suspect likely failure mode in this VTVM, however, is thermal expansion and contraction of the tube's internal structure that occurs when the device is turned on and off. That can also loosen the grids over time. Thermal expansion is the quintessential "irresistible force". A VTVM would likely experience quite a few power cycles in routine bench use.

John

[Johnny10]

Welcome to the forum, jalind
Thanks for reminding me about my VTVM.
Have to get  it out again and see if I finished repair or was waiting for parts.
I started so many projects this year !