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 Post subject: heathkit vtvm model im-32
PostPosted: Sep Tue 28, 2021 7:36 pm 
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I just bought the Heathkit IM-32 vtvm in good condition.
But I wonder why they put 2 capacitor of 0.05uf and on the parts list of IM-11 or IM-28 the value is 0.005uf.
(I couldn't find the shematic diagram of IM-32, but I believe they are the same)

And I would like to know if you would change them. And there is also the capacitor of 0.047uf 1600v but it seems to be in very good condition I can maybe keep it?

(I did DC and AC tests, check resistance and everything seems to be fine

Thank you

Serge :)


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Tue 28, 2021 8:18 pm 
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https://www.nostalgickitscentral.com/he ... a_im32.pdf


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Tue 28, 2021 8:53 pm 
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serge64 wrote:
But I wonder why they put 2 capacitor of 0.05uf and on the parts list of IM-11 or IM-28 the value is 0.005uf.

You are looking at the wrong capacitors in the IM-11 and IM-28. The 0.05 µF capacitors in your photo are in the AC measurement circuitry, around the 6AL5 tube.

In the IM-11 and IM-18, they are 0.02 µF.

In the IM-13 and IM-28, they are 0.05 µF.

The 0.005 µF capacitors are on the grids of the 12AU7. They are the same value in all of the IM-11, IM-13, IM-18, IM-28, and IM-32

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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Tue 28, 2021 9:05 pm 
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serge64 wrote:
And there is also the capacitor of 0.047uf 1600v but it seems to be in very good condition I can maybe keep it?

(I did DC and AC tests, check resistance and everything seems to be fine)

Did you perform a DC leakage test at 1600 volts?

What is your "AC test"?

You checked its resistance, but at what voltage? What voltage does your VOM, VTVM, or DMM apply? 1.5 Volts? 15 Volts?

The only way to be sure it is OK is to test it at full rated voltage.

That John E. Fast capacitor may be a paper-in-oil capacitor, you might be able to sell it as a "tone cap" to a guitarist for enough cash to buy a handful of replacements.

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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Tue 28, 2021 10:30 pm 
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serge64 wrote:
(I did DC and AC tests, check resistance and everything seems to be fine)

Do you mean you tested the capacitor, or that you tested the DC, AC, and resistance functions of the VTVM?

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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Tue 28, 2021 11:10 pm 
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Do not replace the 0.005 ceramic disc capacitors on the grids of the 12AU7. Though they are ordinary Z5U parts, they were carefully selected at the factory for minimum leakage. This is necessary to prevent drifting of the pointer. They are almost certainly still good, and if you were to replace them with other brands or types of capacitors you could end up chasing the meter pointer up and down the scale with the zero knob as the circuit drifts.

As for the other capacitors, the IM-32 was produced in 1962 and 1963. By then Heathkit had largely shifted away from paper capacitors to polyester (Mylar) film. So except for the 20-uF power supply electrolytic, which should be replaced if it hasn't been already, the caps that are in there might still be okay. The only way to know for sure would be to measure their leakage resistance. If either of the two 0.05-uF caps are leaky, the 6AL5 half voltage doubler circuit will not work properly and the AC voltage readings will be off.

As for the 0.047-uF, 1600 volt cap, its role is to block DC voltages from the AC measurements. You can test this by setting the meter to read AC volts, then connecting the AC probe to a high voltage DC source (with the ground clip connected). Set the voltage scale to a high voltage then work your way down. The needle may "kick" as the capacitor charges, but it should settle back to zero eventually. If you continue to get a reading then either the capacitor is leaky or the voltage source you are measuring has some AC in it.

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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Wed 29, 2021 12:52 am 
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For the capacitor 0.047uf.
I put 85v dc in AC max position. and I went down the AC voltage scale.
The needle does not move at all, except that if I go from 5v to 1.5v it moves a little

(The only capacitor I changed is the 20uf and the model 10 diode. I changed it to a 1n4001 diode.)


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Wed 29, 2021 1:19 am 
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I didn't have a dc probe so I took a Boonton probe that was not working and emptied it.
I put 2 resistors of 2M ohm 2 watts in parallel in a box connect to the jack

This is what i need to measure dc ?


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Wed 29, 2021 1:21 am 
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If you disconnect the 85 volt supply and switch from the 5VAC to 1.5 VAC scale, does the zero change? On some meters it changes a little so you have to make sure zero is still zero.

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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Wed 29, 2021 1:34 am 
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For your viewing pleasure.
Manuel Caldeira. Madeira, Portugal.

How to Reform Electrolytic Filter Capacitors on tube radio or amp.
Image
Watch 22:50 https://www.youtube.com/watch?v=ifWfvaIbWZY


Capacitor leakage tester design & build - Part 1
Image
Watch 45:19 https://www.youtube.com/watch?v=0pySErvzUIY

Capacitor leakage tester design & build - Part 2
Image
Watch 18:20 https://www.youtube.com/watch?v=cW9hHjOxmQw

Capacitor leakage tester design & build - Part 3
Image
Watch 31:20 https://www.youtube.com/watch?v=NBUiaw18eOU

Capacitor leakage tester design & build - Part 4
Image
Watch 16:01 https://www.youtube.com/watch?v=X4B79BYIlbY


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Wed 29, 2021 10:31 pm 
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You might want to read my comments on this thread in regard to the large thumbwheel knobs on that meter.

viewtopic.php?f=8&t=385450

Meter does work well; I like the large size meter movement like the one used in the IM-13 meter. The weak point is those knobs; I doubt they expected they would still be in use over 50 years later.


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Wed 29, 2021 10:46 pm 
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serge64 wrote:
This is what i need to measure dc ?

It would be better to put the resistor in the probe. Its purpose is to isolate the capacitance of the shielded cable from the tip of the probe.

As you have it, it does not isolate the cable capacitance. You might as well have installed it inside the meter.

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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Thu 30, 2021 2:05 am 
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I am trying to sort out the parameters for evaluating capacitor leakage. As I see it, it takes two forms.

One is coupling capacitors where leakage causes bias shifts in tube amplifiers. This needs calculation to decide how much leakage is allowable.

The other is power supply filtering, where leakage can raise capacitor temperature due to power dissipation, which could cause thermal runaway.

There are other situations as well, such as timing networks/RC oscillators.

Has anyone here worked out any permissible leakage currents? Or perhaps knows a reference that doesn't skim over the subject by suggesting rules of thumb. The definitive answer probably lies in the capacitor data sheet, assuming the circuit designer took the trouble to see what leakage his capacitor choice specified.


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Thu 30, 2021 3:08 am 
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Someone posted some tables for (allowable) electrolytic capacitor leakage on an older ARF thread; not sure where they were sourced. They were intended I think more to decide if the capacitor should be tossed or not.


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 Post subject: Re: heathkit vtvm model im-32
PostPosted: Sep Thu 30, 2021 3:30 am 
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Location: Montreal (Québec) Canada
stevebyan wrote:
serge64 wrote:
This is what i need to measure dc ?

It would be better to put the resistor in the probe. Its purpose is to isolate the capacitance of the shielded cable from the tip of the probe.

As you have it, it does not isolate the cable capacitance. You might as well have installed it inside the meter.


I thought about doing this but I had only 2X 2M ohm 2 watt resistor in hand and it did not fit into the probe.

I'm going to buy a 2 watt 1M ohm resistor and put it inside the probe. Thank you.

And thank you all for helping me.

Serge :)


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