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 Post subject: Heathkit TC-2 Tube Checker Restoration Parts Substitutions
PostPosted: Mar Sun 28, 2021 6:50 am 
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My TC-2 has not been used in 40+ years so I am planning to replace all out-of-spec resistors as well as the .1uF capacitor before using it again.

Finding replacement carbon resistors was very easy. I will likely order from Mouser.

Finding a suitable replacement for Heathkit part# 2-21A "75K ohm precision resistor" is a bit of a challenge since Heathkit documentation, as I understand it, anyway, does not provide data for power rating or tolerance. Even though it's a resistor, the original part looks more like an old, black, plastic covered, tubular capacitor.

Would Vishay part# CMF6075K000FKR6 "Metal Film Resistors - Through Hole 1W 75Kohms 1%" be a suitable replacement?

Any better suggestions are appreciated.

As for replacing Heathkit part #23-11, ".1uF tubular capacitor" (which is clearly labeled 600VDC) I chose Cornell-Dubliner part# 940C6P1K-F, "0.1uF 600V 10% Film Capacitor". Does that sound ok? Thanks.


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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Mar Sun 28, 2021 2:45 pm 
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The TC-2 is a simple emission tester. Any resistor would have to be WAY out of tolerance to make any significant difference. Nothing you can do to this tester is going to turn it into a precision instrument. :)

That said, any modern carbon film resistor should be fine. Even a metal film resistor would be fine. Any NOS carbon resistor should also be fine. The tester is just testing emission, not operating characteristics, so the usual cautions about using resistors that may have a frequency component to them is meaningless here.

For your precision 75K resistor, why not just use a 2 watt for peace of mind? I did not look, but if Heath didn't specify the wattage, you're just guessing, unless you actually measure the voltage drop across it and apply Ohm's Law to figure out the current. Yes, 1% would be more than adequate for this. Wirewound or film type should work equally well.

For the capacitor, virtually any mylar or film capacitor of 600V minimum reading is fine. Use one of the generic yellow ones rated at 630V and supplied by both our parts vendors over there >>>>

Don't stress over precision. It won't help this tester... it's a decent go-no go tester, as are all the emission testers ;-)

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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Mar Sun 28, 2021 4:41 pm 
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Did you check the 7.5K precision resistor? It is probably fine. The schematic shows it as a 1 watt.
The resistors do effect the meter readings as well as the point that the short lamp lights.

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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Mar Sun 28, 2021 4:48 pm 
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Martino wrote:
Finding a suitable replacement for Heathkit part# 2-21A "75K ohm precision resistor" is a bit of a challenge since Heathkit documentation, as I understand it, anyway, does not provide data for power rating or tolerance. Even though it's a resistor, the original part looks more like an old, black, plastic covered, tubular capacitor.

Would Vishay part# CMF6075K000FKR6 "Metal Film Resistors - Through Hole 1W 75Kohms 1%" be a suitable replacement?

Yes.

The schematic shows it to be a 1 Watt resistor. I expect the tolerance was 1%, but as you say, it was not specified. "Precision" is usually taken to mean 1%, but I suppose it could have been a 2% tolerance resistor. In any case, a 1% tolerance resistor will be fine.

The original 75K precision resistor is probably an old-style carbon film, but it might be a wire-wound resistor. If it is a wire-wound resistor, then it is most likely either open, or still good. You should check its resistance. If it is within a couple of percent of 75K, then it is probably OK. It's unlikely that you have a meter that is precise enough to tell if it is still within a 1% tolerance.

Martino wrote:
As for replacing Heathkit part #23-11, ".1uF tubular capacitor" (which is clearly labeled 600VDC) I chose Cornell-Dubliner part# 940C6P1K-F, "0.1uF 600V 10% Film Capacitor". Does that sound ok? Thanks.

Any modern film capacitor rated 600 volts or higher (630 volts is common) will be fine. The CDE 940C6P1K-F is a pulse rated polypropylene film capacitor, and is way overkill for the replacement, but it will work fine. Mouser gets $2.58 a piece for them. The CDE 104MWR630K at $0.95 or CDE 104MPW630K at $1.12 would be a more economical choice. I'd buy ten of the CDE 104MWR630K at the quantity ten price for $6.35 and put the excess in stock for future restorations.

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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Mar Sun 28, 2021 6:53 pm 
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Thank you, all, for your very helpful comments and suggestions. I am continually amazed by the vast depth of knowledge on this site.

As for that "75K ohm precision resistor", it does test bad and has to be replaced.

One more question comes to mind: how might I reduce the strong, internal musty/moldy smell developed over the last 45 years? I took the clean-looking internals out of the case & kept everything open & exposed to sunlight for a day, but that made little, if any, difference.


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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Mar Sun 28, 2021 7:07 pm 
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The 75K precision resistor is a 1 watt 1% wirewound resistor.

DM


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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Mar Sun 28, 2021 10:47 pm 
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Quote:
One more question comes to mind: how might I reduce the strong, internal musty/moldy smell developed over the last 45 years?

One way would be to take it out of the case and leave it set out in the open air for several days, even better in the hot sun. The idea would be to make it absolutely dry inside.

I'd wash it with soap and water, but that's tricky ... you can't get the transformer wet, and you can't get the meter itself wet. And you don't want to get water inside any sealed or semi sealed components, but if you've got some experience at this it works well. Just mild dish soap, a soft brush, a rinse and a lot of hours in the hot sun. Air blast to blow off the excess water first helps.

otherwise you could just try setting a box of arm n hammer baking soda inside it, and leave it out in the sun closed up. Don't use any chemical de-molding or sanitizing agents unless you can guarantee they can be washed out immediately as above.

And then you have to re-lube everything that moves, including shafts etc.

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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Mar Sun 28, 2021 11:04 pm 
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Location: Greater Boston, MA
FWIW, the 75kohm precision resistor in the TC-1 I bought on the classifieds was actually *shorted.* I replaced it with two 150kohm resistors I had on hand connected in parallel. I don't recall if I used 1% or 5% units.


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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Sep Wed 15, 2021 9:01 pm 
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Joined: Sep Fri 05, 2014 11:10 pm
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One more question about restoring this Heathkit TC-2 if I may:

Also required is a replacement NE-51 neon bulb which is a short indicator in the tube checker. Can I use an NE-2 (soldered into a lamp holder of course) instead of the NE-51? I ask because I can order some NE-2 bulbs from Sal's Capacitors at the same time I order the other needed parts.


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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Sep Wed 15, 2021 9:15 pm 
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I believe the NE-2 and NE-51 are supposed to be electrically identical.


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 Post subject: Re: Heathkit TC-2 Tube Checker Restoration Parts Substitutio
PostPosted: Sep Wed 15, 2021 10:58 pm 
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Joined: Sep Fri 05, 2014 11:10 pm
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Thanks!


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