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Old 22nd Feb 2021, 1:53 pm   #1
aviv_zl
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Default Heathkit TC-2 resistors advice

Hello everyone,

Recently I got Heathkit TC-2 tube checker which was stored for many years. the tester is in a resonable shape, but needs care due to time.

All the small (1W?) resistors are off by more than 10%
The 2.5K 5W resistor is measured 2.46K (OK I guess)
and the 75K "precision" (according to manual) resistor is measured 75.1K.

I plan to replace the small resistors with 1% 1W metal film resistors.
Regarding the 75K- what "precision resistor" would mean back then? 1% 0.1%?
finding 75K 1W axial resistor of less than 1% tolerance is not an easy task. Do you think the measured 75.1K resistance is fine for its purpose?

Another queation- the analog panel meter front plastic is quite yellowish. Can it be decolorized by the Preoxide UV method?

Regards,
Aviv.

Last edited by aviv_zl; 22nd Feb 2021 at 1:59 pm.
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Old 22nd Feb 2021, 3:40 pm   #2
ColinTheAmpMan1
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Default Re: Heathkit TC-2 resistors advice

Quote:
Originally Posted by aviv_zl View Post
Hello everyone,

Recently I got Heathkit TC-2 tube checker which was stored for many years. the tester is in a resonable shape, but needs care due to time.

All the small (1W?) resistors are off by more than 10%
The 2.5K 5W resistor is measured 2.46K (OK I guess)
and the 75K "precision" (according to manual) resistor is measured 75.1K.

I plan to replace the small resistors with 1% 1W metal film resistors.
Regarding the 75K- what "precision resistor" would mean back then? 1% 0.1%?
finding 75K 1W axial resistor of less than 1% tolerance is not an easy task. Do you think the measured 75.1K resistance is fine for its purpose?

Another queation- the analog panel meter front plastic is quite yellowish. Can it be decolorized by the Preoxide UV method?

Regards,
Aviv.
I would have thought that The 75k would be good enough at 75K1 (how accurate is your meter anyway?), as I don't think that 1960s resistors would have been more precise than that (but I could be wrong; it depends on where in the circuit it is).
Regarding the meter plastic front - I have cleaned them with the use of a few applications of metal-polish or similar mild abrasive. The one thing to be aware of is that you can get a build-up of static electricity with this technique, so you will need to disperse that static or the meter needle will be affected.

Colin.
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Old 24th Feb 2021, 11:25 am   #3
Herald1360
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Default Re: Heathkit TC-2 resistors advice

75.1k is only 0.13% high.

You could get one of these:

https://www.ebay.co.uk/itm/75K-OHM-0...item5d9a01b15a

(Holco H2 75k 0.1%)

if you're worried. It would be interesting to see what your DMM makes of it, at least

It's very unlikely that Heathkit would have gone for any better than 0.1%, 1% more likely.
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Old 26th Feb 2021, 7:12 am   #4
aviv_zl
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Default Re: Heathkit TC-2 resistors advice

Thanks for the valuable inputs.
Another question- many tube tester use 0.1uF capacitor for the short/leakage test. I have them both in my Taylor 45D and in the Heathkit TC-2.
If I got it correctly, the capacitor is used as a voltage devider in these tests. If this is the case, I'm not sure why they used capacitor and not a simple resistor. Could you educate me on this point?

Cheers,
Aviv.
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Old 26th Feb 2021, 1:58 pm   #5
factory
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Default Re: Heathkit TC-2 resistors advice

Quote:
Originally Posted by Herald1360 View Post
75.1k is only 0.13% high.

snip...

It's very unlikely that Heathkit would have gone for any better than 0.1%, 1% more likely.
Had a quick look at pictures of my Heathkit curve tracer and the visible precision resistors used are indeed 1%. Others are ordinary 5% or 10% carbon types.

David
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Old 1st Mar 2021, 1:18 pm   #6
aviv_zl
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Default Re: Heathkit TC-2 resistors advice

Thanks David for checking.
Any comments on my question regarding the 0.1uF cap?

Aviv.
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Old 1st Mar 2021, 2:36 pm   #7
factory
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Default Re: Heathkit TC-2 resistors advice

Not sure why the capacitor is series with the neon short indicator for the Heathkit as it has resistors too, the manual states the neon stays lit for short. Whereas something like a capacitor bridge I repaired uses a capacitor (in parallel with neon?) to allow the neon to flash at different speeds to allow you to judge how bad the leakage is.

My older Taylor 45A doesn't have a neon for the leakage test, it uses the meter instead (as does the 45D you mentioned), the valve under test being used as a rectifier with the capacitor across the heater/cathode having the DC across it and the meter reading the leakage from this.

David
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Old 6th Mar 2021, 10:06 pm   #8
FrankB
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Default Re: Heathkit TC-2 resistors advice

Colin,
Back then "Precision" was sort of a nebulous term.
Often, a shipment of resistors was individually checked and the ones that met the 'Precision Percentage" criteria were put into kits as "Precision" types. Sort of like "resistor matching'. (I have some "matched" Tek resistors in a package here somewhere that were "matched" like that.)
So I would say buy a lot of 1W 75K resistors and find the one closest to the value you need. 1% were available back then, typically a special order. I have done this myself when I needed a specific resistance- or get some resistance wire and wind one yourself.
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