Adjust line control Modification to Heathkit tube checker TC-2

Thread Starter

larisme

Joined Aug 26, 2019
3
I have a heathkit TC-2 tube checker. It appears that someone had added a wire-wound power resister that I measure to be 75 ohms in series in the "Adjust line control" circuit of the primary side of the transformer. This resister does not appear in any of the schematics that I can find for the TC-2 (or even the TC-1 or TC-3 for that matter). I have another TC-2 tester and this resister is not in that circuit. In my limited understanding, all that I understand is that it is going to drop some of the voltage across this resistor and thus reduce the voltage across the "Adjust line control" rheostat. Thus it is going to vary the the way the rheostat would function. This rheostat is rated at 200 ohm. Can someone please give me some insight as to why one might add such a resistor? And any recommendations about it. Should I remove it or? Thanks
 

Wolframore

Joined Jan 21, 2019
1,748
I add resistors to a potentiometer when I want to keep the adjustable range in a certain area. So adding it changes the values from 0-200 to 75-275. I don’t know how those work but maybe it was needed to get the correct voltage for that unit. I just looked at the schematics and that pot looks like the adjustment for the meter.
 
Last edited:

crutschow

Joined Mar 14, 2008
24,959
Perhaps the line voltage was high were it was previously used, and the extra resistor was needed to reduce the voltage to the desired value.
 

Thread Starter

larisme

Joined Aug 26, 2019
3
I add resistors to a potentiometer when I want to keep the adjustable range in a certain area. So adding it changes the values from 0-200 to 75-275. I don’t know how those work but maybe it was needed to get the correct voltage for that unit. I just looked at the schematics and that pot looks like the adjustment for the meter.
Thanks Wolframore! Yes it is to zero the meter to the line voltage, but I thought the 200K pot was for that. Maybe it is out of tolerance. I will check that. I have the capacitor and out of tolerance resistors now to change those out. That added resistor looks as if it might have gotten a little hot. Not sure of its rating; it is wire wound with no markings.
 

Wolframore

Joined Jan 21, 2019
1,748
There’s always a trade off between being too large of value hard to fine tune to being too small and not getting you to the correct value.
 
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