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Author Topic: Heathkit TC-2 tube tester DIODE  (Read 1156 times)
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Posts: 110

« on: April 29, 2023, 11:27:06 AM »

I just got this tube tester.

There is NO meter movement whatsoever...its 1 Ma unit.
Installed my ammeter in its movement.

The vintage diode was replaced with a IN5408...which checks bad. appears there are various 'comments' on the 'correct' diode replacement...
such as using a 1N191 and replacing R9 from 1200 to 1000 ohms.

Welcome your comments, please.

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Posts: 19

« Reply #1 on: April 29, 2023, 01:25:30 PM »


Is the 1 mA panel meter OK? You can check for continuity with a modern
DMM that puts out 1mA or less during an Ohmmeter test.

An easy check for panel meter accuracy is to isolate it, and connect a 9V
battery in series with a 9K resistor across the meter to see if it reads
near full scale.  I would guess the panel meter's own resistance
is about 200 Ohms which you can account for when making up
the total of 9K Ohms in series with the 9V battery.

The diode is only used in adjusting the 200 Ohm line voltage adjust pot,
not when a tube is actually tested.  The tube to be tested provides
diode action during testing, allowing use of a d.c. panel meter.

Note that the direction of the diode 'arrow' is shown backwards on the schematic.
I think Heathkit copied Triplett, which also had the same arrow direction error
on their model 3413A tube checker.  The original diode used was a copper oxide
rectifier in both instruments.

A 1N4007 diode in series with a resistor (value to be determined) is fine.
First use a variable pot or rheostat in series with the diode and panel meter..
Insert say a 6L6 tube, meter the heater voltage of the tube
with the TC-2's line adjust pot centered.  Vary the temporary pot
in series with the diode so that the panel meter reads at the 'line voltage'
set point when the 6L6 heater is right at 6.3V.

Then meter the value of the temporary series resistor and replace it with
a permanent series resistor near that value.



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Posts: 110

« Reply #2 on: April 29, 2023, 02:28:21 PM » we took some time to get it going.

After 2 days of 'troubleshooting'...found this.
Got no meter movement as the PLATE control had NO wire on one lug!!
It was to go to 0V on the filament primary.
Curiously, the was a broken wire on the far end of the lever section ON the proper tap that the plate goes to?

All those GOOD rig troubleshoot videoS say: USE YOUR EYES!!!

Regarding the 'diode'...

I had removed the previous diode replacement as mentioned.
Installed a 1N34A...meter working now.

I then reinstalled the original Bradley Labs rectifier.
It worked.

With 1N34A...meter good variation and centered...all filament voltages were 12% to 17% high.
Bradley Rect...meter adj to full CCW..could only get meter just to the right of center...all filament voltages SPOT ON.

Good enough for Mua....only use these things to find tube dud's.


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Posts: 110

« Reply #3 on: April 29, 2023, 04:12:29 PM »

Unless I'm 'off center'...the matter of diode choice is a moot point.

Since we're talking about FILAMENT voltages...having the meter CENTERED is only a VISUAL indication.

Fact can get the EXACT voltage by measuring the pins on an empty socket and use THAT to know
what the voltage is ON the tube being tested since ALL the filaments are connected.

Adjustment is then done using the "Line Adjustment" to get it right on.
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