Heathkit TC-2 Tube tester

[gygagnon]

Hi,

I recently picked up a Heathkit TC-2 Tube Tester and I am going over it before I actually use it and I had a few questions.



I checked all resistors and they all are within 5-10% tolerance of nominal. I assume that is good enough?

In the schematic there is a diode shown (the part list calls it a halfwave rectifier part no 57-6) and I believe I have identified it in the unit. (Shown in the center of the picture below with yellow and red wire)


Do I test this like I would a regular diode? What do I replace it with (1n4001)?

Is this unit any good for general (Good no Good) tube testing?

[easyrider8]

Why would you want to replace it? They very seldom ever go bad. You will find some or the resistors have drifted high and need to be replaced. The capacitor also needs to be replaced. Fix these things and you should be all set.

Dave

[FrankB]

Ah, the good old copper oxide type meter rectifier.

[gygagnon]

Why would you want to replace it? They very seldom ever go bad. You will find some or the resistors have drifted high and need to be replaced. The capacitor also needs to be replaced. Fix these things and you should be all set.

Dave

No Reason, but do I test it the same way I would a regular diode?


Is this a worth while tester?

[FrankB]

I would test it, but it will test different than a silicon or germanium diode.
Its been so long since I had to test one, I don't remember the readings it should have.

Last time I tested one was maybe 25 yrs ago when I re-built a telco PS to run my model 15
TTY unit, then never did get it on the air; as I never built the converter box.

[radiotechnician]

If the line set works properly, the diode is working. That is the only function of that
part in the in the TC-2. In the later versions of this tube checker, that copper
oxide rectifier was replace with a 1N191 diode. However, the value of R9
changed from 1200 ohms to 1000 ohms. The 1N191 is a 75 volt germanium
diode.

That rectifier is a key component that affects the overall calibration
of the tube checker. If the line adjust set is wrong, everything else is
wrong. As to testing the original consider this from Weston.

Weston Coper Oxide Rectifiers.jpg (202.69 KiB) Viewed 14772 times

[Chris108]

These rectifiers can be tested same as any others. You will get considerably more reverse leakage than a modern silicon diode, but this is not a problem as long as the reverse resistance is significantly higher (eg. a couple of orders of magnitude) than the forward resistance.

The thing you cannot tell is whether or not the forward resistance has changed, which is possible if the rectifier has suffered internal corrosion, a weakening of the bracket and rivet that hold it together, or electrical overload. If the forward resistance has changed, the meter will be out of calibration. So the best thing to do is a calibration check.

Set the tester up and test a tube that draws considerable heater power. A 25L6 would be a good choice. After the tube warms up, set the line voltage adjust to the mark on the meter. Measure the heater voltage with an accurate DMM. This can be done from the heater pins of one of the other sockets. The two large pins on the 4-prong socket are a good choice. If the heater voltage is within a few percent of 25 volts, the rectifier is working well and you can leave well enough alone.

If the meter is way off when you set the line adjust, the rectifier is probably gone and you need to replace it. A 75-volt Schottky diode would probably be the best bet, as you are not likely to find new germanium or copper oxide rectifiers available. But if the meter is only off a little one way or the other, it would be better to raise or lower the value of 1,200 ohm resistor in series with the rectifier as needed.

[oldtubesoundguy]

I picked a TC-2 up cheap on EBay and finally got around to looking at it this week. First thing I noticed was a 100mfd/50v electrolytic paralleled across the meter. Don't find this on any schematic. Oddly, the tester works fine with it in the circuit testing a 6v6 with a known emission value and the line setup goes dead center about mid-rotation. Anybody seen this anomaly?

[Chris108]

Putting an electrolytic capacitor like that across the meter was common on many Hickok testers. In those it serves to smooth out the ripple in the voltage reaching the meter so the pointer doesn't vibrate. As long as the capacitor does not become leaky, it will not affect calibration one way or the other. If it works, you may decide to leave it in place, or you could remove it since it is not part of the original design and see if it was making a difference in the way the tester operates. (My guess is probably little to none).

[Fred Scoles]

Also, an electrolytic capacitor (usually from about 10uf up to 100uf, depending on the damping required by the operator) was installed to help dampen or slow down the meter's needle travel speed, in order to make reading the meter easier or to help protect the meter needle from slamming hard to its end peg which could damage the meter. It was common for 1960's lab spectrometers (Perkin Elmer UV/Vis 202, PE 290A & 303 AA's, etc.) to have a meter damping multi-position switch where each sw position had a different value meter shunting capacitor. It was a common repair on those spec's to have to replace the shunt capacitors since those instruments often lasted over 25 years. Different brands and models of panel meters (all having the same fullscale and sensitivity specifications) often have completely different amounts of movement damping, so one production run of Heathkit TC-2 might require meter damping cap, where others may not, since Heathkit often changed brands of panel meters from year to year.

[oldtubesoundguy]

Thanks gents! I also have an Eico 625 that will occasionally whack the meter movement. Might try the cap as a fix!

[mcgyver2822]

I have a TC-2, I replaced the meter rectifier with a 1n4001, now when it is turned on the meter pegs to the left..whats wrong??

[easyrider8]

Wrong diode and installed backwards, read the above posts.

Dave

[radiotechnician]

The TC-2 does not apply DC to test emission. The meter goes up because the tube
under test becomes a rectifier.

The meter sees half wave DC.

The rectifier is only for the line adjust.

TC-2.JPG (30.7 KiB) Viewed 11434 times

[Chris108]

Did it work with the original meter rectifier? If so, put it back and leave it alone! Meter rectifiers were a special breed of device with very low forward drop at low forward currents, linear characteristics over their operating ranges, and high stability. A common power rectifier will not make a good replacement.

Meter rectifiers are normally left in place as long as the meter can be calibrated. If it becomes necessary to replace one, it is often necessary to hand-select a small signal or Schottky diode for linearity, and maybe recalculate some of the circuit resistances so the meter can be brought back into calibration.

And yes, if the meter goes the wrong way the diode is probably in backwards.

[DaleHuff]

Any idea where I can get a replacement meter for this tester?

[easyrider8]

What happened to the meter.

Dave

[mcgyver2822]

The schematic shows a diode installed with the wrong polarity with the meter...an error??

[easyrider8]

The schematic shows a diode installed with the wrong polarity with the meter...an error??

Nope not an error, just the difference between conventional flow notation and electron flow notation.
https://www.allaboutcircuits.com/textbo ... tron-flow/" -="window.open(this.href);return false;

Dave

[radiotechnician]

The schematic shows a diode installed with the wrong polarity with the meter...an error??

Nope not an error, just the difference between conventional flow notation and electron flow notation.
https://www.allaboutcircuits.com/textbo ... tron-flow/" -="window.open(this.href);return false;

Dave

The TC 1, and later versions such as the IT 3117 used a different symbol polarity.

In the case of the IT 3117, there was an actual numbered diode 1N 191 .

Polarizing it was easier. "Connect the banded end of a 1N191 diode (#56-26) to
lug 1 of terminal strip AD (NS)."

The older tube checker kits informed builders by referring to a colored dot or terminal.

Whether or nor it was a convention swap is hard to determine. I will check back on
some more period equipment to see when they changed.

Heath TC1 1949.jpg (177.45 KiB) Viewed 11051 times

HEATH IT 3117.jpg (102.91 KiB) Viewed 11051 times