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Author Topic: Heathkit TC-2 Tube Tester  (Read 17674 times)
N4NYY
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« on: November 11, 2010, 04:59:53 AM »

I aquired one of these from a friend. I have to replace the resistors and cap as they measured way off. I have to do the cleaning with Deoxit, etc.

My question centers around the CR-1 diode and the 1.2K resistor. It measured good (at least I think it is, as it is not a silicon). This website says that you can use a 1N4007 as a replacement, but then have to change the 1.2K resistor accordingly. I typical silicon diode is usually .6-.7V.

What does the value of the 1.2K resistor need to be changed to? BTW, I am not doing the mods that add a calibration control.

Note, he advises using a Knight 600 schematic as he said the Heathkit schematic is wrong.

http://tubesound.com/2008/05/31/knight-600-tube-tester/
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KA5N
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« Reply #1 on: November 11, 2010, 07:19:02 AM »

I assume the rectifier is a selenium rectifier which has a voltage drop of around 5-15 volts .  The small units are often used as meter rectifiers (which it appears that this one is) so the current flow is low.  If the existing rectifier is still good I would measure the voltage drop while the tube tester is working and then replace the selenium with a silicon and increase the value of the 1200 Ohm resistor to match this drop.
Seleniums were the rage for awhile and were used in radios and TV sets as power rectifiers.  They were also used as high voltage rectifiers in portable (and other small) TV sets.  When used as "stick rectifiers"
(HV types) they were just a bunch of "selenium tablets" in series.  One used to be able to buy replacement sticks from TV supply places but with the change to digital TV channels and phasing out of
CRT's I expect this market is just about gone.
Good Luck
Allen
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N4NYY
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« Reply #2 on: November 11, 2010, 08:18:35 AM »

It looks like a bunch of washers glued together. I have never seen anything like it. I assume you test it like any other diode. Assuming, that the voltage drop is 5-15 volts, I would then use the .6-.7 volt of the silicon, and the rest of the voltage is dropped across the resisitor? Correct? Then I would have to use Ohm's law to calculate resistance?
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N4NYY
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« Reply #3 on: November 11, 2010, 08:25:35 AM »

OK, upon a Google, it appears these are heavily toxic when they overheat, and should be replaced. They will fail eventually.
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KA5N
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« Reply #4 on: November 11, 2010, 08:41:01 AM »

Each of the "washers" is a diode so you have several in series.  Since the 5-15 volt number is sort of a guess, you have to sort of go by trial and error.  Yeah they are toxic.  But just don't handle them or breathe the vopors (and you won't have to guess if there are vapors as it will stink like rotten eggs).
When they were used on a wide scale in electronics, I never saw any warnings about them being toxic.
So I handled them and inhaled some of the vapors and it didn't seem to do anything, but in those days
we didn't have OSHA or the EPA and cigarettes didn't cause cancer.  But they aren't wholesome so clip out and get rid of them. 
You can Google "Selenium Rectifier Voltage Drop" and get a lot of hits, and find out there is not cut and dried guide to figuring what the voltage drop is for one, unlike you are used to for silicon, germanium etc.
Anyway tube checkers aren't that wonderful anyway.
Allen
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N4NYY
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« Reply #5 on: November 11, 2010, 08:56:28 AM »

Allen,

It appears that firing it up and measuring the drops would be futile. It appears they change over time, and also the 1.2K resistors has changed value. I am trying to find a manual that list this rectifiers spec, and am coming up empty.

This appears to be parts of the 0-1mA meter circuit (and I have no idea what that is used for in tubes, as my tube experience in minimal). Would this be a "zero" resistor to zero the meter? if so, I can replace with a pot and use that to zero. 

I will clip it out, and replace it. I have no idea as to the resistor needed for this as I need some sort of starting point. Being this is so old and the values have changed greatly, I do not know the original voltage drops. What I think I might do is replace the resistors and then measure the drops. The schematic is not showing me any voltages.
« Last Edit: November 11, 2010, 10:39:24 AM by Vinnie Sallustio » Logged
KA5N
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« Reply #6 on: November 11, 2010, 11:30:41 AM »

"Meter Rectifier"  are the secret words.  This rectifier is so the meter will have a DC input (and hence not flop back and forth from AC).  These are often used in cheap multimeters to measure AC voltage and current.  It is not a zeroing device and a pot won't substitute.
As a starting point put in a 1N4007 and change the 1.2K resistor to say 3.3K and see what happens. 
I don't think you will find specs on the selenium rectifier anywhere but some old manual somewhere, I
wouldn't waste time looking.
Allen
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N4NYY
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« Reply #7 on: November 11, 2010, 11:34:23 AM »

Understood. Thanks!
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W3TKO
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« Reply #8 on: November 13, 2010, 11:16:36 AM »

The link given above provides the info to determine the resistor value, if it is CR-1 you are looking at. This voltage provided affects the center zero meter setting.

Measure 1.2k to see if it is 1.2k, just so you know.

Set filament to 6.3V with line control knob and a DMM.

If needle centers on meter with 6.3V @ filament, CR-1 is good.

If it is working, write down the voltage provided by the 1.2k and CR-1 combo.

Use as is for a while if working to see if all is good.

If meter doesn't center, wire in pot and change resistor value until it does. Write down that voltage.

If you want to change over to silicon, I'd just use the pot and dial in to the voltage you wrote down above.

Some of those low current seleniums can limp along indefinitely it seems.
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N4NYY
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« Reply #9 on: November 13, 2010, 03:10:35 PM »

I think I am going to get a 5K pot. It would just be easier. Hopefully, that is big enough as far as resistance.
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N4NYY
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« Reply #10 on: November 22, 2010, 05:42:18 PM »

How critical is the filament voltage? I cannot set it below 6.46V for the 6.3V setting.
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AC5UP
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« Reply #11 on: November 22, 2010, 07:27:46 PM »

The typical 6.3 volt 300 mil filament common in many tubes is a nominal figure. There are very few tubes that could be considered a precision device by today's standards and certainly not in the small signal / consumer varieties.

That's the long way of saying that 6.46 volts is close enough.

Then I have to ask: Did you measure the filament voltage with or without a 6AU6 in the socket? If not, it's going to come in around 6.3 volts (or less) under load. Pop in a 6L6 and it may drop to 6.00 volts. A 6SN7 should split the difference as the 6AU6 draws 300 mils, 600 mils for the 6SN7 and 900 mils for the 6L6. Point being that if it's possible you may want to test several different tubes as a filament load then split the difference on the filament voltage adjustment.

BTW: I should also mention that running a filament at higher than rated voltage can improve the emission reading (transconductance) of the tube. It can also boil the thorium coating off the cathode and croak the tube if you go too far. A low filament voltage has the opposite effect. The tube is slow to warm up and tests weak. When you get the tester going set up a 6 volt tube and check it with the filament switch on 6.3 and 5 volts... That was the old school way of deciding whether a questionable tube had a future. If it looked OK at reduced filament voltage it probably had some hours left in it.

If not, taps.
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N4NYY
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« Reply #12 on: November 22, 2010, 07:54:39 PM »

No load. I used this site for calibration:

http://tubesound.com/2008/05/31/knight-600-tube-tester/
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AC5UP
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« Reply #13 on: November 23, 2010, 12:27:35 PM »

Most tube testers are relatively simple electrically (spaghetti bowl from hell mechanically) and totally unregulated as far as the test voltages are concerned. From what I saw in the URL you gave the idea is to center the front panel Line adjustment so you have maximum +/- range, adjust the internal line rheostat until the filament transformer reads true on 6.3 vac, then set the panel meter to match the calibration mark for the Line adjustment.

If all that works as it should you can put any 6.3 volt tube in the tester, let it warm for a minute, then set the front panel Line adjustment until the meter hits the calibration mark. You'll have 6.3 volts on the filament regardless of the current drawn by the tube. This also means that when you switch from an 807 to a 6BA6 it's wise to check the Line setting as the 6BA6 draws something like only a third of the filament current.

Anyway... If 6.46 vac is as close as you can get you may want to set the front panel Line adjustment to something like 35% - 40% of full range and see if you can reach 6.3 vac that way. For all you know your line voltage is 120+ and the tester was built assuming 110. In that case you don't need much range to compensate for a low line voltage. More important is that with 6.3 vac on the filament the meter hits the Line calibration mark because that's what you'll use to compensate for big tube / small tube.
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N4NYY
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« Reply #14 on: November 23, 2010, 02:40:47 PM »

I can put a series of high watt resistors in series with the line and the rheostat. I have a bunch of 5 watters, however 55 ohms onl brings me down to 6.3V with the knob fully CCW. I would like to get it where I can adjust below and above each settings. Looks like I got the hang of it now, and only need to make a combo to reach 70 ohms.

BTW, that 5K pot works well in adjusting the needle.

I sincerely thank you for all your help.
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