I have assembled a very poor 'guide' to assist you in repairing your Hickok tube tester. Or, you may be hesitant about buying that 'Handyman's Special' for $50 because the thought of repairing a tube tester frightens you. I would be more afraid of someone who would pay $50 for a non-working Hickok, but that's me. I've done it myself in the past, but only because it was something like an otherwise pristine 752A. You won't be that lucky, but you just never know. Unless one of the power transformers has a short or open winding, most any can be repaired; it just takes a little patience and a lot of luck. If your soldering iron is plugged in, let's tackle that Hickok!

Let's assume you have one of the 'basic' Hickok tube testers; the 533 say. If you have the manual, great. If not, download it (or any similar tester), and study it. What you should see from the schematic is as follows.

• Most Hickok tube testers use a 5Y3 and an 83 rectifier tubes.
• The 83 is there solely to provide B+ to the Plate of the tube under test.
• The 5Y3 is there to provide Screen Grid voltage and Bias voltage to the tube under test.

Of course, we will check the 'fuse', which is often an 81 bulb. If the 'fuse' checks OK, we continue on. Set up the tester for a 6V6 (but do not insert a tube), and check the following. Keep in mind that the 'negative' lead of your VOM should be in pin #8 of the octal socket for any voltage measurement other than the filament. These initial tests are done without lifting the panel!

• Are all filament voltages present? Are they 'correct'?
• By pressing the 'P4' button, do you get a signal voltage on pin #5? (Typically 5VAC)
• By pressing the 'P4' button, do you get a Screen Grid voltage? (Typically <130VDC)
• By pressing the 'P4' button, do you get a Plate voltage? (Typically <150VDC)
• Set the bias control fully clockwise. Measure the DC voltage at pin #5; it should be -40VDC. Now adjust the bias control to '22' (the setting for a 6V6), and check the voltage. It should be -3VDC.

We have now checked the tubes under the panel (without removing the panel screws) and confirmed the bias potentiometer is working. That last part is important, because the bias potentiometer isn't available from Radio Shack today. If one or more voltages are 'missing', we will have to open up the panel, and look around inside. Usually, the rectifier tubes are mounted to a small bracket on the power transformer. Below is a view of the interior to a Hickok 600A, showing how the rectifier tubes are situated.

Some models will make you remove the bracket bolts, and position the bracket in such a manner that voltage checks are possible. By carefully having the bracket positioned so you can do some voltage measurements (use clip leads!), press the 'P4' button, and check the following.

• Do you have <320VACbetween pin #4 and pin #6 of the 5Y3?
• Do you have <340VACbetween pin #4 and pin #1 of the 83?

If all checks turn out OK thus far, you are actually in trouble! This is because you now enter the realm of the esoteric fault. I once had a Western Electric KS-5727-L1 that had multiple faults. Aside from leaky capacitors, the plate voltage was very low. A new 83 didn't cure the symptoms, nor did using 1N4007 diodes instead of an 83 (just to be sure I didn't swap one bad 83 with another bad 83). The plate voltage is fed from the 83 through a small tag board (I had cold solder joints here, so be advised), over to the 'P4' switch, as well as through to the micromho/grid signal range switch! Either switch is a pain to get at well enough to service, and chances are this is where the trouble is if everything else checks OK. There are 47-ohm resistors on three switches (buried very cleverly) that also can act as a fuses. When these 'blow' from trying to test a shorted tube, few people know to look for them. Even if you do know what to look for, you go cross-eyed trying to find them. Other odd-ball faults are as follows.

• I had a 750 that would not light up the tube but the filament voltages checked OK! It turned out to be a high-resistance cold solder joint on the filament transformer. The high impedance of my VOM showed full filament voltage, but with a tube inserted the voltage dropped too low to light the tube up.
• I had a 533 that 'partially' blew the 47-ohm resistor on a switch. The resistor was barely touching its two 'halves' together, and sometimes the resistor halves would not touch, and I would not get a reading. Some days I would get the reading just fine, and those days drove me to drink. Just the act of physically moving the tester back and forth could cause the tester not to work some days, as the two resistor halves became jarred loose.
• If the 'Bias' potentiometer or the 'English' control prove to be faulty, you can always use that tube tester as a door stop. I have had a 1% success rate rebuilding either control, and refuse to even attempt such a 'repair' anymore. These potentiometers are simply not available, except through organ donations from deceased Hickok tube testers.

As a final check; let's assume you have a Hickok that appears to test the 6V6, but does not register a reading on the meter. Check the 'Fuse' bulb as you hold the 'P4' switch down. Does the bulb glow very faintly, and increase slightly as you hold the P4 switch down? If so, the trouble is between the P4 switch and the meter movement. Check all wiring between the P4 switch, the micromho range switch, and the meter movement. There isn't much to go wrong; some wiring, and a few precision resistors. The remaining possibility is the meter movement itself.

That about wraps up all that I can share with you regarding Hickok tube tester trouble shooting. Most testers I have come across have given no trouble over their 50+ years of service, but there is always that one 'dog' to contend with. If I am looking over a prospective purchase, I check to see that all of the panel screws are in place. Missing screws almost always points to the fact someone else took time to 'fiddle' with the innards of that Hickok, and only Jehovah knows what they have 'screwed up'. For $20, I have taken the gamble, if only to have a cadaver to donate organs so that another Hickok may live again. But, thanks to the magic of eBay, even beat-to-crap, non-functioning Hickok tube testers can now fetch $125 or more, and some neurotic tube jockey still feels he is getting a bargain. I recently witnessed a Hickok tube tester with no meter movement, no bias potentiometer, and a case that looked like it had been dragged behind a team of wild horses offered for auction. Don't bother asking what the final selling price was; anyone offering over $1 should have their head examined.