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Calibration of Hickok752A Tube Tester

Calibration of Hickok 752A Tube Tester
003editedsss.jpg

For the past thirteen years I have checked up and repaired over 400 tube testers, emission types and dynamic mutual conductance types, such as Heathkit TC-1's, TC-3's, Century FC-2's, EICO's, NRI's, and hundreds of military tube testers like TV-7/U, TV-7A/U, TV-7B/U, TV7C/U and TV-7D/U tube testers. And over one hundred TV-2 tube testers I imported from Angela Instruments, Maryland, USA. The owner of Angela Instruments had some three hundred TV-2 tube testers of various models, and after all about one-third out of his
inventory was sold to me, each time about ten TV-2's getting delivered.


The ways and means of calibrating Hickok 752A tube testers are not usually described or found in any Hickok's instruction manuals and/or anywhere on the global internet webs.
So I am just trying to write down something about the calibration I have carried out on
a Hickok752A tube tester, which I was asked to repair and calibrate by one of my customers living in Tokyo. He claims that his 752A does not work right even if S5 Gm push button is depressed. I asked him to send it to my work-bench to check up all the details of his tube tester.


His package of Hickok752A was delivered at the end of January, and after all the careful check-up on my workbench, nothing about the push-button trouble was found out; he had said that the push button S5 was broken, and did not start measuring the mutual conductance value of any tubes. Well I am not too sure about how he operated this rather rare sort of Hickok tube tester, but he obviously did not possess any instruction manual, so it is assumed that he just operated as he thought was right.

What was sent to me were a Hickok 752A tube tester and a data booklet, and most likely he tried to use this great tube tester without any guidance.
The beautiful appearance and superb arrangement of the meter and knobs of this 752A
tube tester are beyond description. I understand why this tube tester is very expensive at first glance. Inside and outside this is extremely beautiful as if it were born yesterday!
011edited.jpg

Yes, sir! I have found a website where I can download the instruction manual and schematic of this particular tube tester, and also at the same time I tried to find out some relevant articles on how to calibrate this great tube tester. After hours of browsing various parts of websites. domestic and/or foreign, no meaningful information was obtained, only that there exits no maintenace manual for 752A, and that this tube tester is about the same as TV-7 military tube tester, so that its calibration can be done almost like that of TV-7's. I agree that most Hickok tube testers are the same inside, whether it is for military use or for civilian use. Their circuit diagrams show that most of the Hickok's tube testers are made out of the basic mutual conductance test circuits.
However when it comes to repairing different models of Hickok tube testers, you have got to make use of all your attention and skills in checking and repairing each of them.

How to Calibrate Hickok 752A Tube Tester:

1) After turning on the power switch, turn O-Adjust knob until the meter indicator comes to the mark at the center of the meter.
2) Set the tube tester's knobs for checking 6L6 GT tube; 6.3V 7250-3481, Bias 17.
(=Very similar to TV-7's, HS53481, 72 is equal to HS, and zero after 5 is because 752A has another additional switch for testing dual triodes)
3) Without inserting any 6L6 tube, check Plate voltage, Screen voltage, and Bias
voltage. Normal readings for these must be in the following range:
Plate Voltage: 150+-3V
Screen Voltage: 130+-3V
Bias Voltage: 40v+-3V
You can adust and re-arrange each voltage in the following manner.
As illustrated in Figure 01, you can turn the trimmer potentiometer and adjust voltages
at each circuit.
001edited.jpg

R-5 50k ohm is for adusting O-Adust voltage.
R-10 500 ohm is for minute re-adustment of shunt control.
R-20 50 ohm is for slightly adusting the plate voltage of the rectifier tube.
002edited.jpg

R-27 slide tap resistor is for setting the bias voltage just like R-130 of TV-7 tube testers.

4) Check the shunt resistors;
017edited.jpg

The following are placed at the lower part of the shunt resistor board. ( From left to right hand side)
020edited.jpg

R-31 500 ohm (for X1)
R-32 250 ohm (for X2)
R-33 150 ohm (for X4)
R-34 50 ohm (for x10)
R-35 50 ohm (for x20)
Multiplier is intended for selecting an appropriate shunt resistor for testing a particular
vacuum tube.

At the upper part of the shunt resistor board: (From left to right hand side)
R-21 119ohm
R-23 41ohm
R-24 41ohm
R-26 119ohm

5) After checking all these points, you can set out to actually test the tube tester's testing
capability by inserting reference tubes whose Gm value you already know.

6) If you find the Gm value on the meter different from that of the reference tube, you can rectify by adjusting the shunt resistor/ bias voltage/and if the bias voltage and shunt
values are proper, you can adjust at the final stage by turning R-10 500 ohm trimmer pot,
which is about the same as in Hickok 6000A tube tester.

All in all, the basic working of Hickok's tube tester is about the same. If you can master how to repair or calibrate TV-7 tube tester, you can most possibly repair other kinds of
Hickok's tube testers, and the only problem left is whether you have time to check up
all the details of damaged or dissatisfactorily working tube testers that were produced forty or fifty years ago. It is easy to say but hard to do for anybody. Whether you like it or not, it is there right in front of you, and it is waiting for you to help recover its former status!

All rights reserved.
Written by Philip Y. Shibuya
JA3KEV/AI4LA
February 16 2014
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