Hickock 750

Both 750 I bought, had good basic substance as I got it. No serious damage, and only worked ony by good technicians. Well almost. One had a sticky roll chart, which must have driven the owner crazy, ending up using a big screw driver, violently pushing the wheel, until it all broke down. But that was all of the mistreatment it got, and I could fix that at almost no cost. I had an NOS roll chart, and digged up a new thumb wheel. It seems to me, the later roll charts are simply too big, and these roll not smooth for that reason. For making it an accurate tool for daily use, there was a lot to do stil.

The 750 is a refurbished 532, and Hickok tried to replace the 539C by it. My opinion is, the 750 is one of the best Hickoks there is, and more useful than the overhyped 539C. More about this later.

What is really nice with the 750, it features an adjustable DC voltage of 200V, 100mA. I found both my 750's can not supply full current and full voltage simultaneously, so that must be how they are. When exchanging the 83 rectifier for an 83V, maximum currents exceeds even 100mA, while the micromho readings don't get affected. Perhaps I will change this later, but I think this is going to take some exchange of other components too. At least I need to check this better, and for the moment I will leave it as is. The voltage option lets you test stabilizer tubes, but you can do also other things with it, as you like. If you don't know how to do that, it's better only to test things as indicated on the roll chart. Later on in this article, I will show you how to repeat Funke W19 tests with it. You do need a copy of the W19 test cards.

The 750 differs from the testers like 533 and 534 because it works with different AC test signals, instead of a fixed signal. It has this in common with the 539 series. This allows testing of tubes with very high gain, so no overdrive will occur. This makes a 30.000 micromho scale possible.

They put back in the load test in the 750, which the 539C is missing. I think it was a mistake, to remove that with the 539C. The latter can only measure Transconductance, no load test is done. Also you can't compare the transconductance you measure with the 539C with your tube book, because the plate voltage of the 539C is too low, and simply wrong for most tubes. Next is, the roll chart data of the 539C is lower than for a new tube. That was done to give you a way to pick out the bad tubes, because in the end that's what people want. So Hickok put a non-standard roll chart in the 539C, with lower values than normal. So they printed the reject value on the 539C roll chart, not the "new tube" value. (of course you 539C users, you all know that... hehe). This makes I don't like the 539 so much. To to stop the confusion, Hickok put back in the load test in the 750, and also give standard tube data on the 750 roll chart, same as with older (normal) testers. That was a good decision.

I never found any documentation about how this load (English) test is really done. The word "English" suggest the AVO method, like in the Mk2...4, and CT160. I will check that later, with an oscilloscope. The oldest tester I have, using the AVO so called "patented" method, is.... a German military tester that I estimate from 1938. For the moment let's trust Hickoks experience, giving us the correct "English" dial setting for each tube. Somebody in Cleveland Ohio (where this tester was made..) must have had a huge cabinet full of good and bad tubes, and just tried out the right settings. The roll chart is the software of the tester, so to say.

( I know it was done like this from the book of Harvey "Gizmo" Rosenberg, who described the work of Julius Futterman. Before the war, Julius used to work for a company building tube testers, and tube manufacturers would send him all kind of tubes, to update the roll charts)

Now go to the restoration page of this tester. There were many things to do....

WHAT CAN BE DONE WITH THE 750 AND WHAT IS NOT IN THE MANUAL:

1) FUNKE W19 simulation. Here is something really new. With the 750 it is possible to do the Funke W19 tests with your Hickok 750. All you need is the famous Funke Blue book, which costs you about 30..80 Euro, and a copy of the card set which is on Ebay for 15 Euro. The Funke W19 is extremely popular in Europe, a top model, but it is completely ignored in the English speaking territories, except Asia who love the W19 also. Now don't ask me why that is, but this is where we are. The W19 is mainly an Emission tester, but boy.. it is such a good one. Test results are undisputable. This tester proves what a good Emission Tester can look like. It is at the level of the better Hickoks, in terms or reliability, accuracy and correct test results. So if you have a Hickok 750 (or 539), read here is how to do Funke W19 tests on it!

2) Forward voltage of rectifier tubes. This is of particular use in the laboratory. If you don't know much about tubes, and only want a quick result, you should use the classical test a by the roll chart.

Example: The 80 rectifier to my experience should have a forward voltage of 25..40 Volts at 60mA to be good. Where 25V rarely happens, and above 45 the "bad" range begins. Also note what it does at lower filament voltage. A good rectifier does not significantly change it's forward voltage if you do so. On the other hand, if the forward voltage is higher than normal, and you set the filament voltage one click higher, and the tube significantly improves by this, the tube is worn out. You can try to ge those results in-line with the classical test as on the roll chart. To do this test, just set up the tube from the roll chart first. Then set the "Adjustable Voltage" to zero, and the test know to position "H" for high. Flip the Vr-Mils switch to mils, and slowly increase the voltage until you see the milliamps go up. Check the forward voltage, this is when Vf begins. Then go on until you have 60mA and read the Vf now. Then do the other plate (switch setting is at the roll chart). You will see this is very easy to do, and you learn a lot about the tubes.

3) Forward voltage of Power Zener diodes. Same procedure as with tubes. Only you have to prepare an adapter this. Remove the base from an old octal tube, and connect two wires to plate and cathode. Take the connections for the VR105 tube. Make sure you don't reverse the Zener, and don't boost 100mA into a short circuited Zener, that is probably not so good for the tester. You will be surprised how bad solid state zeners actually are, they have high temperature drift, and bad load regulation compared to tubes like VR150 or VR105.

4) Continuity tester. This one is from the manual actually, but it's nice one. You can use the 750 as a continuity tester, when you set the "shorts" switch for position 4. Then put two probes in the "grid" and "plate" connections. Because the tester has an isolated chassis, this is of no danger. Then the neon lamp will burn, below 200K resistance. Nice for checking switches, connectors, etc.