Hickok 1-177-B bias pot replacement

[Jonny123]

Hi All,

Im wondering if anyone has replaced, or know what to replace the bias pot on the Hickok 1-177-B with, as I can’t seem to find any specifications on the schematics I’ve seen. I’d very much appreciate any help or advice on replacement parts.

[Terry295]

Hi Jonny123

I think I may have some bad news for you. Many years ago I restored an I-177B to health after someone have cooked the mains transformer. During my searches for parts and calibration data it became apparent that there are two components in these testers that are very hard to source. One is the 150R dual gang pot that forms the "L" control, the other is the bias or "R" control

If you have taken the "R" pot out of the instrument you will have seen it has a very special tapper that I feel sure only Hickok knew how to make.

At the time I was repairing my instrument I was advised that if it I should need either of these parts ("L" or "R") my best chance was to find a "junker" and hope what I needed was in good order. Subsequent searches showed me that (if I had enough money) I could get an "L" pot custom made from a US company that would have been better spec. than the original, however this was not the case for the "R" control, they simply were not available.

I have seen some I-177 spares on eBay over the years but they are becoming less common over time. It may be of some help to you to know that both pots are the same from the model 530 to the 600a (But I don't know if there are the same physical size)

During calibration I measured the volts delivered to the DUT as per a cal procedure published by Mr Daniel Shoo for a similar Hickok instrument, I have attached the results. It seems that the voltage when the control is set to 18 is critical, the value here is 3.0 +/- 0.1v .

I hope this isn't too depressing, if you want to chat about this problem PM me. Good luck!

Regards Terry C (ZL4TC)

[Jonny123]

Thank you Terry the list of voltages is a great help

I was afraid that the pot would be difficult if not impossible to find and you have confirmed that ! The band has simply worn almost through and the winding frayed - I had the tester working well with the -3 at 18 and it was wonderful. I think I’m going to look at fitting at pot that works and attach a digital display so I set the bias manually I wanted to keep it original but I’ll compromise the aesthetics for accuracy.

Thanks again Terry for taking the time to respond - I’d be lost without people like you !

[Jonny123]

Just a little update I’ve removed the bias pot and there was no sign of electrical malfunction just simply worn through the fine winding wire between zero and 25 a heavily used area. I’ve got the old pot and I’m taking it to Cambridge university electrical engineering dept to see if they can either refurbish or design something that will work with the set voltages correct on the numerical dial. It will be an interesting project for them and may provide a solution for others as well. I’ll keep you posted on the outcome !

[Jonny123]

Ok update on my tester bias pot I’ve had the pot replaced with a 10 turn 5k ohm pot ant had two meters added 0-10 & 0-50 to set the bias exactly it works very well. Thanks to Jennifer Edwards who is American and an expert in Hickok Testers