Hello all,

So, the day has come in which the bias pot in my TV7-D/U finally failed, and this time beyond repair. I have spent hours today trying to source a replacement and even spoke with Dan Nelson but none is available.

So, guys, do any of you have a spare? I would be very interested!
Also, has any of you come up with any work around to replace this hard to find part with anything else and still keep the unit usable wihtout any major modification?

Thanks!

What value is it and what does it look like?

It's a 3K wire wound potentiometer.

Biggest problem is that it has sa custom tapes which is not linear nor logarithmic... so its a special pot made specifically for these testers. And thats what makes it hard to find...

Here you can see the curve someone measured:
http://www.antiqueradios.com/forums/vie ... 1&start=20

I replaced my bias pot in TV-7 about 6 yrs. ago and not sure but MIGHT have gotten it from "ziggy7" on ebay (think he is in Jacksonville Fl). I know of Dan Nelson too but do not think he had one. They are indeed hard to find. A new "cottage industry" opportunity??

Harold

Hi Harold,

Thank you, I have just sent him a message. He does not have the pot on his listings.
IMO, it does seem to be a nice opportunity... these testers are getting more and more expensive and it really makes sense to fix them.

I never had such difficulty sourcing a part... and I have sourced many old rare parts in my life trust me!

Harold,

Ziggy7 (Roger) just got back to me. He has no pots left

One way around it would be to replace it with a conventional pot, maybe an audio taper and add a voltmeter to set the proper bias. Someone would have to make up a chart that would give the voltage for each division of the setting from a good working tester.

Instead of trying to find or make the right control, wouldn't it be simpler to just find a 3k control that works and make a custom graduated plate?

That would certainly work, although the scale would be very crowded at the low end. I'm not sure how easy it is to get 3k wirewound pots either.

You can use a pot from any other Hickok, even from the 1930s but they're probably not precisely the same.

Well, yes, I know I could fo several things to fix this, like adding a test point so I could connect a multimeter and read the exact bias volts. Or translate the chart to a new scale with a linear or log pot, or even change the meter scale to reflect the new pot scale.
The thing is I like to keep my stuff as close to original as possible, so I dont really like the idea of doing major changes like these. This might be a last resort option though.

Alan, yes, this pot is very common in several hickok testers, but unfortunately, I have no other defective tester to use as a donor unit. And I cant seem to find any source for a similar part anywhere...

You may best be served by figuring out what made it fail prior to putting in a new pot. The classified forum may be a decent place to find a replacement.

That's easy, someone tested a tube with a grid-to-plate short.

Alan,
I know the easy answer is to follow the manual and never test a shorted tube. What I'm thinking is what if a failure occurred in the shorts test circuit? Would there be an easy way to add circuit protection to prevent smoking the 3K pot?
Tks.
Bob

Maybe you could add a PTC (inrush current suppressor) in series with the bias pot current path.

Edit - approx. 18 mA = 1 W into 3 K Ohms. Protection could not be in plate supply current path due to low trip point needed. It would need to go into the bias pot end of the path.

This is the extreme from original, but I wonder if there is a reasonably simple way you could use a digitally programmable potentiometer - controlled by a plain old analog pot (pin 3 up/down below) that at least looks outwardly the same.



Don't kill me - just a thought!

The bias pot is not in the short circuit testing per the schematic, so I can not see it going open from testing shorts. Some of the subsequent Hickok testers use a #49 bias fuse bulb (or fuse), the wiring varies on the model. You could emulate the TV-10 circuit which uses a 1/8 amp fuse in the 0V CT of the 5Y3 cathode, I assume this also protects the screen circuit as wired. Alternatively you could use a fuse or #49 bulb after the bias pot wiper as done on the later Hickok models like the 580 and 752. I have also done this on the Hickok 539 series, on the AVO CT160's I use a 50 ma fuse after the bias pot. I have had several occasions where the bias fuse went under testing (after short testing). The AVO CT160 also has an unobtainable bias pot, although there are some workarounds posted. The difficulty with the many of the Hickok testers is that the bias settings use a 0-100 scale (and a non linear bias pot), as opposed to volts, so one cannot necessarily use an external bias meter without knowing the conversion voltage at each bias setting. Finding/rewiring the bias pot is a challenge. Years ago I purchased a spare bias pot for my 580, and that was after several years of looking.

Actuallly, in my case, there was no electrical problem. I have opened up my pot and somehow, the wiper managed to rip the nichrome wires in one section of the pot. It was a mechanical failure. I will look into the classified section.



Well, that did cross my mind. I even thought of using a micro to do this! But this would be too dramatic and I would really like to keep this TV7 untouched... and anyway, I would be better off stripping the wholhe thing apart and mounting a digital panel and a processor controlled supply and bias if I were to go that route. This is somehting that I may do in case everything else fails. But first I will do my best to find the original pot and save my beloved TV7.

Or, you could "go rogue" and connect the thing to a curve tracer.

There is a website called "TV-7's and TV-7 D/U Data Page" that has chart of numbers vs bias.

RRM

That would be interesting, but I dont have a curve tracer (yet!!).



Hey, that's interesting! That should save me some work. Thanks.


I guess I can forget finding a pot. No one seems to have spares