Whisker on a tuning gang

[Radio Fixer]

Got a Fada 790 radio off the shelf, restored some years ago. This is a very neat US radio having BC (MW) and the full FM band from 88 - 108 even though its from 1947.

At first all was well but then on AM 'snap crackle and pop'. Suspected the minature tuning gang (combined AM and FM) plates, with very close vane spacing fairly early on but could see no badly aligned plates. Also, the problem was still there right across the tuning which it normally isnt for this. And it wasnt with the oscillator as a 'scope showed health right across the band.

So hunted around and whilst doing this all RF gain was lost. 'Knocked around' and there was definetly a prob. around the RF tuning gang. With a meter on it the Ohms would intermittently drop to zero. Suspected the mica under its trimmer cap, it was a bit cracked so I cut a new one. Nope not that!

I was getting to the stage where I thought I might have to remove the gang for a closer look which I didnt want to do as it would have meant disconnecting all the very solid FM wiring and coils from it.

Tried blowing through with an air duster but no change. Then tried my variable high voltage insulation tester on it to see if I could see an arc somewhere but it wasnt dark so that didnt work... or did it. When it was removed and the the meter replaced there were no more shorts. So I can only assume there was a whisker grown from one of the plates which the high voltage fused.

I had read about this but never come across it before. To be on the safe side I charged up a 0.1 mF to 500V and discharged this into the gang a few times.

Once again the radio is playing well...time will tell!

Gary

[Mark Hennessy]

I've certainly see whiskers growing from the frame, but never the plates. I've only ever seen these made from aluminium. Is it possible that the whiskers had originated from the frame? Perhaps not, based on what you say...

If you have a metal that is producing whiskers, it normally produces lots of them, so a careful look under good light should reveal all. I recently took apart a cassette deck made by Sanyo that had a carpet of whiskers on one particular part of the chassis - readily visible to the naked eye...

Either way, well done

[Robert Darwent]

So I can only assume there was a whisker grown from one of the plates which the high voltage fused.

I had read about this but never come across it before. To be on the safe side I charged up a 0.1 mF to 500V and discharged this into the gang a few times.

That is reminiscent of the well known AF117 mode of failure, even the method used to cure the problem is the same.

I can only assume that a fine slither of metal has sheered off the leading edge of one the tuning vanes during use and caused the intermittent shorting.

It does make one wonder though whether the 'whisker' has grown by the same mechanism as they do inside an AF117 can? I believe the general opinion is that the growth is down to the semiconductor, but perhaps it is the aluminium after all which is the culprit!

It'll be interesting to know if it reoccurrs several months down the line, as it often does with 'zapped' AF117's.

Regards

[Joe]

I'm probably showing my ignorance here but how could you discharge a 0.1µF cap across the gang if you'd already blown the whisker with the insulation tester? Wouldn't it just present an open circuit to it?

Cheers

- Joe

[pwdrive]

I'm probably showing my ignorance here but how could you discharge a 0.1µF cap across the gang if you'd already blown the whisker with the insulation tester? Wouldn't it just present an open circuit to it?

Cheers

- Joe

It would depend on what voltage the capacitor is charged up to and the max working voltage of the tuning gang, the closer the plates the lower the max working voltage is with air as a dielectric.

As a footnote: Most will know this but tuning gangs don't like bits between the plates like strands of wire wool or metal filings (thinking restoration/cleaning here)

Lawrence.

[Mark Hennessy]

It does make one wonder though whether the 'whisker' has grown by the same mechanism as they do inside an AF117 can? I believe the general opinion is that the growth is down to the semiconductor, but perhaps it is the aluminium after all which is the culprit!

Hi Robert,

The problem with AF117s is the tin used in the encapsulation.

You could spend an enjoyable hour or two browsing this site: http://nepp.nasa.gov/whisker/anecdote/af...index.html

Cheers,

Mark

[Robert Darwent]

The problem with AF117s is the tin used in the encapsulation.

Yes Mark you're quite correct, I have read that before about the tin plating inside the can but it had gone right of my head until you jogged my memory.

Regards

[Geaney]

Fascinating. Is there a write up anywhere that describes what's happening from a materials perspective at the molecular level? I'd love to know what causes this phenomenon and why the material responds as it does.

I'm reminded that many years ago I worked with a defense establishment and one of the requirements of the control systems was that there should be no changes even at component level for thirty years. I sure hope they check them out regularly!

Cheers, John

[Radio Fixer]

I have looked the best I can with good lights and a magnifier at the gang but its a small chassis, complete with a large frame aerial on the rear so access is poor. Also passed a strip of paper through between some of the plates with it partially open. Perhaps before I box it up I will do it for every plate on there and to the frame as well. And more from an air duster.

If it were not 4 gang, with two being for the FM with 16# wire or thicker and some of that being coils, I would immediately take it off. As it is I'm loath to do it particularly now its working again. We know what the old service shops would have done all those years ago dont we!!

Yes! you are correct Joe if the gap, including the remains of unknown whisper were larger than the breakdown of the 500V charged cap then it wouldnt have gone Zap! But it did and it would have done even without the possible shortened whisker as the spacing is very close. Those 'mericans made really nice components back then. The reason for doing the cap zap was that the insulation tester is a battery powered, inverter design with little current and energy ... no flash seen when hooking that up, but yet it did clear whatever was there...

   

Gary