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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Dec 2004, 11:41 pm   #1
ipcress22
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Default Repair of AF117 Transistors

Further to the UKVR topic on 'intermittent' AF117 etc transistors, I tried to rectify the problem as follows :-

The *outer metal case was unsoldered by carefully pulling the four leads whilst moving the soldering iron around the lower rim of the case. The transistor was placed on a wooden surface during this operation and although the case became hot enough to melt the solder the glass base seemed to prevent any heat conduction damage to the device.

Using a wooden cocktail stick, it was possible to clean the inner glass surface and the areas between the wire connectors ie removing the offending conductive fibres thereby removing the leakage paths.

Silicon grease was then carefully smeared over the pnp junctions and over the inner pin connections before the outer case was replaced and tacked with solder.

Subsequent continuity tests showed that the 'low resistance' between electrodes and screen had been eliminated and that the junctions were still intact.

The process was repeated with all four 'faulty' AF117 transistors in a Hacker Herald RP35 and the set is as good as new.

It is a tricky task but bearing in mind the scarcity of these transistors, it is satisfying to see the original devices in situ'.

The set has been operational for several months but should the same condition recur it should now be possible to remove the metal covers without unsoldering the leadout wires so that a 'decoke' may be effected.

Best of luck and apologies if this technique is common practice.

Ipcress22

PS Phase II - A recent attempt to repeat the operation with a transistor from another set proved more difficult as the metal case was reluctant to separate from the base. (different bonding method?)

The transistor case became excessively hot so the process was aborted, however, this process alone appears to have cleared the leakages so that the device operated satisfactorily. Still worth a try?

Ipcress22

Last edited by Chris_C; 28th Dec 2004 at 1:58 pm.
 
Old 17th Dec 2004, 11:42 am   #2
radiogammon
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Default Re: Repair of AF117 Transistors

Thanks for the suggestion. I for one have certainly not heard of this idea before. Last week, I removed a self-destruct AF117 from a Mini Herald RP17A and replaced it with a OC169. I still have the offending AF117 and will certainly give it a try.

John B.
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Old 20th Dec 2004, 3:58 pm   #3
Darren-UK
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Default Re: Repair of AF117 Transistors

Hmmm.......I've not heard of this either and will have to try it sometime. Normally I either 'blow' the offending hairs out of these transistors by the method described elsewhere within this website - or simply chuck 'em and replace with another.

As for supplies of AFXXXs, I gather up lots of tatty old trannies from boot sales at 1 - 2 each and these frequently yield some good AFs as well as other useful parts. Always worth keeping an eye out for, me thinks.
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Old 22nd Dec 2004, 12:30 pm   #4
See_Mos
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Default Re: Repair of AF117 Transistors

Very interesting technique, I have often cut the screen lead out as in most cases it made no difference to the operation of the set.

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Old 22nd Dec 2004, 6:35 pm   #5
Darren-UK
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Default Re: Repair of AF117 Transistors

Cutting the screen lead is a common dodge which doesn't always work and can cause symptoms similar to misalignment of the IF.
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Old 22nd Dec 2004, 8:54 pm   #6
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Default Re: Repair of AF117 Transistors

I've also found that cutting the screen is not always effective. Recently I looked at a Roberts R200 and one AF117 measured leaky from case to all electrodes so cutting the screen on this would not have worked. The only option was replacement (with a NOS AF116 in this case). For best results the AF12x range can be used as these do not seem to suffer from the same problem.


Rich.
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Old 23rd Dec 2004, 8:49 pm   #7
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Default Re: Repair of AF117 Transistors

This sounds like an interesting approach, but you are very likely to expose yourself to hazardous substances if you open up any semiconductor device. Please take care and consider whether it is worth risking your health over a shorted AF117.
I worked at a manufacturer of semiconductor processing equipment up until a year or so ago; they treated handling of wafers (silicon now of course), dealing with debris, potential exposure to toxic gas etc. very seriously. In fact, nearly every week there would be a toxic alarm and we all had to stand in the car park waiting for the fire service to arrive. Of course there were local legends that stated that eating a hot curry of an evening would be enough to set off the toxic alarm the next day

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