Whiskers in germanium transistors etc - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

YC-156 · 11th Nov 2005, 07:33 AM

Thanks for sharing this, Paul. Nice piece of 'field work'.

Quote:

Originally Posted by m1ecy

Crikey, these guys dont mess about do they!

Either that or they have far too much spare time, and desperatly need a hobby!

I suspect NASA may have an interest in being able to build electronics, which can work reliably for decades. Think about the Pioneer and Voyager deep space probes from the seventies. I also seem to recall there is a satellite, dating to the sixties, in orbit around the Sun, which they still hear from when the angle of its solar panels are just right.

One way to learn about longevity is to study the mistakes of the past, as the NASA science team have done in this case.

Physicists at work, coming through!

Frank N.

Paul Stenning · 11th Nov 2005, 11:53 AM

From their website (which is currently down for maintenance...) they certainly aren't doing this for fun. Although they certainly seem enthusiastic about what they are doing.

Several sattelites have been lost due to tin whiskers from the metal cases to the contacts of small relays in the main control system for example. There were other problems with space craft (less fatal) mentioned on the website, as well as a batch of pacemakers failing (whiskers inside crystal components), computer systems failing (whiskers between IC pins) etc. With the move to lead-free solder more tin will be used, and with closer lead spacings on modern components, tin whiskers are a serious issue.

They are certainly keen to examine any components affected by tin whiskers. The cause is not yet known, so I guess by examining different examples they will gather data to at least find ways to reduce the effect.

Paul Stenning · 11th Nov 2005, 12:01 PM

Quote:

Originally Posted by stretch289

It's a great bit of NASA speak, I imagine the can was probably crushed by the hammer as they tries to open it

They probably used a laser

If you look at the slideshow it has a photo of the "can opener". It works the same way as a pipe cutter.

TNC · 12th Nov 2005, 10:32 AM

Just out of interest, does this problem potentially affect all early transistors?
I am currently working on a radio with the following line-up:
AF102, AF115 (x2), AF116 (x2), OC71, OC44, OC82DM, AD140 - are all these susceptible?
Diodes {OA79 (x5) and OA70} in plastic/glass tubes I assume are affected - am I correct?

Tks,

Trevor.

ppppenguin · 12th Nov 2005, 11:03 AM

The only ones we believe are affected from that list are the AF11x series. Because they have metal cases which are meant to be earthed they are particularly vulnerable.

In theory it's possible for an tin based part to grow whiskers but a large tinned area of metal case is a much bigger hazard than small soldered joints.

Leon Crampin · 12th Nov 2005, 02:30 PM

A most interesting analysis of a well known problem. Metal migration is not by any means a new phenomenon - silver can be very difficult in this respect and can cause havoc in thick film resistor modules and ceramic capacitors. Alloys are generally less troublesome although brass often loses the zinc.

It's fairly obvious to see now why these devices generally short collector to case, as there is a maximum voltage gradient between these two points if the case is positive. So, why not a rescue job? The case connection has to be at earth potential from an RF point of view to be useful as a screen, but why not connect it to the most negative point in the circuit, rather than positive? This may have the effect of making the whiskers grow back.

Accordingly, I have today set up an experiment, using a Bush TR130, which has 3 AF117s and is a very accessible to work on. Starting with a working set, I moved the two screen connections for the IF amplifiers to V- (decoupled). There was no detectable change in performance, but care must obviously be taken to ensure that the transistor cases do not touch the IF cans. I then thought about a "rescue" job, and fitted an AF117 with a short collector to case to the mixer stage. In this case, moving the can connection to V- had no effect, of course. Fitting a 100k resistor in the lead restored normal operation of the set (much as it would if you just snipped the lead), but maintains the bias across the whiskers.

I intend to keep this set permanently powered up to see if the short eventually clears - easily checked by shorting the 100k resistor. I'll let you know how this develops, but it may take a while.

Possibly, to preserve the AFxxx devices for posterity, the case polarity with respect to the other transistor electrodes should be reversed. We'll see.

Paul Stenning · 13th Nov 2005, 11:20 AM

The transistors I sent to NASA for analysis were UNUSED and had still grown whiskers, so it is nothing to do with voltage gradients (since there are none when it is not powered).

If it was that simple, NASA wouldn't be investing in such a large and ongoing research project to find the cause.

stephanie · 10th Mar 2006, 05:45 PM

So has anyone come up with a good modern replacement for the AF117s yet?

Paul Stenning · 10th Mar 2006, 05:49 PM

I beliebe the BF450 is a reasonable replacement. Do a search here for "BF450" and you'll get four threads on the subject.

I have now added a video presentation about this from NASA to http://www.vintage-radio.info/whiskers/

paulsherwin · 10th Mar 2006, 06:03 PM

Stephanie, the answer seems to be that it depends on the circuit. The AF114-AF118 seem to be pretty much the same transistor but selected for particular characteristics and then marked. You often see AF12x transistors on eBay which should work in most cases. The silicon BF450 works in many cases, and Sean managed to sub the cheap'n'cheerful silicon BC214 here: http://www.vintage-radio.net/forum/s...ead.php?t=5884 Probably wouldn't work replacing the mixer/oscillator but you never know.

I still have some dirt cheap BC214s available if you want some - send me a PM if you do.

Best regards, Paul

howard · 10th Mar 2006, 06:06 PM

Hello Stephanie,

I use AF125s to replace AF116/AF117 transistors. Theyre smaller than the originals but are germanium of similar construction and of the same era yet dont suffer from the whiskers problem. I recently bought a large stock of them at a very good price to keep me going for a while

There are few of these still around, usally on eBay. I could let you have three if you dont go for the silicon BF450s which are just as good.

Howard

Steve_P · 12th Mar 2006, 08:47 PM

Paul - if you're still in touch with those people looking into this, then this thread may be of interest:

http://www.vintage-radio.net/forum/s...ead.php?t=7941

Steve P

11th Nov 2005, 11:53 AM	#22
Paul Stenning Administrator Join Date: Dec 2002 Location: Hereford, UK Posts: 7,098	Re: Whiskers in germanium transistors etc From their website (which is currently down for maintenance...) they certainly aren't doing this for fun. Although they certainly seem enthusiastic about what they are doing. Several sattelites have been lost due to tin whiskers from the metal cases to the contacts of small relays in the main control system for example. There were other problems with space craft (less fatal) mentioned on the website, as well as a batch of pacemakers failing (whiskers inside crystal components), computer systems failing (whiskers between IC pins) etc. With the move to lead-free solder more tin will be used, and with closer lead spacings on modern components, tin whiskers are a serious issue. They are certainly keen to examine any components affected by tin whiskers. The cause is not yet known, so I guess by examining different examples they will gather data to at least find ways to reduce the effect. __________________ Paul Stenning Forum Admin/Owner and BVWS Committee Member

12th Nov 2005, 10:32 AM	#24
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex. Posts: 439	Re: Whiskers in germanium transistors etc Just out of interest, does this problem potentially affect all early transistors? I am currently working on a radio with the following line-up: AF102, AF115 (x2), AF116 (x2), OC71, OC44, OC82DM, AD140 - are all these susceptible? Diodes {OA79 (x5) and OA70} in plastic/glass tubes I assume are affected - am I correct? Tks, Trevor.

12th Nov 2005, 11:03 AM	#25
ppppenguin Dekatron Join Date: Dec 2003 Location: North London, N12 Posts: 6,764	Re: Whiskers in germanium transistors etc The only ones we believe are affected from that list are the AF11x series. Because they have metal cases which are meant to be earthed they are particularly vulnerable. In theory it's possible for an tin based part to grow whiskers but a large tinned area of metal case is a much bigger hazard than small soldered joints. __________________ Jeffrey Borinsky www.borinsky.co.uk

12th Nov 2005, 02:30 PM	#26
Leon Crampin Hexode Join Date: Nov 2005 Location: Surrey UK Posts: 470	Re: Whiskers in germanium transistors etc A most interesting analysis of a well known problem. Metal migration is not by any means a new phenomenon - silver can be very difficult in this respect and can cause havoc in thick film resistor modules and ceramic capacitors. Alloys are generally less troublesome although brass often loses the zinc. It's fairly obvious to see now why these devices generally short collector to case, as there is a maximum voltage gradient between these two points if the case is positive. So, why not a rescue job? The case connection has to be at earth potential from an RF point of view to be useful as a screen, but why not connect it to the most negative point in the circuit, rather than positive? This may have the effect of making the whiskers grow back. Accordingly, I have today set up an experiment, using a Bush TR130, which has 3 AF117s and is a very accessible to work on. Starting with a working set, I moved the two screen connections for the IF amplifiers to V- (decoupled). There was no detectable change in performance, but care must obviously be taken to ensure that the transistor cases do not touch the IF cans. I then thought about a "rescue" job, and fitted an AF117 with a short collector to case to the mixer stage. In this case, moving the can connection to V- had no effect, of course. Fitting a 100k resistor in the lead restored normal operation of the set (much as it would if you just snipped the lead), but maintains the bias across the whiskers. I intend to keep this set permanently powered up to see if the short eventually clears - easily checked by shorting the 100k resistor. I'll let you know how this develops, but it may take a while. Possibly, to preserve the AFxxx devices for posterity, the case polarity with respect to the other transistor electrodes should be reversed. We'll see.

13th Nov 2005, 11:20 AM	#27
Paul Stenning Administrator Join Date: Dec 2002 Location: Hereford, UK Posts: 7,098	Re: Whiskers in germanium transistors etc The transistors I sent to NASA for analysis were UNUSED and had still grown whiskers, so it is nothing to do with voltage gradients (since there are none when it is not powered). If it was that simple, NASA wouldn't be investing in such a large and ongoing research project to find the cause. __________________ Paul Stenning Forum Admin/Owner and BVWS Committee Member

10th Mar 2006, 05:45 PM	#28
stephanie Heptode Join Date: Jul 2005 Location: New York City, USA Posts: 894	Re: Whiskers in germanium transistors etc So has anyone come up with a good modern replacement for the AF117s yet? __________________ Stephanie Weil New York City, USA "London calling at the top of the dial" -The Clash

10th Mar 2006, 05:49 PM	#29
Paul Stenning Administrator Join Date: Dec 2002 Location: Hereford, UK Posts: 7,098	Re: Whiskers in germanium transistors etc I beliebe the BF450 is a reasonable replacement. Do a search here for "BF450" and you'll get four threads on the subject. I have now added a video presentation about this from NASA to http://www.vintage-radio.info/whiskers/ __________________ Paul Stenning Forum Admin/Owner and BVWS Committee Member

10th Mar 2006, 06:03 PM	#30
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 7,038	Re: Whiskers in germanium transistors etc Stephanie, the answer seems to be that it depends on the circuit. The AF114-AF118 seem to be pretty much the same transistor but selected for particular characteristics and then marked. You often see AF12x transistors on eBay which should work in most cases. The silicon BF450 works in many cases, and Sean managed to sub the cheap'n'cheerful silicon BC214 here: http://www.vintage-radio.net/forum/s...ead.php?t=5884 Probably wouldn't work replacing the mixer/oscillator but you never know. I still have some dirt cheap BC214s available if you want some - send me a PM if you do. Best regards, Paul

10th Mar 2006, 06:06 PM	#31
howard Dekatron Join Date: May 2005 Location: Godalming, Surrey Posts: 3,032	Re: Whiskers in germanium transistors etc Hello Stephanie, I use AF125s to replace AF116/AF117 transistors. Theyre smaller than the originals but are germanium of similar construction and of the same era yet dont suffer from the whiskers problem. I recently bought a large stock of them at a very good price to keep me going for a while There are few of these still around, usally on eBay. I could let you have three if you dont go for the silicon BF450s which are just as good. Howard Last edited by howard; 10th Mar 2006 at 06:21 PM.

12th Mar 2006, 08:47 PM	#32
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton Posts: 6,342	Re: Whiskers in germanium transistors etc Paul - if you're still in touch with those people looking into this, then this thread may be of interest: http://www.vintage-radio.net/forum/s...ead.php?t=7941 Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?