UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

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.

Closed Thread
 
Thread Tools
Old 11th Nov 2005, 9:33 am   #21
YC-156
Hexode
 
YC-156's Avatar
 
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
Default Re: Whiskers in germanium transistors etc

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.
YC-156 is offline  
Old 11th Nov 2005, 1:53 pm   #22
Paul Stenning
Administrator
 
Paul Stenning's Avatar
 
Join Date: Dec 2002
Location: Cardiff
Posts: 7,349
Default 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 satellites 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
Paul Stenning is offline  
Old 11th Nov 2005, 2:01 pm   #23
Paul Stenning
Administrator
 
Paul Stenning's Avatar
 
Join Date: Dec 2002
Location: Cardiff
Posts: 7,349
Default Re: Whiskers in germanium transistors etc

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.
__________________

Paul Stenning
Forum Admin/Owner and BVWS Committee Member
Paul Stenning is offline  
Old 12th Nov 2005, 12:32 pm   #24
TNC
Hexode
 
TNC's Avatar
 
Join Date: Apr 2003
Location: Westcliff-on-Sea, Essex, UK.
Posts: 375
Default 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.
TNC is offline  
Old 12th Nov 2005, 1:03 pm   #25
ppppenguin
Banned
 
ppppenguin's Avatar
 
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
Default 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.
ppppenguin is offline  
Old 12th Nov 2005, 4:30 pm   #26
Leon Crampin
Octode
 
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,390
Default 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.
Leon Crampin is offline  
Old 13th Nov 2005, 1:20 pm   #27
Paul Stenning
Administrator
 
Paul Stenning's Avatar
 
Join Date: Dec 2002
Location: Cardiff
Posts: 7,349
Default 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
Paul Stenning is offline  
Old 10th Mar 2006, 7:45 pm   #28
stephanie
Hexode
 
stephanie's Avatar
 
Join Date: Jul 2005
Location: New Bern, North Carolina, USA
Posts: 346
Cool Re: Whiskers in germanium transistors etc

So has anyone come up with a good modern replacement for the AF117s yet?
__________________
Stephanie Weil
New Bern, North Carolina, USA
stephanie is offline  
Old 10th Mar 2006, 7:49 pm   #29
Paul Stenning
Administrator
 
Paul Stenning's Avatar
 
Join Date: Dec 2002
Location: Cardiff
Posts: 7,349
Default 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
Paul Stenning is offline  
Old 10th Mar 2006, 8:03 pm   #30
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 18,674
Default 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: https://www.vintage-radio.net/forum/...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
paulsherwin is offline  
Old 10th Mar 2006, 8:06 pm   #31
howard
Nonode
 
howard's Avatar
 
Join Date: May 2005
Location: Godalming, Surrey, UK.
Posts: 2,590
Default 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 8:21 pm.
howard is offline  
Old 12th Mar 2006, 10:47 pm   #32
Steve_P
Dekatron
 
Steve_P's Avatar
 
Join Date: Sep 2003
Location: Bolton, Greater Manchester, UK.
Posts: 6,615
Default 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:

https://www.vintage-radio.net/forum/...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...?
Steve_P is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:22 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.