Tin whisker growth on potentiometer - UK Vintage Radio Repair and Restoration Discussion Forum

3nf · 20th Jan 2013, 7:48 pm

This Cossor 1052 oscillograph had various faults; no Y shift, focus and brightness problems. This was found to be due to a short circuited Y shift pot. On removing it, it was still shorted, so I removed the back cover. The fault then went away. There are tin whiskers inside the cover which were shorting both the ends of the pot together, but not the wiper.

Anyone had this fault?

Refugee · 21st Jan 2013, 7:15 am

There have been many posts about fly by wire car accelerator pedal tin whiskers including lawsuits about cars going full throttle due to the same problem.
Take a web search and have a read.
The pot on the pedal has the same problem.

Radio Wrangler · 21st Jan 2013, 9:46 am

It's a very well known problem and has got a lot worse since the switch-over to unleaded solder. Though the Cossor scope far pre-dates that.

Some applications (aeroplanes but not cars) are still allowed to use lead in their solder.

Tin dendrites will grow along surfaces quite easily, and even upward across fresh air. All they need it the presence of tin and a bit of electric field to impel them.

David

Nickthedentist · 21st Jan 2013, 12:22 pm

No, I've never had it on a pot. Rather scary about car accelerators, as I think the vast majority if not all are "fly by wire" nowadays.

M0FYA Andy · 21st Jan 2013, 1:01 pm

I spent my career as a systems engineer working on military aircraft fly-by-wire flight control systems. I remember a visit back in the mid-80's by a couple of engineers from one of the motor-car manufacturers, I think it was Jaguar, looking for any advice we could give on using similar technology for steering and braking sytems on their products.

They were quite taken aback when it was pointed out that a military aircraft flight control system typically uses four lanes of sensing and computing, with cross-monitoring between lanes to identify and isolate failures. They didn't see how a system in a motor car could ever be more than simplex, on cost grounds.

The thought frightened me then, and it still does!

Andy

ScottBouch · 11th Feb 2013, 10:08 pm

Quote:

Originally Posted by M0FYA Andy

military aircraft flight control system typically uses four lanes of sensing and computing, with cross-monitoring between lanes to identify and isolate failures.Andy

One reason why our Lightning would never be allowed to fly in the UK... Lack of redundancy in the control systems. 15 tonnes of Mach 2 in private hands is another reason!

I'd not come across this property of tin before, it's fascinating! What age do you estimate your pot to be? -ie what growth rate do these whiskers have?

Cheers, Scott

jhalphen · 12th Feb 2013, 6:12 pm

Hello to All, 3NF,

Read the full story on tin whiskers and by none less than NASA itself!

http://nepp.nasa.gov/WHISKER/background/index.htm
http://nepp.nasa.gov/whisker/photos/index.html#semi

Makes you shudder...

Best Regards

jhalphen
Paris/France

kalee20 · 12th Feb 2013, 6:24 pm

Quote:

Originally Posted by jhalphen

... by none less than NASA itself!

... with input from Paul Stenning!

3nf · 12th Feb 2013, 8:27 pm

Hi Scott

The tin whiskers on the Cossor 'scope had 50 odd years to grow. The set has only been running for a couple of hours in the last 5 years, so can't be attributed to an electric field.
The first time I heard of this was when bronze anodes on 100KV electron beam microlithography machines grew whiskers and shorted out the EHT. These bronze anodes were quickly replaced with stainless steel. End of problem.
chris

McMurdo · 14th Feb 2013, 12:36 am

Most car accelerators have 2 pot tracks that have to track each other at all times. If the signals disagree then on goes the engine light and limp mode soon after!

I think military and medical devices must change to ROHS ?

60 oldjohn · 14th Feb 2013, 10:02 pm

Just seen an interesting article on the Net, Please Google, 125mph nightmare.
John.

G8HQP Dave · 21st Feb 2013, 2:58 pm

I don't know if this NPL webinar is of interest to anyone. You need to book, but it is free.

20th Jan 2013, 7:48 pm	#1
3nf Triode Join Date: Jul 2006 Location: Thetford, Norfolk, UK. Posts: 22	Tin whisker growth on potentiometer This Cossor 1052 oscillograph had various faults; no Y shift, focus and brightness problems. This was found to be due to a short circuited Y shift pot. On removing it, it was still shorted, so I removed the back cover. The fault then went away. There are tin whiskers inside the cover which were shorting both the ends of the pot together, but not the wiper. Anyone had this fault? Attached Thumbnails __________________ He's gone to make his meter.

21st Jan 2013, 7:15 am	#2
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Tin whisker growth on potentiometer There have been many posts about fly by wire car accelerator pedal tin whiskers including lawsuits about cars going full throttle due to the same problem. Take a web search and have a read. The pot on the pedal has the same problem.

21st Jan 2013, 9:46 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Tin whisker growth on potentiometer It's a very well known problem and has got a lot worse since the switch-over to unleaded solder. Though the Cossor scope far pre-dates that. Some applications (aeroplanes but not cars) are still allowed to use lead in their solder. Tin dendrites will grow along surfaces quite easily, and even upward across fresh air. All they need it the presence of tin and a bit of electric field to impel them. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done Last edited by Radio Wrangler; 21st Jan 2013 at 9:47 am. Reason: forgot one sentence!

21st Jan 2013, 12:22 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Tin whisker growth on potentiometer No, I've never had it on a pot. Rather scary about car accelerators, as I think the vast majority if not all are "fly by wire" nowadays.

21st Jan 2013, 1:01 pm	#5
M0FYA Andy Nonode Join Date: May 2007 Location: Preston, Lancashire, UK. Posts: 2,511	Re: Tin whisker growth on potentiometer I spent my career as a systems engineer working on military aircraft fly-by-wire flight control systems. I remember a visit back in the mid-80's by a couple of engineers from one of the motor-car manufacturers, I think it was Jaguar, looking for any advice we could give on using similar technology for steering and braking sytems on their products. They were quite taken aback when it was pointed out that a military aircraft flight control system typically uses four lanes of sensing and computing, with cross-monitoring between lanes to identify and isolate failures. They didn't see how a system in a motor car could ever be more than simplex, on cost grounds. The thought frightened me then, and it still does! Andy

12th Feb 2013, 6:12 pm	#7
jhalphen Hexode Join Date: Jul 2004 Location: Paris, France Posts: 350	Re: Tin whisker growth on potentiometer Hello to All, 3NF, Read the full story on tin whiskers and by none less than NASA itself! http://nepp.nasa.gov/WHISKER/background/index.htm http://nepp.nasa.gov/whisker/photos/index.html#semi Makes you shudder... Best Regards jhalphen Paris/France

12th Feb 2013, 8:27 pm	#9
3nf Triode Join Date: Jul 2006 Location: Thetford, Norfolk, UK. Posts: 22	Re: Tin whisker growth on potentiometer Hi Scott The tin whiskers on the Cossor 'scope had 50 odd years to grow. The set has only been running for a couple of hours in the last 5 years, so can't be attributed to an electric field. The first time I heard of this was when bronze anodes on 100KV electron beam microlithography machines grew whiskers and shorted out the EHT. These bronze anodes were quickly replaced with stainless steel. End of problem. chris __________________ He's gone to make his meter.

14th Feb 2013, 12:36 am	#10
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,271	Re: Tin whisker growth on potentiometer Most car accelerators have 2 pot tracks that have to track each other at all times. If the signals disagree then on goes the engine light and limp mode soon after! I think military and medical devices must change to ROHS ? __________________ Kevin

14th Feb 2013, 10:02 pm	#11
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,987	Re: Tin whisker growth on potentiometer Just seen an interesting article on the Net, Please Google, 125mph nightmare. John.

21st Feb 2013, 2:58 pm	#12
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Tin whisker growth on potentiometer I don't know if this NPL webinar is of interest to anyone. You need to book, but it is free.