Is this a record? - UK Vintage Radio Repair and Restoration Discussion Forum

lesmw0sec · 19th Jul 2014, 8:46 pm

I was given a nice Racal-Dana 9921 counter (faulty). Having set about the job I had to replace 1) CD4049, 1) 74LS145, 1)DIL relay, 1) 24V reg. 1) s/c tantalum.

The tant. was no surprise as they have a tendency to do it. That had destroyed the regulator, but I was surprised to have to replace the rest as there was no connection between them and it is certainly the first TTL device I ever had to change.

I wonder what could have caused all those bits to die? Now working very well

Radio Wrangler · 19th Jul 2014, 10:07 pm

It could be coincidence.

CMOS devices havle long been known to fail from corrosion of their aluminium metal top layer by phosphoric acid leached from their glass-passivation coat (glassivation) This needs moisture ingress. cMOS runs cold so humidity isn't driven out and so the devices degrade in use as in storage.

TTL families were thought immune, but that's only if used regularly to dry them out. Long term storage and the passivation and the humidity leaks will do their thing.

David

PETERg0rsq · 19th Jul 2014, 11:29 pm

I have an interest in repairing old Racal test equipment, and one thing you can be sure of is in a non working instrument, there will be several faults!

I put it down to:-

The instrument goes faulty, and it put to one side...for several years...waiting repair.

During this time, the tantalums turn into resistors, and the CMOS just give up the will to live.

We get hold of these instruments, and power them up, ........and see multiple unrelated faults!

Of course all that could be complete rubbish

but it allows me to sleep at night without worrying too much about it

On a 9081 I once recorded 14 different faults (some were related though) during a single repair!

Peter

peter_scott · 20th Jul 2014, 9:10 am

Quote:

Originally Posted by PETERg0rsq

On a 9081 I once recorded 14 different faults (some were related though) during a single repair!

Snap! ...and I think you need to undo and redo about 80 screws to get at them.

Peter

lesmw0sec · 20th Jul 2014, 9:39 am

You could all be right. This was an ex-BT instrument which after years of service was probably stored in less than favourable conditions. I have serviced a number of bits of kit from the same source and all of the instruments seem to have been on the shelf for some time - often with traces of mould starting to appear on the knobs. Very glad to get my hands on this one though.

Andy Day · 20th Jul 2014, 6:59 pm

Things do just die from old age! I recently powered up my old Toshiba Satellite laptop after about a 5 year rest in the loft, (The laptop, not me!). It was perfect when I put it up there. Dead as a dodo. It it had powered up I would have put it for sale at 99p to see what happened. But in my estimation it's not worth the effort.

But for the "record" I once replaced 144 of 214 TTL devices on a Mini computer CPU pcb. All done at a fixed price. One of many "loss leaders", the daft idea that due to poor cost analysis has downed many companies.

19th Jul 2014, 8:46 pm	#1
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,509	Is this a record? I was given a nice Racal-Dana 9921 counter (faulty). Having set about the job I had to replace 1) CD4049, 1) 74LS145, 1)DIL relay, 1) 24V reg. 1) s/c tantalum. The tant. was no surprise as they have a tendency to do it. That had destroyed the regulator, but I was surprised to have to replace the rest as there was no connection between them and it is certainly the first TTL device I ever had to change. I wonder what could have caused all those bits to die? Now working very well

19th Jul 2014, 10:07 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Is this a record? It could be coincidence. CMOS devices havle long been known to fail from corrosion of their aluminium metal top layer by phosphoric acid leached from their glass-passivation coat (glassivation) This needs moisture ingress. cMOS runs cold so humidity isn't driven out and so the devices degrade in use as in storage. TTL families were thought immune, but that's only if used regularly to dry them out. Long term storage and the passivation and the humidity leaks will do their thing. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

19th Jul 2014, 11:29 pm	#3
PETERg0rsq Heptode Join Date: Jun 2009 Location: St Helens, Lancashire, UK. Posts: 641	Re: Is this a record? I have an interest in repairing old Racal test equipment, and one thing you can be sure of is in a non working instrument, there will be several faults! I put it down to:- The instrument goes faulty, and it put to one side...for several years...waiting repair. During this time, the tantalums turn into resistors, and the CMOS just give up the will to live. We get hold of these instruments, and power them up, ........and see multiple unrelated faults! Of course all that could be complete rubbish but it allows me to sleep at night without worrying too much about it On a 9081 I once recorded 14 different faults (some were related though) during a single repair! Peter __________________ SPECIALIST.....Knows everything about nothing EXPERT..........Knows nothing about everything

20th Jul 2014, 9:39 am	#5
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,509	Re: Is this a record? You could all be right. This was an ex-BT instrument which after years of service was probably stored in less than favourable conditions. I have serviced a number of bits of kit from the same source and all of the instruments seem to have been on the shelf for some time - often with traces of mould starting to appear on the knobs. Very glad to get my hands on this one though.

20th Jul 2014, 6:59 pm	#6
Andy Day Pentode Join Date: Aug 2005 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 154	Re: Is this a record? Things do just die from old age! I recently powered up my old Toshiba Satellite laptop after about a 5 year rest in the loft, (The laptop, not me!). It was perfect when I put it up there. Dead as a dodo. It it had powered up I would have put it for sale at 99p to see what happened. But in my estimation it's not worth the effort. But for the "record" I once replaced 144 of 214 TTL devices on a Mini computer CPU pcb. All done at a fixed price. One of many "loss leaders", the daft idea that due to poor cost analysis has downed many companies. __________________ The older I get, the faster I was!