Hi ivp,
Can't you just simply monitor the machine to see what actually turns off
when it fails and work from there. In a case like this I would look at
the output stage, and the signals that control it and make a note of
them and then run the machine and wait until it fails. When it does you
can compare what you had when it was working with when it's not and move
bit by bit to the root cause of your problem. I would start by looking
at the output part of the circuit first. I do this all the time as some
of the stuff I work with is a few years old and complicated, or even
when I have something new but no drawings. You have the advantage at the
moment that the machine works for a period of time before it fails,
hence I would advise against doing anything until you get as much
insight into how the circuit operates as possible.
We have a few old catalogues here, I will have a look to see what I can
find about your transistor.

Best Regards
            Luis
  

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
Of ivp
Sent: 21 December 2009 08:15
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Help identifying transistor

>> from a good refit, clean and resolder
>>    
> Having done a fair bit of repair, this sounds somewhat risky to me...

I agree, although they haven't a lot to lose by at least trying. At the
moment they are renting a machine, at quite some expense, and to
buy a new one will give their accountant palpitations

> Yes, cleaning, adding a bit of flux, and reheating all/most of the 
> joints sounds like a plan (don't forget the flux to help clean up
> oxides)...
> 
> But seriously dismantling a circuit which sounds as seriously put 
> together as this sounds like asking for a trace lift or similar,
unless 
> you are *very* skilled and *very* careful

Generally it's put together solidly but only those transistors are hard
to get at. After a little more research into how ultrasonic welders work
these may be less like audio power types and perhaps more like high-
voltage TV or monitor line output transistors

> Plus, replacing semis 'just because' doesn't seem to me to be very
> useful - who is to say the newer ones are any better than the old ones
> Last I checked, semiconductors generally do not wear out... unless
> they are stressed beyond their design parameters

Well, you could be right, and I suppose (without knowing its service
history, specs or schematic) that if it's gone 26 years without a
problem
then components probably are comfortably rated. Everything looks
original, no apparent board scorching although a couple of power
resistors look very much the worse for wear. All ICs are socketed so
I'll check for bad contacts and so on. The whole back compartment
of the case containing the primary power supply was full, I mean full,
of fluff and dust blown in by the fan, so it could even be as simple as
poor ventilation
 
> Now replacing any electrolytic caps on the board (and anything else 
> which truly will age), that sounds like a plan (and could very well be

> your issue with that old of a unit)

Yes, I have been looking at the caps. Both the electrolytics and non-
polarised are all the old-style pig-tail mount, very large physically
for
their capacity. I note in the vintage radio articles in electronics mags
that the capacitors are the first suspects and the components most
likely to be replaced routinely
 
> In regards to your mystery part... I did do a bit of searching and 
> didn't come up with a helpful cross for that part.  Thought there
might 
> be an old SGS transistor book in my library, but no luck ...

Never mind, thanks for looking
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