Recently, Somebody Somewhere wrote these words
> > These are venerable transistors in a TO5 can. The board is a Siemens
> > part, which they have made 'obsolete' without replacing it. This is
> > hardly surprising, as the devices are batch dated '6940'
> >
> > The board is basically a set of interconnected discrete bipolar
> > logic gates, with some intermittent fault. I can find huge gain
> > mismatches, and I want to replace the low gain devices.
>
> I have to ask why you want to do that. Classically, those circuits
> were designed so that the low gain devices work just fine. Excessive
> gain can cause it's own problems. ??
I have this obsolete board from the Electricity Supply Board's Turlogh
(pronounced Turlock) Hill plant which has an intermittent fault. It has
diodes, resistors, and SST117s. I can eliminate the diodes, the
resistors, and so am left looking at the transistors.
When these boards classically went intermittent, what was the issue? No
capacitors here, beyond a few pF. There's 100pF across one resistor.
The caps don't leak. I can check and exercise them with Analogue
Signature Analysis.
>
> Also, it appears that the SST11x series of JFETs is a clone of the
> J11x series from a different time than the SST117. I think it's
> unlikely that a logic board would have been made from JFETs in 1969.
That's another 'red herring' correctly categorised, then :-)
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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