klem kedidelhopper wrote:
On Mar 11, 7:39 am, Sylvia wrote:
On 6/03/2012 10:53 PM, Sylvia Else wrote:

So if the transistor has a beta of around 40, the behaviour can be
explained by an intermittent open circuit at, or in the vicinity of, pin
10 of F IV.

Belatedly, I realise that this cannot be correct. If pin 10 of F IV were
to become open circuit, there would be no sink for the current through
the diode 1.4 St1, and the bases of AF178 and AF124, and the latter
would rise to the positive rail.

I'm left wondering about the voltage on the collector of AF121. If it
turned out to be -4V (which seems a plausible enough operating point),
then there could be an intermittent short across the windings of FII,
thereby connecting the base of AF126 I more or less directly to the
collector of AF121.

Sylvia.


Given the intermittent nature of this job so far I have trouble
actually saying that it's repaired, However I now have two periods of
time wherein it has operated without a problem for at least 14 hours
continuously, with a few hours powered down in between. It's never
done that since this problem began. So I am starting to feel a bit
more certain about the validity of this repair.

I reset the bias to 1.18V yesterday and last night I noted that it had
drifted up to 1.23V. It's not a big excursion but I will keep an eye
on it though comparing initial turn on with after a few hours running
over the next couple of days or so.

There are a few things I still have to do. All the can grounds
are.currently unsoldered and disconnected from the circuit.
After I log some more time on this radio I'll need to restore those
and see if that makes any difference. I don't expect that it will but
I want to take this one step at a time.

The other thing I want to do is remove FIV one more time and put back
the transistor, (AF126 II) that was originally there. In actuality
that transistor is an AF124 which I purchased from Grundig service in
Manhattan about 35 years ago. The AF126 II was the problem with the
radio when I got it from a friend back around 1975 and not having a
126, Grundig substituted the 124 for me. It never seemed to affect the
alignment so I see no reason to not use it over again. The replacement
transistors that I have been using through out this repair came from a
friend in Sweden and are likely as old as my originals anyway.

So as it turns out I didn't need to replace any of them. Therefore the
alignment shouldn't have been affected, nor should the replacement of
C38 have affected the alignment much if at all either.

Were it me, I would move on to other things.

If the radio repeats the symptoms, then it wasn't an
intermittently leaky C38. At that point, I would
open the investigation once more.

Hey, it's working. Be happy.

It occurred to me that if this were a customers radio, I'd likely own
their home at this point...

More likely, they'd own your shop.

--Winston