Hi Max,
I think the extra 3.9 ohm helps a tiny bit (makes the snap-off current to the TD somewhat stronger) but in my opinion its not the problem. Since at 15.5 V I suspected a wrong state of Q86/Q88 where Q88 should conduct I measured the waveforms at the bases of this comparator pair. The bases switch role with about 1 V voltage difference in both states. Roughly speaking 4.5 V vs 5.5 V and vice versa. The nominal Off value for base Q86 is 5.56 V and the nominal On value for base Q88 is 4.23 V (ignoring base currents).
R86 is easy to access from the bottom side. Less easy was to clip the probe tip at R89.
Hope this helps.
Albert
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On Thu, Jul 2, 2020 at 09:54 PM, unclebanjoman wrote:
Some progress.
Just now (in Italy it's 9.40 p.m.) I carefully measured the parallel R65/R68
with my Fluke 8040A, with a reading of 3.3 ohms. No good.
I carefully soldered a 3.9 ohm resistor, the only value I have, in parallel
with the other two. Not a carbon composition but a metal film one, for a quick
test it's enough.
Result: with +15V and the trim R90 fully clockwise the pulse appears and seems
good. At +15.1 V disappears again. Sgrunt.
I think there are some other resistors out of tolerance. I'll check the whole
"thing" quietly tomorrow night with great patience.
If other resistor's values are needed I will make a single order, the only
place near Italy where I can buy them (carbon composition, 5% 1/4 W) at decent
price is in G.B....
Otherwise I can get SMD resistors but I don't know if they are suitable. The
seller (in Italy) guarantees them up to 1GHz only.
Max
