[sdiy] Question about an expo pair in a Time Machine

Pete Hartman pete.hartman at gmail.com
Sat Jan 25 06:05:59 CET 2014

I'm working on a Blacet Time Machine (the latest of several that I've repaired).

This one appears to have issues in the HF VCO section that drives the
clocks for everything else (the filters and the MN3005 delay).

Digging further, while adjusting the delay pot I got a decent voltage
swing (around 19mV to around 115mV) at the base of the input to the
PNP matched pair (an LS358-71) but there's also an op amp section in
this expo that I don't understand well;  while my working module
swings 2.3V to .91V on the op amp output as I change the delay pot,
this one was steady at a bit over 13V.

Examining more closely, I saw that the base of the other side of the
matched pair had a trace cut so it was floating instead of at ground.

I bridged the cut and I don't believe I shorted anywhere else (checked
with my beeper) but I'm getting the same results: 13V and no voltage
change at that op amp and no correct oscillation at the output of the
VCO (two stages of an LM13600 with necessary parts).

My expectation is that the matched pair is dead, due to the base being
ungrounded while we were trying to figure out what wasn't working.
Unfortunately I don't have a spare, so I'll have to find one.

I'm looking for some confirmation about whether this actually seems
likely to be a way to kill a matched pair or not?

I need to think more about the role of the op amp in this version of
the expo converter too, so I can understand it better.

Here's a link to the relevant fragment of the schematic; the pencilled
in numbers are the nominal values at each end of the delay pot's swing
taken from my working module.


Thanks for any thoughts...


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