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

Pete Hartman pete.hartman at gmail.com
Sun Jan 26 07:12:32 CET 2014

Many thanks to Ian who pointed out that this was in fact a standard
expo converter with that op amp providing the fixed current.  My
pattern matching skills were looking at the circuit diagram and not
thinking about the circuit topology :/.

I found the problem with the clock from the HF VCO.  In addition to
the ground cut on one side, for some reason the tempco had been cut
away from the other side of the matched pair as well, effectively
isolating it from any input voltage...

With that restored, I am now clocking and passing audio.  There are
still some issues to be resolved with regen, but I think I have this
back on track.


On Fri, Jan 24, 2014 at 11:05 PM, Pete Hartman <pete.hartman at gmail.com> wrote:
> 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.
> http://elmegil.dynathome.net/~elmegil/IMAG1303.jpg
> Thanks for any thoughts...
> Pete

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