Hi,
I'm trying to troubleshoot my SH-7 with serial
670xxx (so looks like the first version).
I haven't had it for a long time and the first
thing I had to do was to replace the CV mode switch as it made the pitch
unstable.
I also had to calibrate the VCO2 waveshape for the
triangle wave as it was more of a saw than triangle, but it looks and sounds
good now.
When testing all the functions before putting the
synth in the stand, I noticed I can't get osc sync to work.
Instead of retriggering, it just seems to jump
between octaves.
To test this, I've enabled the sync switch, set
VCO2 to saw and put a slow LFO mod on it (also tried autobend). Only VCO2 volume
in the mixer.
No attack, max decay, max sustain, no release.
Simple.
If I press C3, what I get is a continuous note, as
expected, but something like:
C3----C4--C3----C4--C3----C4--C3----C4-- (with
slightly varying volume, which I can understand)
So, instead of retriggering the wave at every sync
pulse, it just seems to do nothing, until it can retrigger it often enough to
make it 1 octave higher, or lower.
I'm not an expert in analog circuits but I've been
trying to figure out how the VCO frequency generation works, and the reset
mechanism.
It seems the VCOs first generate a much higher
frequency (top octave?) than the final frequency. This is then divided down in
LM3216 (IC407 on VCO2), depending on the range and transpose switches settings.
The sync pulse seems to be affecting the oscillator frequency before the
divide-down takes place.
I think I have found where this frequency octave
jumping takes place though.
It seems to be at IC405 (DN819), which is a 4-pin
IC toggle flip-flop. As the "period retrigger" pulses from the same oscillator
reach the input, it toggles the output.
However, when the retrigger pulses get more
frequent and can occur more than once in a period of the master oscillator
trigger pulse interval, it of course toggles more than once during that period,
causing an increase in pitch.
Perhaps this is the way it should be but it looks
really strange to me.
It could be that this is just used to retrigger,
and not actually control the pitch, as this perhaps is generated separately (on
the lower half of the VCO schematic) and just uses the retrigger pulses
generated by the divider circuit.
I realize this is very difficult to help with, only
going by my description, but does anybody have any suggestion as to what might
be causing sync to not actually sync, but just make an octave jump?
Also, I've read that the SH-7 sync sounds
"different", but I haven't heard it so I'm not sure what I should try to
achieve...
Any SH-7 owners here with experience of failing
sync?
Long shot... :/
Cheers,
Peter