Mystery solved: It was the ENV circuitry that caused the four muted voices on my 707. IC39 ( TL064 ) of the ENV is a quad op-amp handling the envelope signal for the very four muted voices. I changed it with a new TL064 and now the thing is ticking like a charm.
TL064 price ca. one Euro.

Bo

--- In tr-707@yahoogroups.com , "b_dk2001" <b-ankjaeroe@...> wrote:
>
> Hi Oli and Florian,
>
> We definitely have an error pattern here that seems to appear in many 707s, you see, my 707 has muted instruments, too. On mine the HH, BD and SD are totally muted and the MT is partially muted. Otherwise the unit is perfect.
> I've lab-tested my unit (very time consuming) with an oscilloscope in all analog signal paths on the PCB and the erratic behaviour.
> So, Oli, where ever you have your diagnose from, the fault seems to have it's origin in the ENV circuitry. I can confirm that based on my exact lab-tests.
> This is where I am right now. I'll return with further testresults when I get there.
> If we study the svc-manual as for the ENV-circuitry, we learn that it comprises of these components:
> DAC (IC31, Q19),
> latch (IC27),
> Q21-Q28
> IC38 and IC39
>
> The svc-manual has no test procedure for this particular section of the voicing board, so we're "on our own" here, but I'll return when/if I discover something important.
> Electrolytic caps are well known to be error prone, but none is used here.
> Right now I'm testing for correct accent CV output from the DAC (IC31).
> Wish me luck :-)
>
> Best
> Bo
>
>
>
>
>
>
> --- In tr-707@yahoogroups.com , "oli_larkin" <olilarkin@> wrote:
> >
> > Thanks florian. I took out the fader and tested it. Seemed ok. Also the problem is on the seperate out andthe mix out and is there when at high or low volume. My suspicion now is that it is a component in the hihat envelope circuit
> >
> > Oli
> >
> > --- In tr-707@yahoogroups.com , Florian Anwander <fanwander@> wrote:
> > >
> > > Hi Oli
> > >
> > > > Anyone got any ideas what I should check
> > > Either the switch of the HH-single out socket or the HH-fader.
> > >
> > > Florian
> > >
> >
>