[sdiy] Re: Memorymoog VCA/VCF (was 4803 Project Idea)

[Tim Stinchcombe]

Hi Brian, Aaron,

> Output OTA:
>   Even with the control current corrected, there was no audio at the 
> output.
> I traced it through to the OTA at the output and what troubles me is that
V+ 
> and V- at the OTA are identical (phase and magnitude). So the audio is
being 
> zeroed at the output. What really confuses me is that by following through

> the schematic I do not see how they would be different.

The common DC bias level of about 6V will be present at *both* outputs of
the 'instrumentation amp' op amps (i.e. it does *not* get subtracted out),
and so I hope you are looking for the AC signal component being (probably)
less than 10mV or so superimposed on this (and out of phase of course)?

Other than that, perhaps a wiring mistake or blown component? I note that
the service manual is readily available on the web, and as well as saying
how to set up the two trimpots (which for starters I'd set at roughly the
same DC level as the op amp outputs), also includes a nice troubleshooting
section, which might be of use to you!!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk