[sdiy] bizarre Prophet 600 LFO

[Tom Wiltshire]

Bob,

The schematics I've got for it show the freq CVs all come out of a pair of 4051's. They seem to have their address lines connected via an offboard connector - that'd be a suspect, I'd have thought, or the 4051's themselves. The S&H caps is more likely if it were one voice or osc but not others.

If pitch is fine *except* when you add a bit of LFO modulation, that does seem to imply it's a digital/software problem, since the pitch CV is supplied by the uP pre-mixed (e.g. with all modulations applied, pretty much - polymod is done in the analogue domain). So if it works *sometimes*, then the hardware must be ok, since if the hardware were faulty, the pitch would always be whacky. Or is my logic faulty?

A broken modwheel pot or bad cal like Peter suggests would be another possibility. I had a slider on my SH101 which was fine at one end of the track, but which went open circuit if you moved it even a little bit. This had the effect of providing the maximum filter cutoff frequency. You might be seeing something similar with the mod wheel pot.

Some thoughts - hope some of them are some use to you.
Good luck,
Tom

On 29 Dec 2010, at 13:07, dragons wrote:

> Hi,
> I wonder if there is a calibration procedure for the pitch and mod wheels ?
> would be worth checking the sample and hold caps as I think these can sometimes cause problems if the value of them drifts. or am I thinking of another Prophet ?
> 
> regards Peter B
> 
> On 29/12/10 09:57, Bob Weigel wrote:
> > OK as if that isn't bizarre enough...ahem..I just fixed a Prophet
>> 600..had a bad LF357 dac I-V op amp. However...it's mostly working save
>> one thing near as I can tell. LFO works great on PWM and FIlter
>> modulation. But the pitch is all whacky. Once you move the wheel or
>> initial amount at all it immediately starts popping convulsive
>> transients and more modulation than you'd expect at the low amount of
>> modulation wheel you are giving it. I'm thinking the EPROM is corrupted
>> probably. Any other thoughts on that one?
>> 
>> 
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