[sdiy] Prophet 10 op-amp swaps?

[Dave Manley]

Ah, good I was confused by the earlier comments about 560 ohm output resistors.  If you trace through the circuit from these op amps thru the 3280 to the outputs where do you see the oscillation?  Using U<number>.<pin number> is a concise easy way to document.  

Is it really only on the upper eq?  Are the eq settings the same between the upper and lower eqs? Does changing the eq have an effect on the oscillation?

If you have some small (10 to 100) of caps, does connecting them from pin 2 to pin 6 change the oscillation?

You could also try adding a capacitor from U820.3 or U821.3 to ground. Try some values around 0.1 to 10 uF.

In these experiments keep the cap leads short.





Eric Frampton <eric at ericframpton.com> wrote:
>We're looking at the same thing; jump down to pages 29 and 30, the EQ
>section, and you'll see 'em.
>
>
>
>On Jul 31, 2013, at 8:12 PM, Dave Manley <dlmanley at sonic.net> wrote:
>
>> I'm looking at page 27 of the PDF which has the upper and lower eq,
>but not U820 or U821?
>> 
>> Are we looking at different schematics?  The ones from cem3374.com?
>> 
>> Eric Frampton <eric at ericframpton.com> wrote:
>>> Hi Dave -
>>> 
>>> The op-amp(s) in question are U820 and U821, used for the Upper and
>>> Lower EQ's.
>>> 
>>> I've installed sockets for all of these, yep. I figured, worst case,
>I
>>> could always put the original op-amps back in if this experiment
>didn't
>>> work, or if someday I had to sell the keyboard and someone else
>wanted
>>> it completely stock.
>
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