[sdiy] Prophet 10 op-amp swaps?

[Dave Manley]

BTW, what frequency is the oscillation, and which opamps ( U numbers ) have been changed?


Dave Manley <dlmanley at sonic.net> wrote:
>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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