-----Original Message-----
From: Mike Peake [mailto:
peake@...]
Sent: Tuesday, July 03, 2001 11:26 PM
To:
analogue@...Subject: Re: [AH] Jupiter-8: Repairs, Technical Details
At 2:11 PM -0700 7/3/01, Mike Kent wrote:
Hi Mike, I can offer my input from the peanut gallery...
>Is it likely that 1/2
>of a TL082 would fail like this? Is a TL082 a common part easily replaced?
It's certainly -possible- for this to happen as it's a dual opamp. Check the
output on both sides in a full poly program to see if it's the same value.
Also check the inputs for the same. It's a very common part.
>I'm little bit less certain about the source of this problem, but after
>studying the schematics for a while I think this is most likely caused by a
>4558 opamp that all voltage sources to the VCF go through. Do my findings
>make sense? Is it likely that a failed opamp would behave like this? Is a
>4558 a common part easily replaced?
My limited understanding is that in general, the 4558 is not a good part
to use in DC processing circuitry... It's an extremely common and easy
to find part. And it doesn't sound like an opamp problem, more like a
capacitor...
>3. I recently read online about a simple modification for JP-8. Someone
>suggests changing the opamps at the final output amplifying stage of a JP-8
>to a better, low-noise opamp (they suggest NE5532) to reduce background
>noise. This person never tried it on a JP-8 but did it on several other
>Roland products of the same period that suffer from the same type of
>background noise that I notice in the JP-8. Careful setting of output
volume
>and adjustments on my mixer can reduce the noise acceptably, but it would
be
>good to improve it more if possible.
Screw that 5532 crap and get an Analog Devices OP275.
>The JP-8 output stages have 4558 and 4556 opamps. Each voice uses several
>4558 and some TL-082. These particular TL-082 were individually tested for
>slew rate before installation at the factory. Do you think it is worth
>replacing the few opamps in the final output stages? Is worth the trouble
to
>then replace opamps in each of the 8 voices? Can I do these kinds of parts
>changes without damaging the overall warmth and sound of the JP-8?
The 4558 has a terrible slew rate and doesn't belong in the audio path in my
uneducated opinion. The 082 is barely acceptable spec-wise. Note that the
final output sounds like it does because of the parts chosen. The SEM is
filled
with 741s throughout the audio path, but it sounds sooooo good that I'd
never
want to change it. You could get a sharper, crisper output if you redesigned
the output section but would you want it any more detailed than the JP8
already is? An HF slope of +1dB across a wide spectrum can make something
sound bright in comparison to a flat output. On older gear such as flangers
and tape echos, swapping chips can make a big, desirable difference but in
others...let's just say that my Buchla 200 has 741s and 4558s in it's audio
path but I'm not about to swap them. I -may- use snooty opamps in the
matrix mixer kit I've got, but that might be the limit...
Easier, consult a tech,
Mike Peake, Your Psychic Friend
___________
Specifications:
"Do yourself a favor and put a microphone on that thing;
that thing sounds like shit." --DHS