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Subject: FW: [AH] Jupiter-8: Repairs

From: "Verschut, Ricardo" <ricardo.verschut@...>
Date: 2001-07-04

-----Original Message-----
From: Mike Kent [mailto:mikekent@...]
Sent: Tuesday, July 03, 2001 11:12 PM
To: Analogue Heaven AH
Subject: [AH] Jupiter-8: Repairs, Technical Details

Over the weekend I tried to trace a couple of problems with my JP-8. I wrote
this email to a tech friend. But I think he's on vavation so I send it to
the list now too. I welcome feedback from people with more experience than I

1. The modulation button in the bender panel sends LFO to the VCOs to effect
the pitch. When using this performance button the upper 4 voices are
receiving only about 1/3 of the amount of modulation as compared to the
Lower 4 voices.

I've pretty much decided that it must be a particular IC. It is a TL082 dual
OP-AMP. The input source to both sides is common. So the output should be
identical, right?. It seems like one side is only letting through a fraction
of the voltage compared to the other side. I haven't tested this with any
equipment, it's just my theory based on studying the symptoms carefully and
searching the schematics. Do my findings make sense? Is it likely that 1/2
of a TL082 would fail like this? Is a TL082 a common part easily replaced?

2. The VCF Frequency of the Upper 4 voices seems to be considerably lower
than the Lower 4 voices. The difference is definitely bad enough that I am
sure this is a failure, not just a calibration problem. The problem did not
exist in the past. The frequency of the filters slowly rises to match the
lower voices over about 15 minutes as the instrument warms up. It also
corrects itself if I force max voltage through from the VCF pedal input and
by raising the frequency to the max from the front panel; after doing this
the problem often is resolved at least in 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?

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.

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?


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