-----Original Message-----
From: Kevin Lightner [mailto:
heaven@...]
Sent: Thursday, June 01, 2000 2:08 AM
To: Analogue Heaven
Subject: Re: [AH] Jupiter-6 Service Question!
>hey, while u guys are commening on the JP6 stuff
>could u shed some light on my ob8 prob? :)
>
>
>ok, here it is:
>With both oscillators off, and keys are played-
>hiss still bleeds through the outputs!
>& the oscs still sound but at greatly reduced volume..
>:0/
>
>This hiss is audible even with both oscs on -
>you can hear it in the background. It blends
>in with the sound.
>We checked all cables, connection, etc, no probs
>there.. Nothing on the circuit boards appears
>obviously blown. All of the chips have been
>firmly pressed into place.
>
>WOOHOO!
>
>I've swapped out the tlo82 & 3360 on the lower
>& upper boards with known good chips.
>The bleedthru problem remains.
>
>Any idea what could be causing this? It does
>the same thing on upper & lower boards with
>the lower board being more noticable.
>Not specific to one particular oscillator.
>also,
>The bleedthru noise is louder in unison mode
>on the lower voice board (voices 1-4)..
>and is only heard whenever a key is pressed,
>not constant.. hmmmm..
A bad cap possibly? (They commonly make tons of noise in bad MKS80's)
Have you disabled the noise source internally or used a dual trace
scope to compare the raw noise source with the output noise?
You need to verify it's not the noise source bleeding somewhere.
Have you set a scope to AC and checked the power supply rails at high
scope gain?
The noise may be on the DC rails either from a bad power supply or
another component elsewhere.
Problems like these are "divide and conquer" type probs. Isolate
sections as best you can to eliminate them as the source and home in.
A few tips anyways. :)
K