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CS80 Strange Behaviour

 

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Today I had a CS80 in for a service. Basically most of the voices were working, but a couple were dead and there was the usual problem with unequal volume and filter cutoff. Nothing too drastic then. I found two dead oscillator chips, but have some spares, so that fixed that. So, after several hours work, all of panel 1 was sounding pretty decent.
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Then the audio just quit. No output. Nothing. I checked the fuses and power supplies, all appear good. Taking an oscillator card, I can see a key voltage on Pin 2 of the VCO chip which changes as I play the keyboard, yet no waveform output. Take another card, same thing.
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So, how about the white noise? It comes into the M card, but doesn’t leave the filters.
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Next thing – I find that the ground on the M card is referenced to GND2. If I meter between the GND1 and GND2, I see around 2k ohms. Confused??? So am I. So if I measure the 15V rail at the VCO chip, I see around +11V if I take the negative lead of the meter to the metal chassis.
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Where to go from here? Any ideas welcome.
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MTIA
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Lucid.
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I'd check if all the intended ground connections are intact. ?I've seen units where the +6.5/-8.5 (whatever it is LOL) volts, (or the other rail) become "decoupled" from each other when they lose their common ground connection. ?This could be due to a broken wire near a ring terminal, for example a screw on the wood chassis that connects these system grounds together.


jkjelec

On March 14, 2017 at 6:09 PM "'LucidSound' lucidsound@... [yamahacs80]" <yamahacs80@...> wrote:

Today I had a CS80 in for a service. Basically most of the voices were working, but a couple were dead and there was the usual problem with unequal volume and filter cutoff. Nothing too drastic then. I found two dead oscillator chips, but have some spares, so that fixed that. So, after several hours work, all of panel 1 was sounding pretty decent.
?
Then the audio just quit. No output. Nothing. I checked the fuses and power supplies, all appear good. Taking an oscillator card, I can see a key voltage on Pin 2 of the VCO chip which changes as I play the keyboard, yet no waveform output. Take another card, same thing.
?
So, how about the white noise? It comes into the M card, but doesn’t leave the filters.
?
Next thing – I find that the ground on the M card is referenced to GND2. If I meter between the GND1 and GND2, I see around 2k ohms. Confused??? So am I. So if I measure the 15V rail at the VCO chip, I see around +11V if I take the negative lead of the meter to the metal chassis.
?
Where to go from here? Any ideas welcome.
?
MTIA
?
Lucid.
?

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