Rhodes Chroma Polaris

> From: Eric
> 
> I've got a Polaris on the bench with some issues. It came in 
> with a dying membrane, so the first order of business was to 
> replace that. Done and working.
> 
> Once I was able to control the machine again, I discovered 
> that all 6 voices fail autotune.
> 
> I hit Tune All and the computer disables all 6. LED doesn't 
> even blink - just lights once and goes out.
> When I manually re-enable the voices, both oscillators on all 
> 6 voices work and track the keyboard. They're just out of 
> tune and out of scale.
> Checked the calibrations on the PSU, DAC, ADC according to 
> the service manual, all are perfect to 4 decimal places.
> Checked the DAC by sweeping filter and detune, and all bits 
> seem to be working (no glitches).
> Watching the Tune buffer on the output board on my scope, 
> when I hit Tune all I see 6 waveforms flash by, then nothing.
> The output of Z62, the 4051 mux'ing the tuning standard 
> voltages to the DAC, looks right (at least, I can make out 
> different voltages in the pulse train, and they're all around 2.5-3V)
> The output of Z31, Channel Control for the Aux switches, 
> looks right (IOW, as I hit Tune All and probe each output, I 
> can see each one go high for a moment)
> 
> Thinking that perhaps the v.7 EPROMS in there had begun 
> failing, I burned fresh v.11 ones and installed them. No change.
> Everything in the machine seems to be behaving fine (as best 
> I can tell at this point) except auto-tune.
> 
> Dunno if this is relevant, but the headphone output seems a 
> bit noisy - it's a high-piched digital-sounding sort of 
> thing. It's low enough that it would probably disappear in a 
> mix, but it's pretty obvious straight out of the back of the unit.

The most likely culprit is a bad connection to the output board. When
tuning, the signal from each channel is routed to a different line (ALT
OUT) to the output board, where it is squared up and fed back to the main
board on the SYNTH ZCD line. It could even be a bad ground to the output
board, which could account for the audio noise, too.

-- 

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@...