On Mar 21, 2013, at 8:17 AM, mrmuchomusic <
musicalfix@...> wrote:
> HI all, this is my first post on here and my first CS80 repair. I wonder if anyone can shed any light on this repair I am hoping to carry out. This unit was mostly working fine, before the upper frame fell down and shorted out on to the keyboard with a bang and some smoke :-/.
> I have identified and replaced a few blown 4000 series ICs, mostly on the KBC boards.
> When switched to panel mode the only audio output I get is when the sine wave (which has a another tone, falling in pitch behind it) or VCF level faders are raised.
> There are no saw or square wave audio ouputs, although I checked MII and could see them present on the VCO ICs that I have checked, the frequency does vary in sympathy with the keys, they seem to disappear around CP3 on the voice card.
> Both upper and lower voices are the same.
Welcome to the group Andy! This should be easy to trace/fix. Are the power supplies all good now? Do yo have noise? I'm guessing not, which means you can't hear the filter output. CP8 is the filter output which just jumpers to CP3. You said there's no signal there? You're on the right track in tracing it. CP2/CP7 is the output of the high-pass into the low-pass. Anything there? The only other thing I can think of that's common to all of this are the f1/2, q1/2 control voltages coming in. Check these when on panel and see if they follow the knobs.
>
> I'm not seeing much in the way of channel outputs on the DAC on the KBC. I have tested all the logic on the input side of the DAC and I have swapped the YM26700 for the one on the KAS board and there it produces between 0 and 4.2volts, as does the original.
> I would appreciate any further suggestions on places to check.
The KBC is just for the volume/filter curve for the high/low notes. If the Keyboard Control levers near the master volume are all in the middle, there won't be any KBC output.
Good luck and let us know how it goes!
David