Hi Gorden, i was wondering if you'd reply :)
Right, none of the diode pairs light up when i play a note but when i hold down keys and look at the outputs from the 4051 at IC12 on a scope, they all peak at around +4v, sustain at +1 and then drop to about -4v at rest. Theres all kinds of junk at pin 3.
On the 4053 at IC6, pins 1 & 2 seem to have a pulse of around 0.5v when i play a key and pins 12 & 13 do the same but more like 0.75v.
One thing that might show a problem is that while pin 20 of the MSM5232 (CLK1) shows the correct clock input on my scope, at pin 19 which is meant to be CLK2 and have the same waveform all i'm getting is a constant +5V or so.
All of the other test points from the poly800 mk1 schematic seem more or less correct, if not quite as square cornered as on the diagrams.
Thanks for your help.
>
> Okay, either the DCOs aren't running or they are not making it out to
> the filter. Note that I'm working from a Poly-800, not a Poly-800 Mk 2,
> but most of the guts should be the same.
>
> When you play a note, one of the little pairs of diodes in the row of
> eight pairs that stick up should light. This feeds the VCA control
> voltage for each oscillator to the MSM5232. It goes through one of the
> notorious 4051 multiplexers. If you find there's a signal on pin 3 of
> IC12 but not much on the outputs, you've got a dead multiplexer.
>
> IC6 is a 4053 multiplexer, used to switch the square/saw outputs. Check
> to see if pins 1, 2, 12 and 13 have any sort of signal when you play
> notes. If not, guess what? Another stuffed mux.
>
> The only other thing I can think of is that there's something amiss on
> the digital side, and the MSM5232 is never seeing a /WR pulse from the
> address decoder. That's going to be a flowery twat to diagnose without
> an oscilloscope.
>
> Gordon
>