Vintage Synth Repair

I think I said I'd put my money on a duff fifo chip - I win! I'd lost the middle 4 bits of the 11 bit word, which I realised last night when I was looking at the demultiplexed audio and seeing it stratified on the scope - a sure sign of missing bits.

Yet another cheap buy fixed up. That's a Linndrum and a Harmonizer resurrected in the same week. Next up a Binson Echorec - real old school! 

--- In vintagesynthrepair@yahoogroups.com, "Susie" <gardens@...> wrote:
>
> I need to give this a rethink as I'm starting to understand a bit more on how the fifo array works, and I think the noise I'm seeing is of a low amplitude and not the same noise I'm hearing on the output. Rather than 2 fifo arrays, the 6 chips are connected as a single 12 bit 32 word fifo containing 2 sets of data multiplexed down the single fifo. The queue is only long enough to hold a fraction of a waveform, so I think a lot of the logic of 'filling in the gaps' is handled by the cpu feeding the data into the array. The output of the fifo goes straight to the DAC, which I know works as I swapped it with the delay line DAC. The output of the DAC is then 2 multiplexed audio signals, and its the demultiplexing of this signal which is now receiving my attention.
> 
> btw, strictly speaking, this thread is off topic, as it's not a keyboard instrument, but as you can play the harmonizer with a keyboard CV then I'm hoping you'll forgive me.
> Susie 
> 
> --- In vintagesynthrepair@yahoogroups.com, "Susie" <gardens@> wrote:
> >
> > hmm...
> > The behaviour tonight was somewhat different. The noise no longer disappears, but does go up in pitch. I traced the signals coming in to the fifo array and all the clock signals are clean, however there appears to be a lot of noise coming in from somewhere on the various lines that enable the fifo circuitry - I'm still trying to figure out where the noise starts.
> > 
> > Would I be right in thinking that if the input of a nand gate is noisy then the source of the noise is on the input side and not the output side? ie is it possible for noise to leak backwards through the output of a nand gate to the input? I grounded the inputs and the noise instantly disappeared, but maybe all I was doing was disabling the fifo array.
> >
>