Synth-DIY Yahoo! Groups Archives

Korg poly-800 dead display

2019-03-10 by studio_1a@...

Hi everyone, so after replacing many parts and qualifying all voltage rails on the KLM596 board, I fired it up and the display is not turning on. I swapped the original KLM596 board with a spare and the display comes on, so I know the connector/harness and display board are good. I checked all timing signals / address lines from the CPU to the 81c55 with my scope and they are present on the 81c55 but there is no output on PC and PB ports or T/O, so i suspect its b/o unless something is loading it down? any suggestions? all traces and lines were qualified between IC24-IC25-IC30-IC31.

The IC is marked as MSM81c55RS, do I need an exact replacement or will any other variations do?

thank you

Re: [korgpolyex] Korg poly-800 dead display

2019-03-10 by support

It may well be something else that is stopping the display from operating. Such as for example, the CPU bus lines, decoders, latched and supporting DTC's etc need to all be working for anything at all to happen.

Have you checked the troubleshooting procedures and gone through them? And the main circuit descriptions?

You could start by checking the CPU RES_7.5 interrupt line. If the clock going into that isn't working, you won't get any display either.

One thing worth testing. Does pressing a note cause ANY change in the audio output? Does pressing a note result in a MIDI note on output?

/Mike

On Sun, 2019-03-10 at 03:15 +0000, studio_1a@... [korgpolyex] wrote:

Hi everyone, so after replacing many parts and qualifying all voltage rails on the KLM596 board, I fired it up and the display is not turning on. I swapped the original KLM596 board with a spare and the display comes on, so I know the connector/harness and display board are good. I checked all timing signals / address lines from the CPU to the 81c55 with my scope and they are present on the 81c55 but there is no output on PC and PB ports or T/O, so i suspect its b/o unless something is loading it down? any suggestions? all traces and lines were qualified between IC24-IC25-IC30-IC31.
The IC is marked as MSM81c55RS, do I need an exact replacement or will any other variations do?

thank you

Re: [korgpolyex] Korg poly-800 dead display

2019-03-10 by studio_1a@...

Hi Mike,

yes, I've gone thru the troubleshooting procedure and circuit description also checked the waveforms. I do have CPU RES_7.5 waveform and much of the other waveforms listed for each section, but do not have RES_6.5 & RES_5.5 and also have no tone. D/A adjustment procedure #2 says to adjust VR4 to obtain 4V out of IC38-7, but BR9 not outputting anything either and 81c55 PA ports are all Low so, IC38 is not supplying voltage to EG cct. for the tone.

I'm getting lost here..

What shall I check next?