Korg Poly-800 not working

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crafter57
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Re: Korg Poly-800 not working

Post by crafter57 » Sun Apr 28, 2019 2:12 am

Ok, I'm back again with some good news!! The poly-800 is now firing up :) found the problem area to be between BR1 and 81c55, no connections on the Address lines between those two due to the fact that the bottom pads had fallen off(thanks to whoever did that), so I wire jumpered from BR1 and 81c55 address lines and BAM!! it now fires up.. alot of troubleshooting went into finding these issues.
I'm glad I didn't give up, BUT...I got no sound some thumping across the keyboard,
Either IC34 is not decoding or a problem with EG/TG circuits..?
so to try to save some time,
I tried to reload the factory presets but I never get the "Good" confirmation, it either stays in "Load" or "Err"
What's also odd is that while I'm playing the Wav file, the "Load" flickers and then each letter flashes like "L" on its own, the "o" in its own, then "a" and "d" etc... then all light up and stays lit...wierd! (also tried few a cables) never seen that before, also odd is that there is a socket on IC37 with a 4558 op amp instead of the M5223...not sure if this is a proper substitute or not but someone obviously tried swapping it out. I've swapped the CPU with the new one and still the same, so its not the cpu. I've checked the traces all gnd's, voltage and D20, D810 also seems right. Because I'm not sure that there are still sounds/presets I'd like to try to reload those first and if still doesn't work, then attack the other circuits...
what do you think?
Attachments
poly800display.jpg

Mooger5
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Re: Korg Poly-800 not working

Post by Mooger5 » Sun Apr 28, 2019 9:15 pm

Finally! Good news indeed crafter :)

Since the ZX Spectrum I have never found a tape loader that wasn't picky with the audio levels. And, the M5225 is optimized for single supplies. I'd replace that 4558 with the proper part.

The thumping sound is typical of a VCA in need of calibration. I can't access the servisse manual right now, but seeing there is a trimmer for resonance and another for cutoff, there is probably a trimmer for VCA calibration…

Good luck!
Herrare umanum est.

crafter57
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Re: Korg Poly-800 not working

Post by crafter57 » Mon Apr 29, 2019 1:50 am

Mooger5, I was afraid that 4558 part was wrong, seriously I don't know what gets into people's heads replacing parts without looking at the specs anyways, so I'll order the Mitsubishi M5223, it will take a week or two(hopefully less), I also had another mishap, I was caught up in the moment while testing and I forgot to remove the audio cable from my laptop to the poly and touched the D24 and PAF!! MY BAD, the diode blew.. I checked surrounding parts all seem good (I don't know why that happened?). so I have to replace the 1SS53(D24), but I think the 1N4148 may work but have to wait for the op amp anyways... ~Sigh~ more waiting... but I'm inching my way there slowly..
If loading presets doesn't work, I will try to program some sounds so that I can finish the calibration..
Ughh!!..one thing after another..
Keep you posted

Mooger5
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Re: Korg Poly-800 not working

Post by Mooger5 » Mon Apr 29, 2019 10:32 am

If the LM358 is easier to get you can use it instead. Sorry if you already ordered the M5223.
D24 can be replaced by the 1N4148 yes. According to the electronica.pt cross reference.
Just checked, no trimmer for vca cal.
It could be the eg the osc mix or the cutoff set to zero due to lack of presets.
If you have access to the parameters thenby all means program it.
Laptop smps adapters can leK ac through the audio. Next time power it off the bsttery
Hope this helps.
Herrare umanum est.

crafter57
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Re: Korg Poly-800 not working

Post by crafter57 » Fri May 03, 2019 1:49 am

This time its for real, the poly is now working and have sound :)
I ended up replacing D24 with 1N4148 and LM358 and it fired up again. I checked all signals from EG, TG waveforms and Filter and following the flowchart I wasn't getting VCF out, but I found another issue with VR2 & VR5 these were not the correct values??, I removed them and installed originals and got sound...FINALLY!!
I also had to rescue the NJM2069, I found pin1 & pin22 soldered to some other pins..(what a headache..) :banghead:
I haven't tried to re-loading the sounds yet but I programmed 1 sound and it worked, so I'm sure there are no sounds in memory.. I'll wait for the M5223 to arrive.
Just a question about the calibration procedure #3 on pp.21, it says noise level should be adjusted to 0.3Vp-p BUT if you look at the main schematic, block [L] "white noise" at the bottom, instead it shows white noise 3Vp-p
Do you know if its 0.3v or 3vp-p?
after this cutoff and resonance to adjust and I'm done(fingers crossed).

Mooger5
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Re: Korg Poly-800 not working

Post by Mooger5 » Fri May 03, 2019 9:13 am

[L] has to measure 3Vpp at pin 7 of the NJM2069. There's no way whatever the faint noise generated by the transistor with the floating collector reaches only 0.3 after the massive gain added by the two amplifying stages that follow.
The 0.3Vpp is to be read at a different location: pin3 of CN6 on some other KLM board? Directly connected to the output of the KORG chip, most likely.
0.3 might look weak, but it's yet to be summed with two oscillators and several voices of polyphony.
Cheers!
Herrare umanum est.

crafter57
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Re: Korg Poly-800 not working

Post by crafter57 » Fri May 03, 2019 9:38 am

right, I see it now. these are 2 different measurements for noise. CN6 goes to headphone jack and the other white noise to NJM2069 (its been a long day/night). I've programmed 4 sounds (#11-14) in it so I'll finish the calibration tomorrow. thx

crafter57
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Re: Korg Poly-800 not working

Post by crafter57 » Sun May 12, 2019 9:08 am

Well Mooger5, I finished the calibration and reloaded all sounds, seems to be working fine, no issues so far.
not sure if I did step#4 Master oscillator adj. correctly I'll have to re-check it again as I only have a digital chromatic tuner. All other voltage measurements are close to specs. So finally the Poly-800 is resurrected, I'm happy to have saved it.!!
now onto a few mods..

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