Korg Poly-800 not working

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crafter57
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Re: Korg Poly-800 not working

Post by crafter57 » Mon Mar 18, 2019 9:09 pm

I also replaced IC29 by the time I finished it was 3am, tested it and still nothing. I started to read the CPU datasheet just to see which were inputs and outputs, curiously some outputs were not present right from the cpu...note: I socketed the cpu because someone else's attempt to remove it failed miserably...I wonder if the cpu may have been compromised due to heated pins or something.
well I'm waiting for spare parts, so I'll take a break on this one for now.

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Re: Korg Poly-800 not working

Post by Mooger5 » Tue Mar 19, 2019 9:36 pm

Was hoping IC29 would at least allow the display to work.
IC40 generates the main reset signal to the CPU. I think the others (5.5, 6.5, 7.5) are provided by the peripherals so the CPU knows everything´s ready to function as a system. Looks like the other sections of the circuit must not be neglected.
The CPU is a regular microprocessor chip BTW. No masked ROM inside. So it may be easy to find.
The datasheet is useful, but the full version of the Service Manual at the Polynomial site is better, I think.
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Re: Korg Poly-800 not working

Post by crafter57 » Wed Mar 20, 2019 2:39 am

Yes, I do get 4.4v on pin 36 of CPU [Reset] but I still don't get pulse on pin8 [Res6.5], I changed DTA1, that did nothing, Also I read the service manual it says pin4 [SOD=serial out data] is output, so its supposed to send a pulse out to IC28 [/PR] pin4, to enable output on /Q pin6 this will enable the pulse coming in from IC36 pin1. I also changed IC28, but nothing. The pulse from IC36 is present on PIn1, depending on something trivial..? [SOD] is not sending pulse out. maybe this is why cpu is on standby and other signals are missing??

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Re: Korg Poly-800 not working

Post by Mooger5 » Wed Mar 20, 2019 5:16 pm

:roll: I hope the CPU is one of the parts due for replacement!
crafter57 wrote:I still don't get pulse on pin8 [Res6.5], I changed DTA1, that did nothing
DTA1 should transfer the /IRQ from IC26, the MIDI interface. Is there a signal on pin7? If not, then the power rail [A] is probably missing. Or, Timer signal TC missing from IC25 or not reaching IC26. Or a broken track.
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Re: Korg Poly-800 not working

Post by Mooger5 » Wed Mar 20, 2019 5:22 pm

crafter57 wrote:The pulse from IC36 is present on PIn1, depending on something trivial..? [SOD] is not sending pulse out. maybe this is why cpu is on standby and other signals are missing??
IC36 looks to be free-running. Provided the Tempo control on the front panel is connected, I think. IC28 and SOD must be there to synch to MIDI. When ext clock is used, I guess IC36 is ignored.
So another priority for IC26.
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Re: Korg Poly-800 not working

Post by crafter57 » Thu Mar 21, 2019 5:32 am

Ok, its starting to make sense why the missing pulse on RES_6.5[pin 8] then, there is no output pulse on TC [pin 6] from 81c55, thats why /IRQ is not pulsing out thru DTA1 to reset RES_6.5, and looks like RES_6.5 is priority-3 and RES_5.5 is priority-4 and it may not be re-setting until RES_6.5 does(I hope)? It definitely starting to look like IC25[81c55] is b/o. I traced all the lines between chips and qualified the voltage rails as well. I do get a good pulse on RES_7.5. I did order a replacement cpu 80c85 and 81c55, now the task of pulling out the 81c55 and put a socket there and wait until the IC comes in, I'll keep my fingers crossed. Geez.. the possibilities are endless when the display doesn't come on. if you think of anything else for me to verify, please let me know and THANK YOU for reasoning things out.

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Re: Korg Poly-800 not working

Post by Mooger5 » Thu Mar 21, 2019 12:37 pm

WaitDont desolder anything yet. Google The Strange Casr of the Poly800. They repaired one withe same problem as yours. We forgot the adress latch. Ive just now verified the sm and it makesabsolute sense. Replace that firsy. Gpod luck.
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Re: Korg Poly-800 not working

Post by Mooger5 » Thu Mar 21, 2019 6:34 pm

http://retroactivesynth.com/the-strange ... -poly-800/

AD0 to AD7 lines are adress and data lines, multiplexed to save board space and output / input ports. It makes sense that the 81C55 is receiving signals from these lines, but isn´t outputting anything because the ALE control line isn´t engaged. Note the ALE goes from the CPU to the 81C55. Note also the CLK line from IC23 linked to the ALE line. Depending on its state it tells the 81C55 when to read the AD lines as either adress or data. Hopefully once the 81C55 starts reading the AD lines as data the display and D/A functions will work!
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Re: Korg Poly-800 not working

Post by crafter57 » Thu Mar 21, 2019 8:13 pm

You know after I woke up, I still wondered if I didn't overlook something, I will go read that link
and it would make total sense, I will check all of this again later and give you an update

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Re: Korg Poly-800 not working

Post by crafter57 » Fri Mar 22, 2019 3:40 am

So after I replaced IC23, the verdict is....it's still not working..Sigh..
so I will wait for the parts to arrive and hopefully one of those will bring it back to life..
taking 2 tylenols to soothe my headache..

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Re: Korg Poly-800 not working

Post by Mooger5 » Fri Mar 22, 2019 1:11 pm

Why worry. They replaced the 81c55 prior to ic23. It looks like nothing but a new 81c55 will put out the timer code necessary to complete the reset, but ic23 would likely have to be replaced anyway. Helps to keep the focus going. When parts take too long to arrive it starts to drift.
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Re: Korg Poly-800 not working

Post by crafter57 » Tue Apr 16, 2019 4:21 am

Hi Mooger, well I received both the cpu and the 81c55, installed the cpu first..nothing happened, then installed the 81c55 and still nothing happened..I guess some other bug exists on this board one level higher than that website you linked earlier. Still no signal on /TC [timer out] and same missing signals on the cpu as previous. Maybe I should just start removing chips one by one and hope to get lucky..

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Re: Korg Poly-800 not working

Post by Mooger5 » Tue Apr 16, 2019 11:27 am

What. Not even the display?
IIRC I had had ran out of ideas.
On that site they managed to fix the synth only by loading up the patches via audio file. Curiously the service manual hints at that. Maybe something in the code. That was after they threw everything at it.
Try that even without the display. Experiment with different audio levels.
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Re: Korg Poly-800 not working

Post by crafter57 » Thu Apr 18, 2019 4:08 am

I was super excited to try either IC's but then was pretty much crushed that it still failed, i threw the towel at it and walked away!!, the next day I came to my senses and sat in front of it, looked and found an oddity which I wasn't sure how to treat it until I started probing, that open trace with the jumper is [D] +5V and looks like the top portion of the board IC's and [IC3-Vdd2] were not powered, I soldered it confirmed the voltage, DTA1 shows correct voltage now but still something missing CPU RES6.5 (restart interrupt) and still no /TC from 81c55 and still no display but more signals have appeared that were previously missing, so I'll go over the board again minutely to see if any other oddities exist. The prints are general and don't show the changes for all revisions of boards which makes it harder to troubleshoot that jumper/trace is not on all boards revisions.
wish me luck..!!Image
Attachments
IC3 connection.jpg

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Re: Korg Poly-800 not working

Post by Mooger5 » Thu Apr 18, 2019 7:47 pm

Hey crafter. Well done for not giving up. That was a great find.

You said the board had been scavanged, that there were some components missing which had to be replaced. Now that's a zero ohm resistor iow a jumper. Maybe whoever tried to fix the synth had found some "funny resistors that were shorted" and decided to remove them, only to find out there were no corresponding parts anywhere in the drawings, because well they're afterall just jumpers :roll: New rule of thumb: triplechecking everything when restoring a synth that had been a parts donor.

It's a double-sided board. Most ds boards use vias to link both sides, but it's still expensive. Korg used mostly the thru-hole components to act as vias. Note the circles around the leads of some resistors. They signal the lead that's soldered to that side of the pcb. The other lead connects to a trace on the opposite side. So I suggest checking the vias on both sides of the pcb. There are only a few. You'll notice every true via always connects to a trace on either side. If you find what looks like a via but it's linked to only one trace, then there might be a jumper that's missing.

The prints are indeed tough to follow. Follow the schematic instead. If no other parts are missing, including jumpers, check the connections between all pins of all ics and all the leads of all components. It's time-consuming but I'm afraid there's no other way.

Make sure every power rail is actually present. Once you do, well the entire thread started on that premise, so most of the detective work will hopefully be already done.

Good luck!
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