Korg Poly-800 not working

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crafter57
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Korg Poly-800 not working

Post by crafter57 » Wed Mar 13, 2019 6:00 am

Got this poly-800 in a non working state and the display does not come on, actually after further troubleshooting there is no sound either. I've already replaced many caps and transistors that were missing or simply wrong parts, verified all harnesses, connectors, traces & voltages and continuity between IC's. All voltages check out +5,-5 and +9 volts, CPU timing and control signals are present except Res_5.5 & Res_6.5(seq. osc circuit). The 81C55 is receiving those signals from the CPU but what is suspicious is that the output ports are all Low, so IC30/IC31 not getting info to the display board, I think..
Just have a question about the gnd shielding foil below the motherboard, does it have to be soldered to the "G" Gnd pad via a resistor, this one is missing the tab so its not connected to the Gnd pad. Also, the C batteries connectors, D1 and R1 were removed and the backup battery installed.
Well if anyone has any idea of something I may have missed, please let me know I'd like to revive this poly

Any help or suggestions greatly appreciated
thanks

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Re: Korg Poly-800 not working

Post by Mooger5 » Wed Mar 13, 2019 7:32 pm

Yes, the shield should be grounded to drain away most of the RF/EM interference it picks up.

I checked the schematics. Removing those two components does isolate the RAM from the six battery cells. Also the mkII doesn´t have them components.
But the first revision of the mkI seemed to need the bat cells at all times, not just for backup, if I read the service manual correctly. The flowchart also hints at that...
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Re: Korg Poly-800 not working

Post by meatballfulton » Wed Mar 13, 2019 11:51 pm

Mooger5 wrote:But the first revision of the mkI seemed to need the bat cells at all times, not just for backup, if I read the service manual correctly. The flowchart also hints at that...
That's correct from my recollection (been some years since I had my P800).
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Re: Korg Poly-800 not working

Post by crafter57 » Thu Mar 14, 2019 1:08 am

Thanks guys, I'd like to believe thats true but it doesn't seem to hold up simply because I have a spare 2nd KLM596 board with the same setup (removed D1 & R1) and when I simply replace the bad board with the 2nd good one, the synth comes on and works fine even without the C batteries. I even tried the eprom from the bad board into the good board and it also works fine. This is why I'm convinced its something on the board. I doubt that the missing shield would not make it start working.
I've been troubleshooting this for a week now verifying all waveforms and measurement against the procedures from the manual. My thoughts are that the 81C55 is dead because when I compare all signals between both I don't see any pulses in that section as I do from the good board. I ordered a replacement 81C55 but before I receive it, I'd like to be 100% that this may be the cause. Any other thoughts..?

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Re: Korg Poly-800 not working

Post by Mooger5 » Thu Mar 14, 2019 1:15 am

Yes, although now I think it becomes clear in the service manual they´re referring to the six cells as the only power source available, like in a worst case scenario. I had thought the main +5V power bus (A) was dependent on D1 which is a zener. Together with the resistor the diode is only there to regulate +5V for the RAM (F). The external power supply also feeds the backup power bus via D1 and the resistor. But the RAM is also powered by (A). So, provided the DC socket isn´t oxidized, it seems to do well without the six cells...
IDK if this makes sense, but can´t explain any better now...
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Re: Korg Poly-800 not working

Post by Mooger5 » Thu Mar 14, 2019 1:16 am

crafter I´ll read your post tomorrow..
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Re: Korg Poly-800 not working

Post by Mooger5 » Thu Mar 14, 2019 11:57 pm

Well you could have told us you had a working modded board earlier. That would have saved some time. If a modification works, it´s much more efficient to learn why it works, rather than why it wouldn´t, believe me.
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Re: Korg Poly-800 not working

Post by crafter57 » Fri Mar 15, 2019 1:59 am

Well, I came here on this forum asking and hoping for help or advice as I'm not 100% sure on my troubleshooting. So, all my basic measurements already done as I troubleshot for a week doing alot of the legwork checking waveforms and comparing signals so I would be able to answer the basic questions if asked. So yes, I do have another working board but my goal is to understand where the problem is on the original board that came in the poly-800 which may or may not help me?. The 2nd working board is not modded its a different revision altogether which happens not to have D1 & R1 components on the board. The 1st board was scavenged for parts thats why I installed/replaced many of them to the original parts. I'm assuming the 81c55 is faulty but not sure if something else could be causing it not to turn on and cutting off the LED circuits. So I was hoping for advice so I wouldn't have to start replacing chips for no reason.

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Re: Korg Poly-800 not working

Post by Mooger5 » Fri Mar 15, 2019 11:49 am

Not upset. It was sort of tongue in cheek. You did write *removed d1 and r1* in your second last post. I get you meant absent or *removed by designer* and I read that as *removed by user*...
Anyway you're probably on the right track. There is a third group of output ports on that 81c55 that controls a dac, possibly for egs or some other stuff. That would explain the absence of sound.
The res5. 5 and 6.5, not sure what you mean by seq. Osc. But they're reset lines. There should be a signal at startup but very brief. After a shortwhile it stays low. The dso might catch it during powerup. Iinm an interrupted rst line would prevent the 81c55 from starting or syncing withthe other devices. Ill come back later.
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Re: Korg Poly-800 not working

Post by crafter57 » Sat Mar 16, 2019 9:08 am

yup, sorry to jump the gun here..the board apparently has been thru alot of hands before I got it and I'm trying to bring it back to life ..Ugh..not easy.. I'm not getting anything from the DAC, I worked my way backwards to IC38 not getting anything there but thats because IC25 PA port are LOW and so no voltage output. strange thing also, is my /CS is high on the eprom and /CE on 81c55(IC25) and not getting the /RD & /WR from the CPU which I though were there...I'm trying to find the culprit and I'm all over the board :( ..its confusing, spent many hours verifying.. I'm getting some signals but not all.. Does the CPU have a masked rom or is it all on eprom? taking a break..will continue tomorrow..

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Re: Korg Poly-800 not working

Post by Mooger5 » Sat Mar 16, 2019 2:25 pm

Right, any *dashed* control line means it's active when low. So, /wr and /rd are actually active. While /cs and /ce short for chip select and chip enable being high means they're inactive. In other words theyre disabling the chips. The rom and the 81c55 are ignoring incoming data and not outputting anything.
/ce and /cs are adress lines. For them to appear high, I guess the adress chip is ignoring commands from the cpu. It could be internally shorted, passing out the +5v rail through the output ports.
If it feels too warm to touch, replace it.
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Re: Korg Poly-800 not working

Post by Mooger5 » Sat Mar 16, 2019 10:17 pm

The adress chip is IC29. TC40H138. The original parts are sold on Ebay for more than 10 Euros. Modern CMOS equivalents 74HCT138 consume less power and work just as well. And they´re about 50 cents.
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Re: Korg Poly-800 not working

Post by crafter57 » Sun Mar 17, 2019 2:54 am

Mooger5, I checked all address lines all seem to reach their destination chips (I do have a spare 138 incase) I did stumble upon what could possibly be the reason why the CPU is not resetting/restarting, DTA1 (Digital transistor) seem shorted and not sending the reset pulse to CPU (Pin8 - restart interrupt RES_6.5) I never get the pulse on pin-8, also the voltage on DTA1 doesn't match the other board and internal resistance is also off. Now the parts list 2 DTA's on the KLM598 board but it seems like the numbers may be different (KLM596 is A114 suffix B), the 2 on the KLM598 show (A114E suffix N-N). I'm willing to try it because I'm not even sure I can source these DTA's or an equivalent.
Do you think they are the same?

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Re: Korg Poly-800 not working

Post by Mooger5 » Sun Mar 17, 2019 8:09 pm

On the Parts List, KLM896 DTA1 and KLM598 DTA1 and DTA2 are all identical: DTA-114N. DTA114E seems easier to find. The numbers are what matter most. The sufix usually refers to current or voltage rating, etc. Nothing to worry about IMHO.
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Re: Korg Poly-800 not working

Post by crafter57 » Mon Mar 18, 2019 5:53 am

Well, I replaced the DTA1 and still nothing positive. I'm gonna replace IC29 next.. and hope for the best as I can't understand what's going on.

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