Synth-DIY Yahoo! Groups Archives

Poly 800 Mk1 display not working

2015-07-07 by kogz23@...

Hello all,

I have a Poly 800Mk1 with a display that is blank.
I have tested the 7 segment LEDs and they are working.
Tested all the power rails and they appear to be correct.
I am reading around 3000hz on the RES 7.5 pin. Getting 3MHz on the clock out of the CPU.
I am using a DVM's frequency counter to read the measurements, i do not have a scope at the moment.
I am only seeing 0.7MHz on the ALE pin of the CPU when it looks like it should be 5.5MHz.
Also I am getting no reading on the Timer out pin of the 81C55.

So that is as far as my troubleshooting has got me. There is sound coming from the Poly, but the keyboard appears to do nothing. Sounds like the plane taking off effect that precious posts have described.
Any ideas as where I should look/measure etc, as I'm stumped at the moment!

Thanks in advance for any ideas

Al

Re: [korgpolyex] Poly 800 Mk1 display not working

2015-07-07 by Michael Hawkins

0.7 Mhz on ALE is about right.

The swooshing sound means the D2A sample and hold circuits are not being refreshed. So this looks and sounds like a non functioning compute unit. That is, the problem is somewhere in the CPU/RAM/ROM/chip select/bus driver section.

So a check of each of the chips and their outputs in the chip select logic and various DTC's (I wish Korg never used those things) and all of the bus buffers etc is in order. Look for data bus, address bus, logic/control bus signals.

If you have a ROM reader that by all means have look at the 2764 EPROM and check for data in the ROM. But those chips are quite robust and almost never die.

Check +5 and -5 volt rails and make sure they are within 10% of 5 volts. Check that those rails have no AC component. Nice and flat. Check your wall wart and make sure it's the right voltage and has 300mA or more of current capacity.

Check for physical damage to the main board. Look for battery acid leakage damage too.

/Mike

From: "kogz23@... [korgpolyex]" <korgpolyex@yahoogroups.com>
To: korgpolyex@yahoogroups.com
Sent: Tuesday, July 7, 2015 3:08 PM
Subject: [korgpolyex] Poly 800 Mk1 display not working

Hello all,

I have a Poly 800Mk1 with a display that is blank.
I have tested the 7 segment LEDs and they are working.
Tested all the power rails and they appear to be correct.
I am reading around 3000hz on the RES 7.5 pin. Getting 3MHz on the clock out of the CPU.
I am using a DVM's frequency counter to read the measurements, i do not have a scope at the moment.
I am only seeing 0.7MHz on the ALE pin of the CPU when it looks like it should be 5.5MHz.
Also I am getting no reading on the Timer out pin of the 81C55.

So that is as far as my troubleshooting has got me. There is sound coming from the Poly, but the keyboard appears to do nothing. Sounds like the plane taking off effect that precious posts have described.
Any ideas as where I should look/measure etc, as I'm stumped at the moment!

Thanks in advance for any ideas

Al

Re: [korgpolyex] Poly 800 Mk1 display not working

2015-07-08 by kogz23@...

Thanks for the in depth reply! A lot of what you said is unfortunately above my head. The testing you describe, is that possible with a DVM? Mine does have a logic reader and frequency counter. I just don't know what I would be looking for when checking the different bus signals you list.
Anyway, I found the problem. IC23 pin 11, clk, was not connected to pin 30 of the CPU. Made the connection and the display came to life. Now to test the machine.
Thanks again,
Al

Re: [korgpolyex] Poly 800 Mk1 display not working

2015-07-08 by Michael Hawkins

Well, you seem to be making significant progress. And nothing to do with any of my suggestions. The CPU just didn't have a clock! I should have included that.

Let us know how it goes.

/Mike

From: "kogz23@... [korgpolyex]" <korgpolyex@yahoogroups.com>
To: korgpolyex@yahoogroups.com
Sent: Wednesday, July 8, 2015 12:38 AM
Subject: Re: [korgpolyex] Poly 800 Mk1 display not working

Thanks for the in depth reply! A lot of what you said is unfortunately above my head. The testing you describe, is that possible with a DVM? Mine does have a logic reader and frequency counter. I just don't know what I would be looking for when checking the different bus signals you list.
Anyway, I found the problem. IC23 pin 11, clk, was not connected to pin 30 of the CPU. Made the connection and the display came to life. Now to test the machine.
Thanks again,
Al

Re: Poly 800 Mk1 display not working

2015-07-09 by kogz23@...

Well after initial testing there are a few problems.
I can not load the patches into the machine. The display reads TAPE when I try, but never goes to GOOD.
I can save patches though.
As well as that not all the keys work and DCO2 seems shot.
I would like to load the patches though before I go any further trying to fix the other problems.
Any idea where I should be looking to fix the patch loading issue?
Thanks in advance
Al

Re: [korgpolyex] Re: Poly 800 Mk1 display not working

2015-07-09 by <backshall1@...>

It should say LOAD, not TAPE. It says TAPE when you switch tape enable on, but then push 4 (LOAD) before you start playing the patch file.

Don B.

From: mailto:korgpolyex@yahoogroups.com

Sent: Wednesday, July 8, 2015 8:38 PM

To: korgpolyex@yahoogroups.com

Subject: [korgpolyex] Re: Poly 800 Mk1 display not working

Well after initial testing there are a few problems.
I can not load the patches into the machine. The display reads TAPE when I try, but never goes to GOOD.
I can save patches though.
As well as that not all the keys work and DCO2 seems shot.
I would like to load the patches though before I go any further trying to fix the other problems.
Any idea where I should be looking to fix the patch loading issue?
Thanks in advance
Al

Re: Poly 800 Mk1 display not working

2015-07-09 by kogz23@...

Doh! Thanks, yes, embarrassingly that was the problem!
Managed to fix the keyboard by replacing IC34. The chip was shorting.
Got DCO 2 back to life, tracked it down to IC18 missing a connection.
Now I have a problem with the pitch bend. I get correct readings at IC10 pin 7 as described in the service manual. But the pitch bend seems to do the reverse of what you would expect.
Reading pin 7 of IC35 with the pitch bend in centre I get 0.7v, with it up it goes to -3V, down it goes to +3.5V. Is this behaviour correct? As I side not the synth sounds alot better tuning wise when the pitch bender is disconnected. I have read the resistance of the pitch pot and it seems to be functioning as you would expect.
Thanks again,
Al

Re: [korgpolyex] Re: Poly 800 Mk1 display not working

2015-09-29 by Vincenzo Vincenzo

Hello,
I'm sorry to bother you and excuse my English, I translate with google translator ..
I have the same problem of pitch bend with my Korg Poly 800 MK1, you managed to solve?
how you solved?
I thank you in advance for your response.

Hello.

Il Giovedì 9 Luglio 2015 22:42, "kogz23@... [korgpolyex]" <korgpolyex@yahoogroups.com> ha scritto:

Doh! Thanks, yes, embarrassingly that was the problem!
Managed to fix the keyboard by replacing IC34. The chip was shorting.
Got DCO 2 back to life, tracked it down to IC18 missing a connection.
Now I have a problem with the pitch bend. I get correct readings at IC10 pin 7 as described in the service manual. But the pitch bend seems to do the reverse of what you would expect.
Reading pin 7 of IC35 with the pitch bend in centre I get 0.7v, with it up it goes to -3V, down it goes to +3.5V. Is this behaviour correct? As I side not the synth sounds alot better tuning wise when the pitch bender is disconnected. I have read the resistance of the pitch pot and it seems to be functioning as you would expect.
Thanks again,
Al