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

From: Michael Hawkins <korgpolyex800@...>
Date: 2015-07-08

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