Synth-DIY Yahoo! Groups Archives

Thread

another dead 800

2015-11-27 by mmdatech@...

Hi,

I got a dead poly800 with a non working

display and power supply sections.

After replacing Q1,Q3,Q18, I got the correct voltages

on all ic's but still no response from keyboard.

The only unusual behavior I can account for is the lack of

signals after the address latch ( IC29 ) in the Y3 - Y7 paths

hence no signals to the keyboard or tone generator IC's.

( IC29's Y0 - Y2 seem to be ok)

The address latch and 8085 pins a13 - a15 were tested and

found to be ok using the 8085 built in binary counter mode.

am I dealing here with a bad cpu ?

Re: [korgpolyex] another dead 800

2015-11-27 by Michael Hawkins

not necessarily and probably not.

Check for master CPU clock and a whole lot more to check after that to get Poly CPU going. Static memory, EPROM, address bus latches etc. They all need to be checked. The DTC's also.

/Mike

From: "mmdatech@... [korgpolyex]" <korgpolyex@yahoogroups.com>
To: korgpolyex@yahoogroups.com
Sent: Friday, November 27, 2015 8:25 AM
Subject: [korgpolyex] another dead 800

Hi,

I got a dead poly800 with a non working

display and power supply sections.

After replacing Q1,Q3,Q18, I got the correct voltages

on all ic's but still no response from keyboard.

The only unusual behavior I can account for is the lack of

signals after the address latch ( IC29 ) in the Y3 - Y7 paths

hence no signals to the keyboard or tone generator IC's.

( IC29's Y0 - Y2 seem to be ok)

The address latch and 8085 pins a13 - a15 were tested and

found to be ok using the 8085 built in binary counter mode.

am I dealing here with a bad cpu ?

Re: [korgpolyex] another dead 800

2015-11-28 by mmdatech@...

Thanks for the reply,

I've already checked the 8085 clock out ( 3Mhz ) and Res7.5 signal ( 3000hz )

and replaced the eeprom testing both firmwares posted in the files section.

Still no go...

However, another clue is the lack of signals at the A,B,C ports of the 8155 IC

while still getting the desired Timer out ( 500Khz ) signal when the eeprom is installed.

What is the best strategy for testing the DTC's and the 6116 ?

Re: [korgpolyex] another dead 800

2015-11-28 by Michael Hawkins

Check all of the address and data lines for continuity from the 80C85 and the outputs of IC 23 and IC29.

Check for low signals on the outputs of IC29. If you don't see any brief lows except for Y0, Y1 and Y2 then there's a reasonable chance that the software memory check is stopping the CPU from progressing anywhere.

If you see lows on Y3-Y7 then the RAM check is probably coming up OK and you'll have to look further.

An oscilloscope is immensely useful.

/Mike

From: "mmdatech@... [korgpolyex]" <korgpolyex@yahoogroups.com>
To: korgpolyex@yahoogroups.com
Sent: Saturday, November 28, 2015 3:52 AM
Subject: Re: [korgpolyex] another dead 800

Thanks for the reply,

I've already checked the 8085 clock out ( 3Mhz ) and Res7.5 signal ( 3000hz )

and replaced the eeprom testing both firmwares posted in the files section.

Still no go...

However, another clue is the lack of signals at the A,B,C ports of the 8155 IC

while still getting the desired Timer out ( 500Khz ) signal when the eeprom is installed.

What is the best strategy for testing the DTC's and the 6116 ?

Re: [korgpolyex] another dead 800

2015-11-28 by mmdatech@...

Hi Mike,

I have attached a screenshot from my logic analyzer showing the IC29 outputs

( channel 0 = Y0 , channel 1 = Y1 and so on ).

There are little or no signals on Y3 to Y7 so it must be the memory check .

Does that mean the problem lies with the 8155 or the 6116 ?

your help is greatly appreciated :)

Re: [korgpolyex] another dead 800

2015-11-30 by mmdatech@...

I managed to write a small program to test 8085, 8155 and led drivers

by lighting them all at once. It seems to work but the startup is flaky, it doesn't

show up every time I switch the power on.

any ideas on what I should test next ?

Re: [korgpolyex] another dead 800 [1 Attachment]

2015-11-30 by Gordonjcp

On Mon, Nov 30, 2015 at 10:35:28AM -0800, mmdatech@... [korgpolyex] wrote:

> I managed to write a small program to test 8085, 8155 and led drivers by lighting them all at once. It seems to work but the startup is flaky, it doesn't
> show up every time I switch the power on.
>
>
> any ideas on what I should test next ?
>
>

Doesn't start up every time? That sounds like evidence, to me...

--
Gordonjcp MM0YEQ

Re: [korgpolyex] another dead 800

2015-11-30 by Michael Hawkins

check the reset circuit.

check power supply is stable and on voltage. +5V needs to be within TTL tolerance levels which is about 10% so even a crappy multi-meter should read close enough to be within 10%.

check that the write, read and chip select signals are reaching the RAM 6116.

check all of the data and address lines are at the RAM.

How did you assemble the 80C85 code? Other than me and one other person, no one on this yahoo group has ever created a working piece of 80C85 code.

sometimes chips fail, but rarely. let's keep assuming that it isn't the RAM itself, not just yet.

/Mike

From: "Gordonjcp gordon@... [korgpolyex]" <korgpolyex@yahoogroups.com>
To: "mmdatech@... [korgpolyex]" <korgpolyex@yahoogroups.com>
Sent: Monday, November 30, 2015 1:40 PM
Subject: Re: [korgpolyex] another dead 800

On Mon, Nov 30, 2015 at 10:35:28AM -0800, mmdatech@... [korgpolyex] wrote:
> I managed to write a small program to test 8085, 8155 and led drivers by lighting them all at once. It seems to work but the startup is flaky, it doesn't
> show up every time I switch the power on.
>
>
> any ideas on what I should test next ?
>
>

Doesn't start up every time? That sounds like evidence, to me...

--
Gordonjcp MM0YEQ

Re: [korgpolyex] another dead 800

2015-12-01 by mmdatech@...

I just bit banged the B & C busses on the 8155 after calculating

the correct addresses. If it's of any interest, I'll happily share the code here.

The reset timing looks a bit funny, Can I replace the IC40 ( pst-518 )

with a regular RC circuit ? or maybe a schmitt trigger ?