Author Topic: Replacing Z80  (Read 2833 times)

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Offline Jbliss

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Replacing Z80
« on: January 15, 2019, 01:15:56 pm »
Hi All,

Doing a restoration project of an old soundtracs Console with Midi Mutes,
trying to repair the Computer section which consists of
Two Z80s ;
Z0840006PSC Z80 CPU 2D
Z84C0010PEC Z80 CPU HU

They look both to be Z80s CPU's Would a new Z80 like this be ok to replace them with?
https://au.element14.com/zilog/z84c0008peg/mcu-8bit-z80-8mhz-dip-40/dp/1081890
Cheers
JBLISS
 

Offline Daxxin

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Re: Replacing Z80
« Reply #1 on: January 15, 2019, 06:55:59 pm »
Hi , you mean overclock the board? i dont think that's easy
 

Offline bsudbrink

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Re: Replacing Z80
« Reply #2 on: January 15, 2019, 07:48:03 pm »
Are you sure the Z80 is bad?  I've restored a number of Z80 based vintage computers and, regardless of the abuse they've seen, I've never come across a dead Z80.
 
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Offline Jbliss

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Re: Replacing Z80
« Reply #3 on: January 15, 2019, 11:12:45 pm »
Are you sure the Z80 is bad?  I've restored a number of Z80 based vintage computers and, regardless of the abuse they've seen, I've never come across a dead Z80.

The Z80 seems to have locked up, Do you have any tips on trouble shooting Them ?
 

Offline jancumps

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Re: Replacing Z80
« Reply #4 on: January 15, 2019, 11:41:55 pm »
They don't have persistent memory or fuses - don't lock up. They are working or defect.
 

Offline bsudbrink

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Re: Replacing Z80
« Reply #5 on: January 16, 2019, 12:12:58 am »
First thing I would check is to make sure it has a good clock.
 

Offline Squiddaddy

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Re: Replacing Z80
« Reply #6 on: January 16, 2019, 01:39:17 am »
My first thoughts too. Make sure you have good power and a good clock. Check for bad electrolytics.
 

Offline amyk

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Re: Replacing Z80
« Reply #7 on: January 16, 2019, 01:45:41 am »
It's easy to breadboard a circuit that will just get the Z80 to execute NOPs, which can at least verify its basic operation.
 

Offline Jbliss

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Re: Replacing Z80
« Reply #8 on: January 16, 2019, 09:26:16 am »
Thanks all so far, just double checking they are both Z80's Just different versions right?
so they should have the same pinouts?
Cheers
JBliss
 

Offline bsudbrink

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Re: Replacing Z80
« Reply #9 on: January 16, 2019, 04:33:48 pm »
Yes, those are both Z80 CPUs.  The text "below" the "Z80 CPU" line is the production information.  For example, the line "9541   2D" is interpreted as the two digit year "95" (for 1995), the two digit week "41" (the 41st week) and the production source code (I don't know the codes, but I would bet "2D" is a facility in Singapore).  I'm guessing now, but I think the text "above" the "Z80 CPU" line is some kind of wafer identification number, that is, what wafer was this chip cut from.
« Last Edit: January 16, 2019, 04:52:25 pm by bsudbrink »
 

Offline Flenser

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Re: Replacing Z80
« Reply #10 on: January 17, 2019, 03:11:34 am »
You can test your existing chips using this technique posted by Julian Ilett to flash LEDs using just a bare Z80:
 
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Offline LapTop006

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Re: Replacing Z80
« Reply #11 on: January 18, 2019, 04:24:41 am »
My first thoughts too. Make sure you have good power and a good clock. Check for bad electrolytics.

... and then the program ROMs, especially if they're EEPROMs.
 

Offline Jbliss

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Re: Replacing Z80
« Reply #12 on: January 18, 2019, 11:22:59 am »
Hey All,

Thanks again so I am having no luck with trouble shooting this circuit, so my last hope is opening it up to the community.
any Help would be appreciated!!!!

The circuit that is a part of an old 1980s Soundtracs PC Midi Mixing Console. This is the computer section. It controls all the mutes of the console.
All the clocks Lines and Voltages look to be present.

Connector 11 (TEST POINT) Shows
RXCLK = 500khz Clock signal
CLCK = 4 Mhz Clock Signal
INT = HIGH
HALT = LOW
WA = 1Mhz Clock
RD = HIGH

Currently have no response from the keypad and No data on the 7 segment displays. :(
I have a Scope so can upload waveforms if need be.

Could anyone point me to a source where I could Purchase the Z80 Parts if this need to be done further down the path. 
Z0840006PSC Z80 CPU 2D
Z84C0010PEC Z80 CPU HU

Thanks !!!
 

Offline glarsson

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Re: Replacing Z80
« Reply #13 on: January 18, 2019, 11:46:59 am »
I found it weird to have two Z80 CPU so close to each other. The schematics you posted now explains that but adds another problem. In the schematics IC15 is the Z80 CPU but IC16 is a Z8470, more commonly known as a Z80 DART (dual asynchronous receiver transmitter). Why have the DART been replaced by a CPU and what damage has that caused?
 

Offline Jbliss

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Re: Replacing Z80
« Reply #14 on: January 18, 2019, 11:49:18 am »
I found it weird to have two Z80 CPU so close to each other. The schematics you posted now explains that but adds another problem. In the schematics IC15 is the Z80 CPU but IC16 is a Z8470, more commonly known as a Z80 DART (dual asynchronous receiver transmitter). Why have the DART been replaced by a CPU and what damage has that caused?


Ahhhh! It was like that When I received the console second hand! However it did work in this configuration? is that possible?

Thanks JBLISS
 

Offline glarsson

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Re: Replacing Z80
« Reply #15 on: January 18, 2019, 11:52:22 am »
The DART is connected to the MIDI port, so definitely no MIDI without a DART.
 

Offline Jbliss

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Re: Replacing Z80
« Reply #16 on: January 18, 2019, 11:54:20 am »
The DART is connected to the MIDI port, so definitely no MIDI without a DART.

Hi Glarsson,
Ok that makes sense the Mutes where working never used the MIDI. At the moment moment if I can get just Mutes going again it would be amazing! Thanks heaps for that !!
 

Offline RoGeorge

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Re: Replacing Z80
« Reply #17 on: January 18, 2019, 12:08:09 pm »
LOL, that was funny!   Somebody inserted another chip there.  I wonder if that was just a mistake, or straight trolling!
:-DD

It probable didn't do any damage, Z80, 8255 and TTLs are pretty resilient, but don't power the board any more.

Find the proper chip first.  I bet the board will simply start running after that.  Usually, cross-reference part numbers are working just fine, so don't spend a fortune for the identical part.  Equivalent parts are good enough:
http://www.cpu-world.com/Support/Z80.html

Of course, check the other components too, just to be sure they correspond to the schematic.
« Last Edit: January 18, 2019, 12:10:53 pm by RoGeorge »
 
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Offline Jbliss

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Re: Replacing Z80
« Reply #18 on: January 18, 2019, 12:18:29 pm »
LOL, that was funny!   Somebody inserted another chip there.  I wonder if that was just a mistake, or straight trolling!
:-DD

It probable didn't do any damage, Z80, 8255 and TTLs are pretty resilient, but don't power the board any more.

Find the proper chip first.  I bet the board will simply start running after that.  Usually, cross-reference part numbers are working just fine, so don't spend a fortune for the identical part.  Equivalent parts are good enough:
http://www.cpu-world.com/Support/Z80.html

Of course, check the other components too, just to be sure they correspond to the schematic.


 :D :-DD It Makes a lot of sense now! However it did Run with it before??? I have just ordered a new part :) However probing around I have discovered all the Data Buss Line on the (Actual lol) Z80 are at 3.6v ??

Cheers Bliss
 

Offline glarsson

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Re: Replacing Z80
« Reply #19 on: January 18, 2019, 12:24:25 pm »
Now back to the test connector.
The four signals INT*, HALT*, RD* and WR* are all active low (bar over, or * as I used). If the CPU is running then both RD and WR (not WA) should have high frequencies showing read and write activity. INT and HALT should be high with possible short low pulses depending on how they are used in this design. A constant low on HALT is not healthy. A constant high on RD is consistent with a halted CPU, but the 1MHz toggling of WR is not.
 
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Offline RoGeorge

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Re: Replacing Z80
« Reply #20 on: January 18, 2019, 12:38:36 pm »
However it did Run with it before??? I have just ordered a new part :) However probing around I have discovered all the Data Buss Line on the (Actual lol) Z80 are at 3.6v ??

Z80 have a three-state data bus.  According to the read and write pins, the data bus can be a HighZ, an input or an output at 0 or 1.  Even with an oscilloscope, it can be tricky to properly visualize the level of 1s.  The data bus can go up to about 4-5V, seen on an oscilloscope.  Sometimes you may find pull-up resistors, or bus driver circuits, but I couldn't spot any in the schaematic you posted.  (We will assume here nobody is trolling us with the schematic, too!  ;D )

There are a lot of 8255 ports, and any extra chip is a capacitive load for the bus.  This is probably close to the maximum fan-out, 3.6V could work just fine.  Since it is working, as long as the power is in the range of +5V +/-0.25V, it's OK.
 
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Offline Jbliss

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Re: Replacing Z80
« Reply #21 on: January 18, 2019, 12:48:40 pm »
Now back to the test connector.
The four signals INT*, HALT*, RD* and WR* are all active low (bar over, or * as I used). If the CPU is running then both RD and WR (not WA) should have high frequencies showing read and write activity. INT and HALT should be high with possible short low pulses depending on how they are used in this design. A constant low on HALT is not healthy. A constant high on RD is consistent with a halted CPU, but the 1MHz toggling of WR is not.

Thanks guys You have all been hugely helpful so far! would it be worth Getting a new Z80 Just to be safe ?

https://www.ebay.com.au/itm/ZILOG-Z80A-CPU-Microcontroller-Central-processing-unit-40-Pin-Dip-Z8400APS/113524787684?hash=item1a6e9ac9e4:g:CUYAAOSwBPNXSRQD:rk:3:pf:1

Thanks JBLISS
 

Offline rhb

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Re: Replacing Z80
« Reply #22 on: January 18, 2019, 12:51:20 pm »
Make a free run harness.  Take a 40 pin socket and bend the data lines at 90 degrees outward.  Hardwire a NOP instruction (00h) on the data lines.

Then insert the free run harness into the socket and the CPU into the free run harness.

When powered up the CPU will sit and cycle through all the addresses.  You can then check for address line faults with a dual trace scope Start with A15 and A14.  A14 should be twice the speed of A15, then proceed to A14 & A13

A 16 bit logic analyzer would be easier.  If you have one verify that all addresses between 0 and 65365 appear once and once only.

Also look for solder flux residue anywhere on the board.  If you see any clean it off with isopropyl alcohol and a soft brush.  I've now made 7-8 repairs just by cleaning off flux residue.  At one end of the spectrum an HP34401A DMM and at the other a motion activated LED night light and a TV remote.

The 34401A arrived from eBay completely wonky.  Just random digits on all settings.  I opened it up and found a 1 cm brown spot of flux residue around an LF357 that had been replaced.  I cleaned it off and it has worked flawlessly ever since.  It reads within a few ppm of my other 34401A.
 
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Offline glarsson

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Re: Replacing Z80
« Reply #23 on: January 18, 2019, 01:09:24 pm »
Don't buy a new Z80 CPU until you have tested the two you already have. Have you removed the "bonus" CPU and tested what happens with an empty IC16 socket? That might work depending on how the firmware reacts to a missing DART. It can't be worse than it is now. If you need to buy anything it's certainly a DART.
 
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Offline sorenkir

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Re: Replacing Z80
« Reply #24 on: January 18, 2019, 01:51:25 pm »
Hi,
A few years ago I have successfully replaced a defective Z80 (one address pin had weak TTL levels), which in fact was a Mostek MK3880, with a NEC D780C.
Michel.
 


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