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Author Topic: Dead EP128, please help! (Read 3892 times)

Offline kokkiklhs

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Dead EP128, please help!
« on: 2022.March.29. 14:52:51 »
Hi folks, I am posting these on behalf of a friend...
A few days before he has fired up his 128, which was working fine until 2-3 months ago and he got this:

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We've got no experience at all on such EP issues, if it was e.g. a Spectrum, I'd say a dead Z80... But what do YOU say, as specialists? It might be a common issue, easily recognizable by experienced eyes... Please help, we don't want to start butchering on RAM ICs or other components, before we get your opinions! :roll:
I am mentioning here that the only things done so far are voltage measurements (OK) and ROM test, which worked without problems on another, healthy machine!

Cheers from Greece!!!
:)

Offline Zozosoft

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Re: Dead EP128, please help!
« Reply #1 on: 2022.March.29. 17:12:47 »
This display means: Nick chip running in uninitialised video memory, soo the ROM not started.
Reset not stuck, because if Dave chip dead then Reset stuck low -> Nick not running -> just black screen)
Most possible failures:
- dead Z80 (especcialy if it is Zilog type, I found many dead, all are Zilog, Made in Philippines, with a near data codes)
- partially dead Dave, which is not genarete ROM signal, can be substituted with few 74xxx ICS.
- onboard RAM, less chance because the ROM can make black screen at start even when onboard RAM faulty, just hang up later

Do you have a EPROM programmer? I wrote a short test ROM, which is try to test memory bytes, and make a beepig sounds of readed bits. Replace the original ROM to this, and if you got a beeping machine then know CPU and ROM access are working.

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #2 on: 2022.March.29. 19:00:14 »
This display means: Nick chip running in uninitialised video memory, soo the ROM not started.
Reset not stuck, because if Dave chip dead then Reset stuck low -> Nick not running -> just black screen)
Most possible failures:
- dead Z80 (especcialy if it is Zilog type, I found many dead, all are Zilog, Made in Philippines, with a near data codes)
- partially dead Dave, which is not genarete ROM signal, can be substituted with few 74xxx ICS.
- onboard RAM, less chance because the ROM can make black screen at start even when onboard RAM faulty, just hang up later

Do you have a EPROM programmer? I wrote a short test ROM, which is try to test memory bytes, and make a beepig sounds of readed bits. Replace the original ROM to this, and if you got a beeping machine then know CPU and ROM access are working.

Wow, thanks for the detailed reply and for the multiple suggestions... Let's hope it is the "easy" Z80, I will tell my friend to try piggy-backing another CPU over the soldered one, to see if nothing changes. And yes, he does have a programmer, so he could try the solution with the alternative testing ROM!!

Offline Zozosoft

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Re: Dead EP128, please help!
« Reply #3 on: 2022.March.29. 20:55:37 »
Ok, there are the test ROMs. These are don't use any RAM for the program running, no stack used, all variable in Z80 registers. (The original EXOS ROM use stack in unverified memory, if the onboard memory bad, system crashed before can report RAM error.)

The OLD version just beeping, the another are try to initialise screen, the character screen memory addressed to the tested memory.
Bytes tested with 00h,FFh,55h,AAh values, after byte write read back, and make beep sound from bits (low tone 0, high tone 1).
Connect speakers to the tape output (or use from monitor connector via SCART cable), for the case if internal speaker don't working)

If machine start beeping then good life sign :ds_icon_cheesygrin:

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #4 on: 2022.March.30. 17:46:23 »
Ok, there are the test ROMs. These are don't use any RAM for the program running, no stack used, all variable in Z80 registers. (The original EXOS ROM use stack in unverified memory, if the onboard memory bad, system crashed before can report RAM error.)

The OLD version just beeping, the another are try to initialise screen, the character screen memory addressed to the tested memory.
Bytes tested with 00h,FFh,55h,AAh values, after byte write read back, and make beep sound from bits (low tone 0, high tone 1).
Connect speakers to the tape output (or use from monitor connector via SCART cable), for the case if internal speaker don't working)

If machine start beeping then good life sign :ds_icon_cheesygrin:

Gee, thanks a lot!
:cool:
I will update as soon as my friend burns the EPROMs and performs the tests!!
Meanwhile, he changed C4, no difference at all.
He also told me that he noticed that, after some reset/power on attemps, the vertical stripes pattern changes from black-white to only blue/or only yellow-white stripes, or even multicolour-white. Does this mean something?

Offline Zozosoft

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Re: Dead EP128, please help!
« Reply #5 on: 2022.March.31. 09:05:05 »
He also told me that he noticed that, after some reset/power on attemps, the vertical stripes pattern changes from black-white to only blue/or only yellow-white stripes, or even multicolour-white. Does this mean something?
No, it is just about the random memory garbage at power on.

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #6 on: 2022.April.02. 19:43:17 »
No, it is just about the random memory garbage at power on.

Oh, and I thought that only the Spectrum did that!!! :mrgreen:

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #7 on: 2022.April.03. 07:55:36 »
Update, after burning both diagnostic EPROMs and testing:
Still the same Black/White vertical stripes and no sound at all... The speaker is in good working order, it's been tested, too...
So, what's next to try out?

A very nice Sunday to you all!
:)

Offline Zozosoft

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Re: Dead EP128, please help!
« Reply #8 on: 2022.April.03. 16:27:23 »
Then ROM not executed :cry:
Need a scope or TTL probe. Check Z80 activity: M1, MREQ, RD, address pins, etc Also check Z80 clock and reset input.
If no activity, and it is have a clock input (and reset not stuck) then probably Z80 dead.
If Z80 active, then check ROM CE pin for a ROM enable signal.

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #9 on: 2022.April.04. 06:04:24 »
Then ROM not executed :cry:
Need a scope or TTL probe. Check Z80 activity: M1, MREQ, RD, address pins, etc Also check Z80 clock and reset input.
If no activity, and it is have a clock input (and reset not stuck) then probably Z80 dead.
If Z80 active, then check ROM CE pin for a ROM enable signal.

I shall report back when I have news of further testing! Thanks for your help and support so far, it is really quite valuable!!
:)

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #10 on: 2022.April.12. 22:52:25 »
Well, my friend used a Chinese lp1 logic probe set to ttl  and pulse settings for testing.
The results are as follows:

On the z80 the a11 a12 a13 pin , mreq , m1 light the 0 & 1 level. The a14 a15  pin flashing the 0 &1 level.
The memory cas , ras light 0 & 1 level . The rom ce light also the 0 & 1 level. Also on z80 the A10 to A0 also light together 0 & 1 level.
The only clear state is clk 1 reset 1 and rd 0 .

Do these give any clue about the possible fault?
Cheers to all readiing this!
:)

Offline Zozosoft

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Re: Dead EP128, please help!
« Reply #11 on: 2022.April.14. 09:17:22 »
The only clear state is clk 1 reset 1 and rd 0 .
These are very-very strange!
CLK 1 :?: Then no clock? But there are activities on other pins! Need also a 0&1 pulsing.
What is the situation on U18 pins?

Reset 1 are ok, need a little 0 pulse when pressing reset button.

RD 0 ??? Also need a pulsing.

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #12 on: 2022.April.16. 13:26:08 »
Wow, I shall inform my friend to do further testing and provide details, then I will get back with news!! Thanks again!!!

Offline kokkiklhs

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Re: Dead EP128, please help!
« Reply #13 on: 2022.April.18. 19:21:18 »
Update from my friend Panos, with latest tests:
U18
pin1 hi
pin2 hi
pin3 pulse
pin4 hi
pin5 ,6  lo & hi
pin7 lo
pin8 lo
pin9 lo 
pin10 no signal
pin11 lo
pin12 hi
pin13 hi