Author Topic: Repairs to a CPC6128  (Read 7863 times)

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Offline Audronic

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Repairs to a CPC6128
« on: 08:47, 03 November 15 »
Hi All


Part 2 of the faulty CPC's purchased.
I have a CPC 6128 which gives me the following screen.(A lot better than the 464 - gave me Nothing)
That's a Colour Monitor. Nothing gets hot, The 5 volts is good. I have not connected the 12 volts yet. NO Stuck lines on CPU. ------->

Ray






« Last Edit: 08:50, 03 November 15 by Audronic »
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Online gerald

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Re: Repairs to a CPC6128
« Reply #1 on: 09:58, 03 November 15 »
This screen is typical from RAM fault.

Offline Bryce

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Re: Repairs to a CPC6128
« Reply #2 on: 10:13, 03 November 15 »
Yup, I second Geralds diagnosis and a Like for the seriously cool retro scope!

Bryce.

Offline Audronic

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Re: Repairs to a CPC6128
« Reply #3 on: 13:36, 03 November 15 »
Thanks Chaps for the pointer to RAM.

For a CPC 6128 is the lower Ram 0000 to ffff 
   IC 127 - IC 134.
or IC 119 - IC126 as Pin 15 /cas?, is stuck HI on this set ??.
This is progress :-[   
Ps:-   Yep my old Cro that does a fair job, has some sync problems but -------
Ray
« Last Edit: 07:58, 08 November 15 by Audronic »
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Offline Bryce

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Re: Repairs to a CPC6128
« Reply #4 on: 13:50, 03 November 15 »
It can only be the lower 64K of RAM, the upper RAM isn't needed for the computer to boot properly.

There's a trick involving bridging the PAL to swap the upper and lower RAM for testing purposes. It's described somewhere here on the Forum, but I can't find it, maybe someone else can point you to this?

Bryce.
« Last Edit: 13:52, 03 November 15 by Bryce »

Offline Audronic

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Re: Repairs to a CPC6128
« Reply #5 on: 11:09, 05 November 15 »
Thanks Bryce I found the article and made up a shunt to suit, But No Boot

Where the project is up to :-

The 40010 Gate Array is OK Swapped with an other.
The Z80 changed Still the Same.
The 40025 Rom 32K signals all look OK.
Have bypassed the Pal chip IC 118, using Bank 1 SAME problem

BUT the Memory Table below this is for Bank 0  Bank 1 is worse ?
Time to remove ALL of Bank 0.
Ray

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Offline arnoldemu

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Re: Repairs to a CPC6128
« Reply #6 on: 11:40, 05 November 15 »
Bypass the PAL entirely and put the 6128 into 64KB mode.
Or is this what the bypass using the 2nd bank does?
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Offline Audronic

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Re: Repairs to a CPC6128
« Reply #7 on: 11:54, 05 November 15 »
@arnoldemu


The Bank 1 is worse than bank 0
The bypass was done with the help on the forum .
Need some help fixing CPC6128 not booting
Amstrad CPC6128 Pal Bypass IC 118
If the main bank of ram is failing, you can switch to the second one:
You can now test Bank 1
      - remove the PAL from its socket
      - on the socket :
      - link pin  9 to pin 19 (A14 to A14out) (Eq to LK5)
      - link pin  8 to pin 12 (A15 to A15out) (Eq to LK6)
      - link pin  7 to pin 16 (NCAS to NCAS1)
      - link pin 17 to pin 20 (NCAS0 to VCC)

If your CPC boot, then either the PLD or ram bank 0 is failing.

You can now test Bank 0 by changing :-

      - link pin  9 to pin 19 (A14 to A14out) (Eq to LK5)
      - link pin  8 to pin 12 (A15 to A15out) (Eq to LK6)
      - link pin  7 to pin 17 (NCAS to NCAS0)
      - link pin 16 to pin 20 (NCAS1 to VCC)
I had some headers that i wired up as above, due to faulty ram in BOTH banks it was unable to work.

Ray
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Offline Bryce

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Re: Repairs to a CPC6128
« Reply #8 on: 12:12, 05 November 15 »
Yup, looks like you may have several damaged RAMs. An entire bank swap is probably the easiest way to go.

Bryce.

Offline Audronic

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Re: Repairs to a CPC6128
« Reply #9 on: 07:17, 07 November 15 »
Hi Bryce


Yes I decided to do that, Clipped out the chips, and then i had the following thoughts :-


""  I have cut out ALL of the Memory for Bank 0 ??

Cut 128 Pins free.
Extract 128 Pins.
Clean out 128 Holes.
Solder in 8 - 16 Pin sockets, 128 Solder joints.

Are you keeping count.

We are up to 512 Transactions and this is only for BANK 0.

In Hindsight :-
I should have just removed the 5 chips marked in red and replaced them in sockets to test that the error was removed.
OR even just removed 1 Chip IC127 and re tested PIN 14.""

I have since acquired this desolderer, which saves me 128 transactions. Phew.

PS :- Still Desoldering

Ray





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Offline Bryce

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Re: Repairs to a CPC6128
« Reply #10 on: 11:43, 07 November 15 »
Hi Bryce

Yes I decided to do that, Clipped out the chips, and then i had the following thoughts :-

""  I have cut out ALL of the Memory for Bank 0 ??

Cut 128 Pins free.
Extract 128 Pins.
Clean out 128 Holes.
Solder in 8 - 16 Pin sockets, 128 Solder joints.

Are you keeping count.

We are up to 512 Transactions and this is only for BANK 0.

In Hindsight :-
I should have just removed the 5 chips marked in red and replaced them in sockets to test that the error was removed.
OR even just removed 1 Chip IC127 and re tested PIN 14.""

I have since acquired this desolderer, which saves me 128 transactions. Phew.

PS :- Still Desoldering

Ray

I don't need to keep count, I've done this operation probably 30 to 40 times in the past. However, it's still the best solution because if 5 of them failed then the other 3 will fail too quite soon and trying to remove and replace these after you've installed a socket left and right of the broken chip is much more annoying than removing them all together in the first place.
I learnt my lesson on a Spectrum 48K lower RAM, where the ICs are as close together as you can get them (the sockets actually touch each other!) Very fiddly trying to remove one last chip from the middle of a row of sockets.

Bryce.

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Re: Repairs to a CPC6128
« Reply #11 on: 16:37, 07 November 15 »
That repair is a great opportunity to have all the memory in nice sockets instead of soldered to the motherboard  :) . I always liked that idea  :)

Offline Audronic

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Re: Repairs to a CPC6128
« Reply #12 on: 04:47, 08 November 15 »
Yes it is good that the Ram's are now socketed..
BUT  Help Please Photo shows it all.
Not Booting completely ?
Ray

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Offline AMSDOS

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Re: Repairs to a CPC6128
« Reply #13 on: 05:31, 08 November 15 »
Yes it is good that the Ram's are now socketed..
BUT  Help Please Photo shows it all.
Not Booting completely ?
Ray


No BASIC 1.1 by the looks of it, someone else will have a better suggestion I'd imagine.


The colours are correct though, so Firmware must be being initialised.
« Last Edit: 05:33, 08 November 15 by AMSDOS »
* Using some of the hardly used Amstrad compilers :D
* I use Firmware in my Assembly code :P
* Have interpreted some BASIC 1.1 programs for BASIC 1.0. :)

Offline Audronic

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Re: Repairs to a CPC6128
« Reply #14 on: 07:51, 08 November 15 »

@AMSDOS
Thanks for the info on the colours.
Weird effect, if i tap the probe on or put my finger near  on Pin 1 FDC chip IC201 Z765 (Reset - FDC reset line ) it jumps to Ready
Some progress
Ray
« Last Edit: 07:55, 08 November 15 by Audronic »
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Offline radu14m

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Re: Repairs to a CPC6128
« Reply #15 on: 09:23, 08 November 15 »
well done :)


maybe is a soldering point unstable ...check it.

Offline Bryce

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Re: Repairs to a CPC6128
« Reply #16 on: 16:36, 08 November 15 »
Suggestions:

- Dry joint somewhere on the FDC circuitry - Causing something to float?
- Dodgy capacitor?

Bryce.

Offline Audronic

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Re: Repairs to a CPC6128
« Reply #17 on: 23:57, 08 November 15 »
@radu14m
@Bryce
Thanks I will check for Dry Joints around the FDC area.
Another thought as i normally only use a CRO and not a Probe.
The CRO will not pickup an Open circuit (Dry Joint) on a pin it will indicate as a LO.
So i will have to use Both.

Thanks   Ray
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Offline Audronic

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Re: Repairs to a CPC6128
« Reply #18 on: 05:44, 09 November 15 »
The saga continues :-

Yes a DEAD Chip (well mostly dead) IC 203 a 74HC240. The MotherBoard has been Socketed and replaced with a temporary 74LS240 (Thats all i have in the spares bin).
The machine now wakes up to the command prompt  BUT.

The Keyboard circuitry is UnWell.

Keyboard Faults ?

Not working

Y10 line ??
-=========-
Z    Nothing
X    Nothing
V    Nothing
,    Nothing
.       vx ],    That what I get on screen  ???
Spacebar    Nothing
Control
-==================-
DEL     also does not work.

Late addition :- Swapped the keyboard from a known working 6128,  Same problem. Oh well its not the Keyboard,:- Investigate.

Using the Probe has been a very useful tool.

Ray (Still smiling)
« Last Edit: 07:54, 09 November 15 by Audronic »
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Offline arnoldemu

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Re: Repairs to a CPC6128
« Reply #19 on: 10:05, 09 November 15 »
great investigation. :)

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Offline Bryce

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Re: Repairs to a CPC6128
« Reply #20 on: 10:35, 09 November 15 »
Wow, that machine really has had a hard life. At least with the keyboard problem it's a simple diagnosis. The 72LS145 has failed.

Bryce.

Offline Audronic

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Re: Repairs to a CPC6128
« Reply #21 on: 10:44, 09 November 15 »
@Bryce
You won't believe this i just finished making up a table for the IC 101 74ls145
and this is what i got :-
Pin 1 is unwell

Amstrad CPC6128  IC 101 74LS145

Decoder (Open Collector)
Pin 1 ??? ??? ?
Pin OutIN/OUTNameActivity
1OUTQ0LO - PULSE  ??? ?
2OUTQ1PULSE
3OUTQ2PULSE
4OUTQ3PULSE
5OUTQ4PULSE
6OUTQ5PULSE
7OUTQ6PULSE
8GROUNDLO
9OUTQ7PULSE
10OUTQ8PULSE
11OUTQ9PULSE
12INP3LO - PULSE
13INP2LO - PULSE
14INP1LO - PULSE
15INP0LO - PULSE
16VCC +5 VOLTSHI
Ray
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Offline Bryce

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Re: Repairs to a CPC6128
« Reply #22 on: 11:36, 09 November 15 »
Yup, I did say it was an easy one :)

Bryce.

Online gerald

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Re: Repairs to a CPC6128
« Reply #23 on: 11:55, 09 November 15 »
Yup, I did say it was an easy one :)

Bryce.
Note that sure.
The wrong line is driven by Q9 (pin 11)
Q0 is the line selected between two keyboard scan done at 50Hz. It is mostly low except when other lines are scanned (few ms)


Offline Audronic

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Re: Repairs to a CPC6128
« Reply #24 on: 23:34, 09 November 15 »
@gerald


Thanks for the comment, I am unclear as to what you are saying could you expand the comment Please.


Thanks   Ray
Procrastinators Unite,
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I keep telling you I am Not Pedantic.
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