Repairs to a CPC464.

[Audronic]

Hi All

I have just picked up a Faulty CPC 464.
There is No Video out Nothing ?
Is there a list and or Order of things to check.
I have changed the Z80 (No Change) It has a 40010.
Mother board is Z70200 - MC00020 1984 it looks in good condition.
I have had a look on the Wiki and was unable to find anything.
I have a CRO / Multimeter etc .

Thanks in advance    Ray

[Bryce]

First commandment of repair - "Thou shalt measure voltages"... Check that you have a steady 5V to all ICs. Test that the 40010 is getting a 16Mhz clock signal and that it's outputting a 4Mhz clock signal. Check that the power switch contact are clean (bridge the switch). Check that the connector to the tapedeck is connected and making proper contact.

Bryce.

[00WReX]

Everything Bryce said. 

If the 'ON' LED is not even illuminating, my money is in the On/Off switch.
I have had a couple of CPC's with those symptoms.
I usually (without power connected) simply flick the switch on ad off quite a few times...that may just clean the contacts a bit.
Another way to check the switch is to actually bypass it.
Where the tape / data cassette connects to the main board also is where the +5v is switched.
Simply shorting those, when the 5v is applied to the board will bypass the switch.

[attachimg=1]

Cheers,
Shane

[Bryce]

I don't recommend doing the bypass at the tapedeck connector because the GND pin is also just beside the "switch on" pin and being slightly off (which is easily done) could mean you've just destroyed your PSU.

Bryce.

[Audronic]

Thanks

The 464 Powers up, light on,  volts every where, Xtal works , have finally got the pinout for the 40010.

I have made up a Plug with a Switch (Normally Closed) to Pins 1 and 3 and extended the earth 0 Volts from pin 2 to be used as a common for the CRO/Multimeter.
The reason for the normally closed switch is that if i press it it will turn the power off and on release it will power up the 464 a bit like a reset.

Ray

[Bryce]

Ok, so let's test some more...

Does the computer beep when the DEL key is held?

Bryce.

[Audronic]

Pressing the Delete key Does Not produce a Beep.


?? What Brand / Model Number Frequency Counter do you use Please. I may as well purchase one.


Ray
 

[Bryce]

Hmmm, tricky one. You've swapped the Z80, so at this stage it can only be the reset circuitry or the GA. Not good. Can you check some of the data lines and see if data is being sent on boot? Heave you reseated the GA and cleaned the pins on the IC?

Bryce.

[gerald]

Could be the ROM as well

[Bryce]

True. It would be still nice to know what's on the data and address bus and whether they are "stuck" somewhere.

Bryce.

[Audronic]

Test Results
Should the Reset Line Be HI ??

Error on the Table  A14 LO. After fitting Reset Switch it toggles LO to HI and stays there.

Ray

 

[Bryce]

Hmmm, Pins 11 and 13 (HSync / VSync) are really low all of the time? That would suggest that the CRTC hasn't fired up.

Bryce.

[Audronic]

Hi Bryce


I will have a look around the HD6845sp.
The A14 line is a pain as it goes to the ROM 40009, and IC 109 74LS153 ( I have disabled A14 going to pin 6  BUT the A14 Line is still unwell)
it also goes to the HD6845 pin 25..


It looks like some new sockets __ Drat.
I was wondering can i burn a 27C265 to replace the 40009 ?? is there a link i can have a look at Please.


Thank goodness i am Retired and have some spare ? time.


Thanks   Ray 

[Bryce]

Yes, you can burn the ROM to a standard 27C256.

Bryce.

[Audronic]

Progress :-

I have found IC 109 (74LS153) Pin6 overloading the A14 Line Holding the line permanently low ?

Waiting on a Slow boat from China.




Ray

[Bryce]

Interesting. What state does the CPC boot to if you disconnect just that pin of the 153?

Bryce.

[Audronic]

Hi Bryce


By cutting the pin free, A14 now is active (not kept low) still no display.
Went to my local IC shop and purchased some 74ls153's , put a socket in, put the chip in, A14 now low again . Something is loading the line ??
I have found a 6845 in an old "Excalibur 64" in a socket, i will transfer that over soon.

Patience Ray.
Ray

[Bryce]

So the 153 wasn't faulty. Good to know, I've never seen one of those go bad yet.

The 6845 doesn't have access to A14, so it won't be that. I'm not sure what A14 should be doing during startup, maybe it's meant to be at 0V?

Bryce.

[Audronic]

Hi Bryce


On the circuit i have Pin 25  /cs  is marked as A14 ??




Ray

[arnoldemu]

if a14 is held high, then the lower rom will not be executed on boot up.

it needs a15=a14=0.

So no display is probably because it's not executing the OS/lower rom!

EDIT: Ok I see A14 is held low.

CRTC will accept i/o, gate-array will not (so no colours and mode set).

Upper rom will not be accessible, this may block start up too.

[Bryce]

Oops, you're right, I forgot about that signal, I was thinking only of the "MA" address lines on the other side.

Bryce.

[Audronic]

Gotcha  all sorted.


What was faulty - 1 6845 (IC 108) and an open circuit in the A14 line Pin 25 ( Suspect a VIA Open Circuit ).
Hard wired under board from Pin 4 CPU (IC 111) to pin 6 on 74ls153 (IC 109) which connects to Pin 30 on 40010 (IC 116).


Happy Ray.

PS:-  Thanks for all the helpful hints.

Now i can have a look at the Faulty CPC6128 that came with the Faulty CPC464. 

[00WReX]

Nice one.
That would have been very satisfying when the boot screen appeared.

Cheers,
Shane

[Bryce]

Congratulations!

It's always very satisfying when the boot screen suddenly appears, no matter how many CPCs you've repaired before.

Bryce.

[Audronic]

Thats why the Photo's have a Gotcha Label.
Thanks for the support and various comments.
BUT :- 
The Cassette player has no output.
Are there any Links to simplify the repair process.
The Belt is good, it responds to the remote control (Relay works) and Starts,  does all the correct actions BUT No Output.
Spools fast in both directions.

Any help would be appreciated.

Thanks  Ray