cpc 464 with no display

[eggman14]

Hi All,

i owned a cpc464 with colour screen back in the eighties and have fond memmories of this.
i picked one up lately and now have got as far as knowing my screen is definetly good.
The Screen flicks when i switch the power button but no basic start screen appears.
I tested the screen with another unit from a friend so i know the screen is good.
i suspect that most of the guts are indeed working as i have some output.
My thoughts are poss zilog processor or 6845 rom.
Any tips or info would be welcome as im new to repairing anything of this age im more familiar with modern equipment but love these machines.
Also some good spares sites would be appreciated.

Thanks in advance

[Bryce]

Ok, first questions:

Does the LED on the keyboard light? If not, measure whether the plug from the monitor to the keyboard is supplying 5V (Negative on outer ring). If 5V is there. Open the CPC, disconnect the cable from the power switch and short the pins of the connector (on the PCB side). If the CPC runs, the switch needs to be dis-assembled and cleaned. If the CPC doesn't run: Turn the volume up and press the delete key. Does the CPC Beep? If yes, you have a video problem, probabaly the 6845 (CRT Controller, not a ROM). If it doesn't beep: Check the electrolytic capacitors on the board, check that the main ICs (Z80, 8255, RAM) are getting 5V to there supply pins and check if any ICs are getting hot. When you tested all that, you should be pretty close to knowing what's wrong.

Let us know how you get on...

Bryce.

[eggman14]

Thanks for the info, i have power into the keyboard tape deck etc the screen twitches but no data.
I will check the pins on those 2 ics this week.
I noticed that the basic rom on my machine has a peculiar heatsink on it a strap under the rom and thermal paste and a plate on top, is this standard as the few boards i have seen did not have this?
Once again thanks for the reply and i will post when i find something.

[Bryce]

That's not the basic ROM, that's the gate array and it was standard to have a heat sink on version 1.

Assuming that's what you have, here's a Picture to show you what's what:

Bryce.

[eggman14]

Thanks for your help Bryce,

i have checked the voltages at z80 and 8255 from a pinout i found on this site and they have 5v dc.
the array as you call it seems quite hot and did have what seems like fairly new thermal paste on it.
Is this a part that is really available and as its hot is it the only likely faulty part?

Thanks

Eggman

[eggman14]

Hello,

i have fully removed the board from the base and noted the following.

1) there is discolouration to the component 16000 ks84g on the underside of this component ( right hand side of the array )
2) there appears to be some kind of liquid mark on the board near incoming power jack.
3) also i noticed some dark patches near to the 3.5mm jack plug but they do not appear to be too bad.

is this unit a scrapper or would it be economical enough to repair?

[eggman14]

another picture

[eggman14]

last picture

[arnoldemu]

The GA will be hard to find.

I do have 2, but I can't say if they will work or not, and the way they have been stored means they may not.

Where are you in the world?

Ok, when you start the machine, what does it show on the display?
I don't think it'll make a difference, but have you reseated the z80?

if you have a scope, check the input to the wait pin on the z80, see if there is a clock on it.

[eggman14]

there is no response to the zilog being re seated i tried it but no good.if the arrays are hard to come by i can get more cpc464 's from ebay as there are a few on there for sale i was intrigued toknow what was wrong with this one.
i am still interested in what has actually happened to this one though.
Any other idea's?

thanks for your help and im in the uk

[eggman14]

sorry i dont have a scope.

[Bryce]

Without a scope it's kind of difficult to do any further tests. I definitely think it's worth saving though, every CPC is worth saving. It might be a simple 20cent capacitor that needs to be replaced, or just a dry joint. But it's hard to say. Depending on where you are, there are people here who could take a proper look at it. I'm in Germany, so if you're near you could send the PCB to me. I can at least tell you what's broken, then you can decide whether it's worth saving.

Bryce.

[CanonMan]

sorry i dont have a scope.

Are you anywhere near Stoke-on-Trent?

[arnoldemu]

thanks for your help and im in the uk

near sheffield?
I don't have a scope, but I have 2 gate-arrays (extracted from other 464's - but they may be broken) that could be swapped.

[eggman14]

im based in coventry but work all over the uk.sorry for the delay in getting back to you all and thanks for the responses.
The array appears to get quite warm and the thermal paste appeared to be quite new.
I am running a v2 cpc464 ok so im good but the faulty unit is a v1.
Im thinking a scope is the way forward for this unit and would be willing to donate this v1 unit  to anyone as although im really intrigued to find out what is wrong i have so many projects running at the moment and a young family to keep me busy that im struggling to find the time and not having the instrument is a massive nuisance.
Once again thanks to you all for your help and if you leave me a message i will try to drop the unit to you when im next in your region.

[ralferoo]

For what it's worth, I'm in Coventry too, so I could take a quick look sometime. I'm a bit busy too though, so I probably can't spend much time on trying to fix it, but I might be able to help diagnose the problem and possibly source you replacement bits you might need.