Amstrad CPC464 - Repair

Vnman

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So I just got myself an Amstrad 464 for pennies, it advertised as "dead".

this is what I did -
  • connected power supply and nothing, no light nothing.
  • opened the 464 up and found that the power switch is covered with "fluff" clean the switch and got the red light.
  • removed the 40007(?) just above the RAM chips and founf that one row of the socket was covered with thermal paste. Cleaned the socket and put new heatsink, powered on and still black screen.
  • found on the power socket that the three solder points are "dried" will re-solder.

Questions -

The board is the long board with no "jumpers", is this the version one board?
Is the 40007 fried? where can I get a replacement?
The RAM chips are socketed, are these original or the previous owner has tinkered with the board?
 
Hi,
the very first versions of the 464 had real wires (not flat membrane) coming from the keyboard to a long white connector and the PCB will have a part No. Z70100 MC0001A. Version 2 had an unpopulated space for an alternative GA (40010) and the PCB part No. will start with Z70200 MC.... . There was only these two long PCB versions, version 3 was the short PCB. RAM was never socketed on any of them, so I would assume someone has tried to fix this in the past.
The 40007 may be fried, but it could be many other things too, so I wouldn't go looking for a 40007 until you are sure.
If you don't have the equipment or knowledge to find the fault, you could send the board over to me for repair.

Bryce.
 
Thanks for the offer.

Since I got it for cheap I will try to repair it myself. Just need a few pointers.
 
Ok. Choose one of the TTL chips somewhere on the PCB and check that they are really getting the 5V they need. After that try reseating the Z80 which should also be socketed.

If possible, could you post a picture of the screen you are getting?

Bryce.
 
my board is Z70100 no jumper wires and no MC marking on the board. Just reflow the power socket and still black screen, no sound when pressing the keyboard. The cassette deck seems to be ok, rewind and forward work.

will check the voltage next....
 
IMG_20170320_095219_edited.jpg

IMG_20170320_095226_edited.jpg
 
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That's a pretty early model. Looks like someone did some modding to it, the heatsink is definitely not what Amstrad originally used and there are several ICs that aren't normally socketed.

Bryce.
 
That's me with the heatsink, before it was the aluminium heatsink and found that there was too much thermal paste applied that it covered the whole bottom row.

4.8v on the power supply, 4.7v on the sound chip, 4.8v on the z80 & 4.4v on the 40007.
 
They're borderline voltages. What's the voltage on the plug supplying the voltage? I'd recommend cleaning the contacts inside the power socket and cleaning the contacts inside the switch. You could also try shorting the pins that go to the power switch to override the switch completely and see if the computer then starts.

Bryce.
 
Hi,
also CRT controller can often cause black screen, ir runs pretty hot and may be faulty..
be aware, do NOT buy HD46505SP as it works partially. (looks like it works good in Basic on first look, but on some games like Turrican - there is a thick black horisontal line in the middle of the screen. What You need - is HD46505RP.. give it a try, these chips are not expensive usually..
regards

-Viktor
 
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The keyboard isn't beeping when he presses "DEL", so the issue (at least one issue) is in the computer itself, not the CRT.

Bryce.
 
So what's next?

replace RAM chips?

I noticed that the RAM chips all have 4.5v supplied to them. 2 are a bit warm but not red hot compared to others.
 
Is there 5V at the input jack? Possibly some RAMs have shorted inside. As they are socketed, remove them all and see if the voltage rises back to 5V.

Bryce.

Gesendet von meinem Motorola DynaTAC 8000X mit Tapatalk.
 
got some spare time to try and repair this board.

removed all the RAM chips and now the voltage rises to 4.94v.
 
it means that Bryce was right, at least one ram chip is damaged and causes a short, just insert them by one and check the voltage, than you will see wich one is defective..
 
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The new RAM chips arrived from China (KM4164B-10), replaced the old ones (TMM4164P-4) and still does not help. Still black screen and no "beep" when pressed the "DEL" key.
The voltage supplied to the RAM chips dropps to 4.91V.

Ordered new Z80 (Z0840004PSC) so it is a waiting game....
 
The board is REALLY simple so you could just change every chip and it will eventually work.
If you want to diagnose **ANY** CPU-controlled device you need a logic probe as a bare minimum. You don't need an oscilloscope, they are WAY over-rated and only useful if you know what the waveform should be and most people don't.
Using the logic probe check the Z80 pin 6 for a clock and pin 26 for the reset (low at power-on then high after about 1/2 second, then it should stay permanently high)
if you have the clock and reset signal it's probably not a bad Z80 (those chips are almost bulletproof).
Probe other pins on the Z80 and you might see some activity. If so the computer is running at least partially. if you see no activity you may have a bad logic chip somewhere or a bad ROM.
To check the ROM you need an EPROM programmer. If the read is good (matches known CRC32.... just type the CRC32 into google and it will come up with a reference to MAME if it's known) then the ROM is ok. If not, get an EPROM of the same type and program a new ROM.
After that, to know what is wrong you need to use a logic probe and check the logic chips comparing the inputs and outputs to logic chip datasheets, or just change every chip one at a time. To remove the common logic chips, just cut the pins at the top near the plastic. Once the chip is out use a soldering iron and tweezers to pull out each leg, then use a solder sucker to clear the holes. fit a new dual wipe socket and logic chip. there's plenty of vids on youtube explaining exactly that.
obviously don't cut out any chip if it's not common. do not try to remove the chips in one piece because you will fail miserably and damage the PCB at the same time. Just cut out the logic chips and replace them, they are only a few cents each.
If you upload HQ IN-Focus and bright pics I could probably help with specifics and/or PM me and I can walk you through some things and how to check them. The pics you uploaded are only good for printing out and then using as fancy toilet paper.
 
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What a crazy piece of advice! Admittedly, the 464 is relatively simple compared to something like an Amiga, but it still has 25 ICs with more than 500 pins between them. Why on earth would anyone take the brute-force approach and start replacing them all? Not only is it a waste of good ICs, it would also take ten times longer than diagnosing the actual fault. And even then, the problem could be as simple as a damaged capacitor and all the chip swapping was for nothing.
I agree that a logic probe can be useful to diagnose such a system, but I wouldn't underestimate the value of a scope, especially as logic probes can give very misleading information. A noisy clock signal looks perfect on a logic probe, but terrible on a scope.
My guess is that the Z80 has failed, which happens quite often on the Amstrad as both the data and address bus is wired directly to the expansion port with no buffering or protection in between. When it fails, both the clock and reset will look fine, but one or more of the bus lines will be stuck high or low.

Bryce.
 
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I'm with Bryce on this one.
Seen many CPC's with Dead Z80's.
A scope is very valuable as it allows comparison of two signals at the same time, phase or timing errors are easily spotted..

A logic probe can get you there but a scope will get you there faster.

:)
 
Thanks for the tips guys.

I know that the cpc is simple machine, that is why I want to try to repair on my own.
 
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