Help, a CPC 464 which has trouble starting!

[LaurentInTime]

Hello,

Let me introduce myself (first post on this great forum). I'm a french guy and I bought last year a CPC 464 with a green screen monitor which worked fine. It's a first hand initialy bought for Christmas 1984: at the time, the french CPC version was out of stock and some QWERTY models were imported from England.
So, I played with my new old CPC: M4 board, GBS8200 for video, external power supply 5V 2A... and it was fun.

Suddenly, the CPC froze during a game (no more screen displayed and a continuous sound instead of the music). It was impossible to restart normally.
So I tried with the original screen and the result was the same: a strange flip screen was displayed. No reaction with the keyboard, no sound...

I carefully disassembled the CPC: the motherboard was spotless and I started a serie of tests without success. Just once, the BASIC reappeared: time to turn off, plug in the keyboard and try again, it was KO again.

A few months later (during these last days), I decided to try again. And surprise! Everything was working fine again... for one or two hours...

So I started a new serie of tests. The problem is on the motherboard (same result if I bypass the tape drive, disconnect the keyboard, change the screen...) but it was not difficult to find...

Moreover, I discovered a strange behavior (I hate random problems...). Sometime, The BASIC reappears again with a low speed display. The keyboard can work again for few secondes.
If I restart, it's KO again. Sometime, it works again: first a stange border, then the BASIC reappears few seconds after...

I suspect a gate array (40007) problem...

What do you think about this case?

Thank you very much for your help.

Regards.

Laurent

[RetroCPC]

Laurent,

I'd start with the basics first...

Could be PSU issues - low voltage on the PCB due to a bad power switch / connectors - If you have a multimeter and confirm the 5V on the PCB...

Oxidized power switch contacts are a common issue... especially if not been used for a long time - Cycling the power switch (with no power input) rapidly many times to "wipe / polish" the switch contacts might help...

The CPC mainboard does not have much in the way of Bulk PSU decoupling (large value electrolytic capacitors) - but I would check the few it does.

If PSU looks good then possible:-

A bad Memory IC? swapping memory IC's with known good would shortcut debugging..

Maybe even a bad Clock Crystal - (if you have a Scope confirm the clock is present and correct).

If non of the above then your into swapping the Gate Array etc...

I'm currently located in mainland Europe, If you have no luck then I can repair it for, you can just ship the mainboard to me to save shipping costs (ESD protected - PCB wrapped in cooking foil if you dont have a better solution).

[LaurentInTime]

Hello,

Thank you very much for your answer.

- You are right: I see 2 electrolytic capacitors and they are known to have an expiration date... I will try to buy new ones and change them (it may take some time but I'm in no rush).

- For the memory IC, I tried the RAM test and all was green... BUT the CPC was just come back OK again at this moment (I sould install again the RAM test program on my M4 Board and test again).

- For the Clock Crystal to test, I have to find it on the motherboard (I'm a newbie for the electronic part ;-)... I will try the next week-end.

- At the moment, I don't have a backup Gate Array for test.

As a last resort, I will be happy to be able to send it to you for a rescue attempt.

I add here some additional elements:

- Same result with a BASIC 1.1 loaded with the M4 Board.

- I tested 5.2V with a voltmeter at the screen output (CPC not connected)
- 5.2V on the motherboard (CPC OFF)
- 5.1V on the motherboard (CPC ON, same result for a OK or a KO state) and only 1A for the charge

- And the more interresting this evening:
If I start the CPC and it is KO, I just have to wait (30s, 1min) for it to finally start. If I run a game for example with a moving intro and with music, the game freezes then restarts then freezes again and restarts again... (10 times in a row this evening before I shut down).

Thank you again. Next tests next week-end.

Regards.

Laurent

[Bryce]

Hi LaurentInTime,

you need to measure the 5V rail somewhere on the PCB such as the pins of a logic chip. Measuring the voltage at the input doesn't really tell you anything. If the power switch is dirty (very likely problem here) you won't see this measuring at the input.

Bryce. 

[LaurentInTime]

Hello Bryce,

Thank you for your answer.

I forgot to tell you where the electric tension was taken.
- 5.2V on the motherboard (CPC OFF) : "-" to the electrical input connector, "+" to the CP001 PIN 3
- 5.1V on the motherboard (CPC ON) : "-" to the electrical input connector, "+" to the CP001 PIN 1

But you are right, according to your advice, I measured again at the pin of logic chips:
- 5.0V on the motherboard (CPC ON): "-" to the electrical input connector, "+" at RAM level (from IC117 to IC124) PIN 8
Again, same result for a OK or a KO state.

I didn't search the problem from the power switch because I didn't use it for the tests (PIN 1 and 3 of CP001 directly connected).

Now the computer continuously goes from a KO state to an OK state, then KO, then OK, etc.
So, I tried the RAM test program again:
It's strange: the borders appear green (either RAM is OK), then the borders disappear (aren't they supposed to be either green or red?). Then they turn green again, then disappear, etc.
During these RAM test, I always got 5.0V at the RAM level.

In any case, for any launch of the RAM test, the screen is well displayed (no flipping).

If you have an idea...

Regards.

[gerald]

Hello Bryce,

Thank you for your answer.

I forgot to tell you where the electric tension was taken.
- 5.2V on the motherboard (CPC OFF) : "-" to the electrical input connector, "+" to the CP001 PIN 3
- 5.1V on the motherboard (CPC ON) : "-" to the electrical input connector, "+" to the CP001 PIN 1

But you are right, according to your advice, I measured again at the pin of logic chips:
- 5.0V on the motherboard (CPC ON): "-" to the electrical input connector, "+" at RAM level (from IC117 to IC124) PIN 8
Again, same result for a OK or a KO state.

I didn't search the problem from the power switch because I didn't use it for the tests (PIN 1 and 3 of CP001 directly connected).

Now the computer continuously goes from a KO state to an OK state, then KO, then OK, etc.
So, I tried the RAM test program again:
It's strange: the borders appear green (either RAM is OK), then the borders disappear (aren't they supposed to be either green or red?). Then they turn green again, then disappear, etc.
During these RAM test, I always got 5.0V at the RAM level.

In any case, for any launch of the RAM test, the screen is well displayed (no flipping).

If you have an idea...

Regards.

Did you just try to re-seat the Gate Array in its socket ?