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Help with CPC6128 not booting

Started by RobertM, 09:45, 11 February 14

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RobertM

Hi all,
        My CPC6128 has been in storage for some time as I don't have an original monitor. I wanted to convert it S-Video and perhaps make a uC SD floppy emulator. I don't know if it ever worked.

No good lol. It doesn't boot. It did once or twice out of about 20 power cycles.

I get a mostly blank screen with a square where the active video normally is. ie white screen and black border.

I am using the composite out to a TV and a small 5 Volt switch mode PSU rated at 3 Amps.

I noticed the board was a bit dirty and there was corrosion in some of the IC sockets (CPU) so I replaced three IC sockets and cleaned the board with metho. I used snap off SIL machined pins with gold inserts.

The three sockets I replaced were for the CPU, a small PAL (Programmable Logic Array) and another chip marked 40010.

Thinking the problem may be noise from the SMPSU, I put a 100nF cap across the 5 Volt input and upgraded the 47uF 5 Volt filter cap to 220uF.

I am guessing that the CPU is running as the registers in the 6845 need to set by the CPU to get video sync and I have sync.

The 5 Volt rail is at 5.05 Volts.

My next guess would be the DRAM chips but I don't want to socket 16 chips if I don't need to.

Does anyone have some experience or advice / suggestions that mat save me some time fixing this??

Any help would be greatly appreciated.

Bryce

A screenshot would be helpful. There's a good chance it's RAM, but you wouldn't have to swap all 16 RAMs, the CPC only needs the first 64K to boot, so only 8 of them would cause this problem. Are any of the RAMs hot?

Bryce.

Badstarr

I have a 6128 I repaired about a year ago, amongst other things, the main issue was a bad Gate Array, the IC marked 40010. If you rule out the RAM as Bryce suggests, then the GA would be the next place I would look.

Mine did the same thing showing a white screen at boot until I desoldered the GA and put a socketed replacement in, after that it booted normally the rest was some dead logic to control the FDD. I suppose if enough corrosion has built up on the GA pins it might cause some issues so maybe start by checking and cleaning them if necessary.

Good luck!
Proud owner of 464 GTM64 6128 GTM65, GX4128 and a 464/6128 Plus Hybrid a 20 year long ambition realised! :-)

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