CPC 6128 DISC MISSING BUT THE DISC DRIVE IS GOOD

[Rabs]

Well this is fun.

So I can't get over the fact that a simple INP from &FB7E returns &FF. Surely this is just a simple IORQ, CS and RD. So I have decided to tackle that. What I don't understand is why it looks like CS and RD are not in sync.

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So I decided to take action, desolder IC207 (one of three ICs involved), test it and see what happened. You guessed it, it passed and the problem remains.

At least I am getting plenty of practice at desoldering  and at this rate the whole board will be socketed. 

[Rabs]

Below is a working CPC showing  CS and RD, which I guess looks correct, RD is in blue.
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[Rabs]

I am hoping I have found the problem.

The fact that an inp(&FB7E) returned &FF implied to me that a simple IO read was not working. So I traced each chip involved with CS and RD, removed and tested. That's ICs 207, 210 and 203. All passed  but the last one. Not sure I believe the chip tester. Anyway new IC coming next week, so I will find out then.

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[Rabs]

All fixed, disk drive now works and loads, casette now works and loads.

So failed IC203 caused a problem with CS.

Symptoms;

• CAT returns that it cannot find the disk.
• But the disk drive is good and works in another CPC.
• If you ignore the CAT error 3 times the CPC cashes with a red screen.
• INP(&FB7E), the FDC status, returns &FF, but should return &80.

Root cause;

• IC203 74HC240 failed, causing errors when generating CS for the FDC.