CPC 6128 DISC MISSING BUT THE DISC DRIVE IS GOOD

[Rabs]

OK, any ideas appreciated.

I have a CPC 6128, which boots fine but cannot find a disc. It does not crash, just returns Bad Command when I cancel after any disc command.

The disc drive is good and works in another 6128, so not the drive. The disc spins but the head does not move. I have visually inspected the pcb and no obvious signs of corrosion around the disc controller or associated logic.

I tried to load the diagnostics program from tape, as I do not have a ROM card yet, and the odd thing is the file is not detected. I can hear the data but nothing, no read error, nothing. Can this be related?

Any ideas? Where do I start?

[Bryce]

A bad command could mean that the OS ROM is not initialised ie: the 40015 has failed. However, if the drive starts spinning after entering a command it means that the ROM reacted but the drive didn't. This means that one of the buffer IC's to the drive (74LS32) has failed / didn't pass the command from the 765 (disk controller) to the drive or the flat cable to the drive has a problem. You'll need to test the outputs of the buffers IC's first. If these are good, the the cable has a problem.

Bryce.

[Rabs]

Great thanks, I will have a look at the circuit diagram from  the service manual and trace the logic gates and cable.

What about the tape? Red herring or unrelated?

[Bryce]

Great thanks, I will have a look at the circuit diagram from  the service manual and trace the logic gates and cable.

What about the tape? Red herring or unrelated?

Red herring I would guess. The two loading systems have almost nothing in common on the hardware level.

Bryce.

[Rabs]

A bad command could mean that the OS ROM is not initialised ie: the 40015 has failed. However, if the drive starts spinning after entering a command it means that the ROM reacted but the drive didn't. This means that one of the buffer IC's to the drive (74LS32) has failed / didn't pass the command from the 765 (disk controller) to the drive or the flat cable to the drive has a problem. You'll need to test the outputs of the buffers IC's first. If these are good, the the cable has a problem.

Bryce.

Thanks, so I have traced all the lines from the back of the ribbon cable to IC203, 206 and 208 and all have good continuity. So I guess not the ribbon cable. Of all the lines pin 11 "side 1 select" from IC 206 is floating around 1V where the inputs to this gate are both 0V. Does this sound like I am on the right track?

[Bryce]

Sounds like you found a possible culprit.

Bryce.

[Rabs]

Sounds like you found a possible culprit.

Bryce.

Thanks much appreciated. Will be a while before I replace the IC. Also think you are correct the tape issue looks like something else. The signal out of the op amp into the PIO looks a bit odd. Need to look at this circuit a bit more and compare with a working CPC. 

Thanks again for the help.

[Bryce]

The tape issue is most likely dirt on the head, a mis-aligned head or the tape turning too slow or not uniformly. Tape loading issues are usually mechanical problems, not the electronics.

Bryce.

[Rabs]

The tape issue is most likely dirt on the head, a mis-aligned head or the tape turning too slow or not uniformly. Tape loading issues are usually mechanical problems, not the electronics.

Bryce.

The cassette player and tape work fine on another CPC 6128, so don't think it is the casette itself.

[Rabs]

OK, learning on the job so to speak. Back to the disk drive issue. My initial diagnosis of the floating output on IC206 is wrong as this IC is a 74LS38 which has open collector outputs. So I guess it is just in a not connected state. Does this make sense? I guess I need to keep looking.

[Bryce]

I assume then, that it was only floating because the drive wasn't attached? If a drive is attached it shouldn't be floating.

Bryce.

[Rabs]

I assume then, that it was only floating because the drive wasn't attached? If a drive is attached it shouldn't be floating.

Bryce.

Sort of. I was wondering why the CPC needed a side 1 select signal. Well I guess it does not. The "not used" written on the schematic gave the game away a little. 

So after looking at the schematic in the service manual, for several hours, and trying to work out why it does not line up with my pcb, I realised it doesn't and my pcb is a later revision Z70290.  Oh what fun I had.   

I have the manual with the amendments now, but looks like the circuit diagrams are just that, amendments.

Thanks for the help.

[ajcasado]

Sort of. I was wondering why the CPC needed a side 1 select signal. Well I guess it does not. The "not used" written on the schematic gave the game away a little.

I guess that the side signal is only used with an external floppy with two heads conected and an OS ROM that supports two sided discs.

[Rabs]

Ran the diagnostics ROM and it does not find the FDC. All the other tests pass.

[Rabs]

Loaded FutureOS and  run the System Monitor, on the advice of @GUNHED, and &FB7E is returning &FF. On my working CPC it returns &80.

I have been tracing the logic and checking voltage levels and comparing with my working CPC and nothing different yet.

But then. If I keep retrying the CAT command, sometimes I see the head move a bit and sometimes I get the same red screen as @llopis in one of his videos when the FDC is removed. So looks like the FDC is failing somehow.

I am going to replace the FDC but I have never desoldered before so need to practise off CPC first. Any advice appreciated.

[GUNHED]

Loaded FutureOS and  run the System Monitor, on the advice of @GUNHED, and &FB7E is returning &FF. On my working CPC it returns &80.

When the value &FF is read, then the FDC can for some reason not be accessed. Now it can be a problem with a small chip or the FDC itself. But the hardware guys will know better.

[Rabs]

Loaded FutureOS and  run the System Monitor, on the advice of @GUNHED, and &FB7E is returning &FF. On my working CPC it returns &80.

When the value &FF is read, then the FDC can for some reason not be accessed. Now it can be a problem with a small chip or the FDC itself. But the hardware guys will know better.

Thanks. Hopefully the red screen is a key pointer.

[Rabs]

Sorry maybe a daft question. What's the difference between a upd765ac and a upd765ac-2? Are they compatible?

[Rabs]

So back, practiced desoldering and removed and replaced the fdc and yes you guessed it, not the fdc. At least I have learnt to successfully remove a 40 pin chip and not even so much as bent a leg 😀.

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[Rabs]

So quick recap.

• Disk drive is good.
• But cat does not find a disk.
• If you keep retrying, eventually you get a red screen as if the fdc is not there
• Diagnostics rom is not detecting the fdc but everything else passes, including the rom and ram.
• Continuity checks all pass.
• Logic gates all look OK on a scope.

So must be some logic to the fdc, right? But I have checked on my scope and they look OK.

I keep going back to the tape not working and I know they are not related but the op amps are associated with power, I think.

So what next?

Test Everything Again?

Go back to basics, check power, check continuity, check logic?

[Bryce]

The op-amps are not in any way involved with the power supply circuitry (other than requiring power themselves).

I'd start going through the FDC pins and monitor what each one does when you do a CAT.

Bryce.

[Rabs]

The op-amps are not in any way involved with the power supply circuitry (other than requiring power themselves).

I'd start going through the FDC pins and monitor what each one does when you do a CAT.

Bryce.

Thanks, I will concentrate on the FDC.

[Rabs]

I think there is something more fundamental going on and I don't think the problem is the disk circuitry or tape (I traced the tape in from the op amp to the PIO and it looks good).

Here is the thing, INP(&FB7E) returns &FF should return &80 for a type A FDC and does on my working CPC.

BUT what about another port, say INP(&BEFF), which I think is the CRTC status, also returns &FF but returns &20 on my working CPC. 

So is the whole IO READ an issue?

Z80?

[Rabs]

Swapped the z80 and not that    still same.

[Rabs]

The op-amps are not in any way involved with the power supply circuitry (other than requiring power themselves).

I'd start going through the FDC pins and monitor what each one does when you do a CAT.

Bryce.

OK I have started to look at what happens when I run CAT and given the odd behaviour with the CRTC and PIO, I have looked for ICs which may have common address lines associated. So I have started with IC210, NAND gate with Schmitt Triggers,  the output of which drives the ROM OE. Normally pin 3 is high, so not enabling OE but when I run CAT it follows pin 2.

But look at the pic, it goes low but then look how it returns high, is this a problem.

You cannot view this attachment.

Yellow is pin 3 OE, blue is pin 2 A15