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?
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.
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?
Quote from: Rabs55 on 19:44, 29 May 22Great 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.
Quote from: Bryce on 19:22, 29 May 22A 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?
Sounds like you found a possible culprit.
Bryce.
Quote from: Bryce on 08:15, 31 May 22Sounds 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.
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.
Quote from: Bryce on 18:42, 31 May 22The 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.
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.
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.
Quote from: Bryce on 19:35, 01 June 22I 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.
Quote from: Rabs55 on 21:07, 02 June 22Sort 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.
Ran the diagnostics ROM and it does not find the FDC. All the other tests pass.
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.
Quote from: Rabs55 on 18:01, 17 June 22Loaded 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.
Quote from: GUNHED on 19:36, 17 June 22Quote from: Rabs55 on 18:01, 17 June 22Loaded 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.
Sorry maybe a daft question. What's the difference between a upd765ac and a upd765ac-2? Are they compatible?
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 😀.
IMG-20220715-WA0001.jpg
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?
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.
Quote from: Bryce on 13:03, 15 July 22The 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.
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?
Swapped the z80 and not that :( still same.
Quote from: Bryce on 13:03, 15 July 22The 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.
pic_26_4_IC210_IN2_OUT3_ROM_OE.jpg
Yellow is pin 3 OE, blue is pin 2 A15
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.
pic_31_2.jpg
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. :)
Below is a working CPC showing CS and RD, which I guess looks correct, RD is in blue.
pic_32_1.jpg
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.
20220723_150850.jpg
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.