Commodore 8050 dual disk drive - Page 8 - UK Vintage Radio Repair and Restoration Discussion Forum

SiriusHardware · 31st Aug 2023, 10:34 pm

That isn't the smoking gun I was hoping for, but even so the output you are seeing from U11/8 does not conform to the truth table for that type of gate. You may wish to wait for Tony to concur but it looks to me as though the U11 10/9/8 gate has a faulty output (stuck low).

Do you have any 74LS32s? I can't remember whether you have ever needed to replace one in anything before.

ScottishColin · 31st Aug 2023, 10:38 pm

No - but they're cheap enough. I'll get on with ordering anyway.

Colin.

Quote:

Originally Posted by SiriusHardware

That isn't the smoking gun I was hoping for, but even so the output you are seeing from U11/8 does not conform to the truth table for that type of gate. You may wish to wait for Tony to concur but it looks to me as though the U11 10/9/8 gate has a faulty output (stuck low).

Do you have any 74LS32s? I can't remember whether you have ever needed to replace one in anything before.

TonyDuell · 1st Sep 2023, 4:40 am

Yes, it looks like that 74LS32 has failed. It's what I would have guessed, actually as the motor didn't run either when the control input (from the digital board) was low or when you inserted a disk. That OR gate is the common bit in the digital side of the motor control circuit.

I would fit a 14 pin DIL socket, in case you have to replace the IC again ever. There would be no issues with stray capacitance, say, in this circuit. Use a turned-pin socket, they are more expensive but a lot more reliable.

ScottishColin · 1st Sep 2023, 10:23 am

OK - they're on order. I'll remove and pop a socket in later on while I'm waiting.

As U7 has already been replaced before I had this device, I'm just wondering what that chip does?

I'll probably go back to the printer while I'm waiting for delivery of the 74LS32s.

Colin.

TonyDuell · 1st Sep 2023, 10:34 am

U7 is a dual monostable (timer) IC and is used to generate the read data pulse (page 12 of the .pdf service manual).

ScottishColin · 1st Sep 2023, 12:49 pm

Removed and socketed.

A few photos showing the history of this device:

1. A nice engineer's note showing that it had a new board fitted in 1983 (although it doesn't say which one)
2. The original company sticker showing the maintainence company from Doncaster
3. Picture of the inside-top cover shwoign where the main board has got a bit hot over the years
4. U7 replaced at sometime with a nice red socket
5. The 2114's with holes in some of the legs - I've not seen this before.

Colin.

SiriusHardware · 1st Sep 2023, 1:30 pm

Shame the engineer didn't sign it as well (when I used to work on certain other bits of equipment I used to 'sign' the PCB so I'd know if it was the same one which came back to me at some point in the future. By an odd coincidence I now own two of those 'signed' units which weren't mine originally). One of them I do know the history of, but the other, I have no idea where it had been in the time between my repairing it and it ending up in my ownership.

Once the drive is up and running I think you might look again at those heat stressed diodes - problem there is that when the solder joints and PCB have been heated so much and for so long, the solder turns into something which is not quite solder any more and is much more difficult to melt than solder should be, so any attempt to remove and replace the original diodes could be quite damaging to the PCB and solder pads. If you were going to try it would be better to snip the legs off the original diodes close to their bodies and desolder and remove the individual legs, rather than try to get the diodes out in one piece. I'd also mount the replacements a bit higher off the PCB to allow air to circulate under and around them.

Or, just leave well alone...

ScottishColin · 4th Sep 2023, 5:35 pm

Replacement 74LS32 arrived, popped in and both drives now work. Drive 0 seems a little temperamental - needs more than one try sometimes so I'll clean the head and take it from there.

This is the best 8050 thread I have found anywhere out there so I hope it's of use to others in the future.

In summary, I resocketed the larger chips (but don't know if that was necessary), replaced 2 x 2114s which were found to be faulty by testing them in my PET and replaced a 74LS32 which had a stuck gate.

Thanks all for your help. Another one fixed.

Colin.

SiriusHardware · 4th Sep 2023, 6:00 pm

Glad to see progress, maybe Tony might have some idea about how to diagnose the reason for the fickleness of drive 0. Maybe it's not running at a steady speed, or maybe the head is not seeking reliably, something along those lines.

Anyone with paper or editable electronic copies of the service manual might find it useful to pencil in '4 red flashes on 8050 = 2114 RAM fault?' as it may help someone somewhere down the line. It might be useful to know which two of the 2114s caused the 4-flash fault, if you happened to make a note of it.

ScottishColin · 4th Sep 2023, 7:04 pm

The two 2114s I removed were UC4 and UD4.

Also, I have subsequently found thie attached document - see the very last page for advice on four flashes on an 8050.

https://drive.google.com/file/d/1eAv...ew?usp=sharing

Colin.

SiriusHardware · 4th Sep 2023, 7:16 pm

Oh, brilliant find, if only we'd known about that before. Definitely a keeper.

Mark1960 · 4th Sep 2023, 7:19 pm

Quote:

Originally Posted by SiriusHardware

Anyone with paper or editable electronic copies of the service manual might find it useful to pencil in '4 red flashes on 8050 = 2114 RAM fault?' as it may help someone somewhere down the line. It might be useful to know which two of the 2114s caused the 4-flash fault, if you happened to make a note of it.

I use foxit pdf reader, which has a typewriter function that allows you to add text notes to pdf documents. These are added as text boxes that you can move if they end up covering something on the document that you want to see later. Sometimes need to print the pdf to a new pdf for the typewriter function to work, depending on how the original pdf was created.

SiriusHardware · 5th Sep 2023, 12:45 am

From the same document under '8050 faults' I also noticed this among the fault case histories:-

Quote:

"Drive 1 OK but drive 0 had intermittent directory and loading. Checked 12V reg. and found ripple, replaced and checked OK".

Looking at the diagram it appears that there are indeed separate 12V regulators and 12V supplies ("12V A" and "12V B") for the two drives, so it might be worth putting the scope on each 12V rail in turn and observing what happens on that rail when its respective drive unit activates and runs. If the one for drive 0 dips or wobbles more than the one for drive 1, this may be an area worth investigating.

TonyDuell · 5th Sep 2023, 4:27 am

There are 2 12V rails, 12VA supplies the motors in one drive. 12VB supplies the motors in th other drive and the read amplifier.

It is not clear to me (as we've been confused by this in the past) which drive is a bit unreliable. If it's the one that shares its supply with the read amplifier then conceivably worn brushes/commutator in the spindle motor of that drive could be putting noise onto the supply line and upsetting the amplifier.

It certainly can't hurt to 'scope the 12V supply lines when the motors are running. You could also swap the drive units round and see if the fault moves with the drive. My initial thought is that it's a dirty/misaligned/worn head. Maybe one drive got more use than the other. You could also try formatting a blank disk in the dodgy drive and see if it's reliable with that. If so, it points to head radial alignment.

SiriusHardware · 5th Sep 2023, 8:06 am

Quote:

Originally Posted by TonyDuell

It is not clear to me (as we've been confused by this in the past) which drive is a bit unreliable

It would appear to be drive 0, the one which was not working at all before replacement of the 74LS32, which is still a little bit eccentric.

Quote:

Originally Posted by ScottishColin

Drive 0 seems a little temperamental - needs more than one try sometimes so I'll clean the head and take it from there.

ScottishColin · 5th Sep 2023, 8:37 am

Drive 0 is the unreliable drive.

I'll take the ideas above and have a look - thanks.

Colin.

SiriusHardware · 5th Sep 2023, 9:27 am

I would try scoping the 12V supplies first - easy to do, and you might get lucky, you might immediately find that the 12V supply to drive 0 looks unstable compared to the equivalent supply for drive 1.

ortek_service · 5th Sep 2023, 10:30 am

Yes, its good news that its been got going with very-few IC's needing changing (unlike the Commodore PET'sa themselves).

Intermittent operation on one drive did rather sound like a mechanical issues, that something might be sticking / require lubrication etc.

But if there are separate power rails for the two mechanism, then worth checking / trying to swap these.

ScottishColin · 5th Sep 2023, 2:01 pm

These readings are taken with my meter - am I better of with a scope for this test?

Measured on P6/3 for Drive 1 and P6/5 for Drive 0:

P6/3 (Drive 1) - 12.04V no drive activity / 11.96V when drive is spinning
P6/5 (Drive 0) - 11.79V no drive activity / 11.67V when drive is spinning.

Colin.

SiriusHardware · 5th Sep 2023, 2:35 pm

It is interesting that the drive 0 voltage is on average a little bit lower than the drive 1 voltage but the voltage for the '12V' is not usually too critical, not like the 5V, which has to be 5V +/- only a small amount. It could be that drive 0 is running a tiny bit slower than drive 1, due to the marginally lower supply voltage.

What we are more interested in for now is how 'steady' the 12V supply for drive 0 is. Ideally, when viewed on a scope it should be a steady flat line at 12.0V no matter whether the drive is idle or active, because these are regulated supply rails, the implication being that they shouldn't vary too much even as the load varies.

Start with the scope on DC input mode as usual, put the '0V' trace right down on the bottom line and set the scope to 2V / Div. If you then attach the scope probe to the 'drive 0' 12V rail and initiate some drive activity the trace should hold steady at the 12V mark (about 6 divisions up the screen), and should not vary very much no matter what the drive does. In practice it may sink a little bit whenever the drive is running. What you should not see are any big dips, wobbles, or variations on the 12V line.

Disconnect the scope probe from the 12V line, move the trace to the centre line of the screen, set the input mode to AC instead of DC and set the sensitivity to 0.5V / Div.

Reattach the scope probe to the 12V rail, the trace will momentarily bounce upwards off the screen and then it will re-centre. With the scope set up this way, the large 12V DC offset voltage on the supply rail will be ignored and the scope will only show you any additional variations / wobbles / spikes / noises which are happening on that rail. Try 'exercising' the drive again, does that cause any large blips on, or movement of, the trace?

31st Aug 2023, 10:34 pm	#141
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,102	Re: Commodore 8050 dual disk drive That isn't the smoking gun I was hoping for, but even so the output you are seeing from U11/8 does not conform to the truth table for that type of gate. You may wish to wait for Tony to concur but it looks to me as though the U11 10/9/8 gate has a faulty output (stuck low). Do you have any 74LS32s? I can't remember whether you have ever needed to replace one in anything before.

1st Sep 2023, 4:40 am	#143
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,067	Re: Commodore 8050 dual disk drive Yes, it looks like that 74LS32 has failed. It's what I would have guessed, actually as the motor didn't run either when the control input (from the digital board) was low or when you inserted a disk. That OR gate is the common bit in the digital side of the motor control circuit. I would fit a 14 pin DIL socket, in case you have to replace the IC again ever. There would be no issues with stray capacitance, say, in this circuit. Use a turned-pin socket, they are more expensive but a lot more reliable.

1st Sep 2023, 10:23 am	#144
ScottishColin Octode Join Date: May 2012 Location: Perth, Scotland Posts: 1,616	Re: Commodore 8050 dual disk drive OK - they're on order. I'll remove and pop a socket in later on while I'm waiting. As U7 has already been replaced before I had this device, I'm just wondering what that chip does? I'll probably go back to the printer while I'm waiting for delivery of the 74LS32s. Colin.

1st Sep 2023, 10:34 am	#145
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,067	Re: Commodore 8050 dual disk drive U7 is a dual monostable (timer) IC and is used to generate the read data pulse (page 12 of the .pdf service manual).

1st Sep 2023, 1:30 pm	#147
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,102	Re: Commodore 8050 dual disk drive Shame the engineer didn't sign it as well (when I used to work on certain other bits of equipment I used to 'sign' the PCB so I'd know if it was the same one which came back to me at some point in the future. By an odd coincidence I now own two of those 'signed' units which weren't mine originally). One of them I do know the history of, but the other, I have no idea where it had been in the time between my repairing it and it ending up in my ownership. Once the drive is up and running I think you might look again at those heat stressed diodes - problem there is that when the solder joints and PCB have been heated so much and for so long, the solder turns into something which is not quite solder any more and is much more difficult to melt than solder should be, so any attempt to remove and replace the original diodes could be quite damaging to the PCB and solder pads. If you were going to try it would be better to snip the legs off the original diodes close to their bodies and desolder and remove the individual legs, rather than try to get the diodes out in one piece. I'd also mount the replacements a bit higher off the PCB to allow air to circulate under and around them. Or, just leave well alone...

4th Sep 2023, 5:35 pm	#148
ScottishColin Octode Join Date: May 2012 Location: Perth, Scotland Posts: 1,616	Re: Commodore 8050 dual disk drive Replacement 74LS32 arrived, popped in and both drives now work. Drive 0 seems a little temperamental - needs more than one try sometimes so I'll clean the head and take it from there. This is the best 8050 thread I have found anywhere out there so I hope it's of use to others in the future. In summary, I resocketed the larger chips (but don't know if that was necessary), replaced 2 x 2114s which were found to be faulty by testing them in my PET and replaced a 74LS32 which had a stuck gate. Thanks all for your help. Another one fixed. Colin.

4th Sep 2023, 6:00 pm	#149
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,102	Re: Commodore 8050 dual disk drive Glad to see progress, maybe Tony might have some idea about how to diagnose the reason for the fickleness of drive 0. Maybe it's not running at a steady speed, or maybe the head is not seeking reliably, something along those lines. Anyone with paper or editable electronic copies of the service manual might find it useful to pencil in '4 red flashes on 8050 = 2114 RAM fault?' as it may help someone somewhere down the line. It might be useful to know which two of the 2114s caused the 4-flash fault, if you happened to make a note of it.

4th Sep 2023, 7:04 pm	#150
ScottishColin Octode Join Date: May 2012 Location: Perth, Scotland Posts: 1,616	Re: Commodore 8050 dual disk drive The two 2114s I removed were UC4 and UD4. Also, I have subsequently found thie attached document - see the very last page for advice on four flashes on an 8050. https://drive.google.com/file/d/1eAv...ew?usp=sharing Colin.

4th Sep 2023, 7:16 pm	#151
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,102	Re: Commodore 8050 dual disk drive Oh, brilliant find, if only we'd known about that before. Definitely a keeper.

5th Sep 2023, 4:27 am	#154
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,067	Re: Commodore 8050 dual disk drive There are 2 12V rails, 12VA supplies the motors in one drive. 12VB supplies the motors in th other drive and the read amplifier. It is not clear to me (as we've been confused by this in the past) which drive is a bit unreliable. If it's the one that shares its supply with the read amplifier then conceivably worn brushes/commutator in the spindle motor of that drive could be putting noise onto the supply line and upsetting the amplifier. It certainly can't hurt to 'scope the 12V supply lines when the motors are running. You could also swap the drive units round and see if the fault moves with the drive. My initial thought is that it's a dirty/misaligned/worn head. Maybe one drive got more use than the other. You could also try formatting a blank disk in the dodgy drive and see if it's reliable with that. If so, it points to head radial alignment.

5th Sep 2023, 8:37 am	#156
ScottishColin Octode Join Date: May 2012 Location: Perth, Scotland Posts: 1,616	Re: Commodore 8050 dual disk drive Drive 0 is the unreliable drive. I'll take the ideas above and have a look - thanks. Colin.

5th Sep 2023, 9:27 am	#157
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,102	Re: Commodore 8050 dual disk drive I would try scoping the 12V supplies first - easy to do, and you might get lucky, you might immediately find that the 12V supply to drive 0 looks unstable compared to the equivalent supply for drive 1.

5th Sep 2023, 10:30 am	#158
ortek_service Octode Join Date: May 2018 Location: Northampton, Northamptonshire, UK. Posts: 1,196	Re: Commodore 8050 dual disk drive Yes, its good news that its been got going with very-few IC's needing changing (unlike the Commodore PET'sa themselves). Intermittent operation on one drive did rather sound like a mechanical issues, that something might be sticking / require lubrication etc. But if there are separate power rails for the two mechanism, then worth checking / trying to swap these.

5th Sep 2023, 2:01 pm	#159
ScottishColin Octode Join Date: May 2012 Location: Perth, Scotland Posts: 1,616	Re: Commodore 8050 dual disk drive These readings are taken with my meter - am I better of with a scope for this test? Measured on P6/3 for Drive 1 and P6/5 for Drive 0: P6/3 (Drive 1) - 12.04V no drive activity / 11.96V when drive is spinning P6/5 (Drive 0) - 11.79V no drive activity / 11.67V when drive is spinning. Colin.

5th Sep 2023, 2:35 pm	#160
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,102	Re: Commodore 8050 dual disk drive It is interesting that the drive 0 voltage is on average a little bit lower than the drive 1 voltage but the voltage for the '12V' is not usually too critical, not like the 5V, which has to be 5V +/- only a small amount. It could be that drive 0 is running a tiny bit slower than drive 1, due to the marginally lower supply voltage. What we are more interested in for now is how 'steady' the 12V supply for drive 0 is. Ideally, when viewed on a scope it should be a steady flat line at 12.0V no matter whether the drive is idle or active, because these are regulated supply rails, the implication being that they shouldn't vary too much even as the load varies. Start with the scope on DC input mode as usual, put the '0V' trace right down on the bottom line and set the scope to 2V / Div. If you then attach the scope probe to the 'drive 0' 12V rail and initiate some drive activity the trace should hold steady at the 12V mark (about 6 divisions up the screen), and should not vary very much no matter what the drive does. In practice it may sink a little bit whenever the drive is running. What you should not see are any big dips, wobbles, or variations on the 12V line. Disconnect the scope probe from the 12V line, move the trace to the centre line of the screen, set the input mode to AC instead of DC and set the sensitivity to 0.5V / Div. Reattach the scope probe to the 12V rail, the trace will momentarily bounce upwards off the screen and then it will re-centre. With the scope set up this way, the large 12V DC offset voltage on the supply rail will be ignored and the scope will only show you any additional variations / wobbles / spikes / noises which are happening on that rail. Try 'exercising' the drive again, does that cause any large blips on, or movement of, the trace?