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Hi guys,

A working 6128 that I own for ~20 years started giving me trouble. A floppy drive related command will wait for a couple of seconds and then return with a "Bad command". There is absolutely no drive activity in the meantime.

In my ignorance I assumed that the problem should be around the floppy controller chip, a cut connection or a chip that died. I was also looking for an excuse to get a desoldering gun and had a few spare parts so you get where this is going.... I ended up reflowing that part of the board (everything to the right of the memory chips) and added sockets and new 74LS chips, 74HC240, FDC. I managed not to kill it, but nothing changed.

Anyone has any idea where to focus next, a potential culprit for this error maybe? Does it make sense to reflow the rest of the board or to change more 74LS chips?

Thanks in advance for any suggestions, I don't have the knowledge to do any targeted move myself.

Quote from: konc on 09:36, 16 March 22A floppy drive related command will wait for a couple of seconds and then return with a "Bad command". There is absolutely no drive activity in the meantime.

So the floppy does not even spin up?

Quote from: Poliander on 09:47, 16 March 22

Quote from: konc on 09:36, 16 March 22A floppy drive related command will wait for a couple of seconds and then return with a "Bad command". There is absolutely no drive activity in the meantime.

So the floppy does not even spin up?

Correct, there isn't any reaction/attempt from the floppy whatsoever. As if you never hit return to be 100% clear. 

This also happens if the head cannot move. I found a (German) blog article where the head was blocked and this caused the same symptoms. (https://www.jungsi.de/retro-neu-der-sammlung-schneider-cpc-6128/)

This can also happen if you have a bad AMSDOS ROM.

Bryce.

Hello again,

When in doubt post on a nice forum and you'll get proper leads.

Quote from: eto on 10:33, 16 March 22This also happens if the head cannot move.

This made me think differently. It turns out I have a 1.1V drop when the Amstrad is on. Which is enough for the computer to start, but not for the floppy drive part to do anything. To clarify:

1647940729840.jpg

I measure at the power jack, it's 5V (ok, 4.9). I turn on the Amstrad, it's 3.8V.
I changed the power jack, removed (jumpered) the switch, disconnected the SCART cable, disconnected the floppy drive. Still 3.8V.
At first I thought the PC PSU I'm powering the Amstrad from is dying. Tried another, the same. Brought a friend over with a proper bench PSU, the same. We increased temporarily the voltage to 5.6V so that the Amstrad gets 4.5V (again the same 1.1V drop) and everything is working correctly. I loaded and played 2 games!

So something inside the Amstrad is causing this significant voltage drop, but also it seems that nothing is completely shorted. Given that I reflowed and added a bit of new solder to every single solder joint there is on the board no matter how irrelevant it looked, the capacitors are all new except the bipolar one, and that no chip is burning or is even remotely hotter than it should, I suspect a marginal connection due to oxidation somewhere. Please correct me if this makes no sense.

But since the machine is working I can't focus on a single chip/route (not that I know how anyway...) and nothing looks suspect to my untrained eyes, so I have no clue how to proceed. I'm also afraid at this point that it might be somewhere hidden.

corroded power jack? or corroded power switch?

Quote from: eto on 12:08, 22 March 22corroded power jack? or corroded power switch?

Quote from: konc on 10:57, 22 March 22I changed the power jack, removed (jumpered) the switch, disconnected the SCART cable, disconnected the floppy drive. Still 3.8V.

Unfortunately no, that would have been nice and easy.

Are the wires from the PSU too thin?

Bryce.

Quote from: Bryce on 15:10, 22 March 22Are the wires from the PSU too thin?

Bryce.

Just for curiosity and to learn something: Wouldn't that need to be very, very thin to cause a 1.1V drop?

What (in general) could cause a voltage drop at the points where the voltage is measured?

My first thoughts would be:
1) some (significant) resistance/load between the PSU and the points where the voltage is measured => cable, connector, soldering points, crack in the track?
2) the CPC drawing more power than the PSU can deliver, so the PSU drops the voltage => reason?

Is that right? Anything else? Sorry, my "knowledge" in electronics is only what I learned from this forum over the last year, and what I remember from school ;-) so I'm really not sure if this is right or covers everything.

That's pretty much correct, plus possibly a partial short inside a chip. But if the wire from the PSU is too long or too thin, that's a resistor too.

Bryce.

Quote from: Bryce on 15:10, 22 March 22Are the wires from the PSU too thin?

Bryce.

That's a very good point that I never thought of, ok not exactly the wires being very thin but something wrong with the power wire.
You see the only thing that remained the same while trying a second PC PSU and my friend's bench PSU was my little adapter. Maybe there was something wrong with it? It's not new either, I've been using it for all these years that I own this 6128.

So I quickly made a new one just to check the 5V, it's made from a dead PSU that was able to drive 20A through these wires and I connected it directly to a brand new PC PSU, just to rule out the wiring. This is what powers the Amstrad now, a new adapter, a new/different PSU and definitely not long or thin wires.

1647977514865.jpg

1647978342145.jpg


And damn it, the power wires are ruled out. Still 3.8V at the socket  :( I guess I'm looking at the bad scenario where I have to find a bad track, maybe hidden under the solder mask, or a semi-shorted chip without a clear lead since with a bit more juice everything seems to work?

Is it possible to measure the "Current" being drawn by the Motherboard ?

Keep Safe
Ray

I don't suppose you own a thermal camera? A 1.1V drop at 1.5A is producing 1.65W of heat which should be easily visible on a thermal camera.

Bryce.

Quote from: Audronic on 00:25, 23 March 22Is it possible to measure the "Current" being drawn by the Motherboard ?
Keep Safe
Ray

Unfortunately not directly, at least I don't know how. My multimeter doesn't measure current and the friend with the bench PSU isn't that close to bring him over again. However, I do have a super cheap and obviously terrible quality chinese power adapter. It claims to be 5V/1.5A but I bet it can't even deliver that. Well with this adapter the Amstrad turns on and I measure 3.7V at the socket, which means (given the 1.1V drop) it gives 4.8V at no more than 1.5A. I don't know if this is enough to reach any conclusion, with a short it would draw a lot more than 1.5A or is it within possible range?

Quote from: Bryce on 11:35, 23 March 22I don't suppose you own a thermal camera? A 1.1V drop at 1.5A is producing 1.65W of heat which should be easily visible on a thermal camera.
Bryce.

No I don't. And after a bit of googling, getting one doesn't seem probable  :)

Ok we cannot go any further.
? what country are you in ?
there may be somebody nearby who can help ?

Keep Safe

Ray

Understandable, at least we made sure that I didn't miss anything important until now. 
I'm in Greece, at some point I'll visit someone from local forums with a bench PSU that can measure current, or promise a lot of beers to the guy who already came once, or even get one myself if I can justify it and I'll report back.
For now thank you all for the leads and ideas, they did put me in the right track.

Quote from: konc on 20:53, 23 March 22

Quote from: Audronic on 00:25, 23 March 22Is it possible to measure the "Current" being drawn by the Motherboard ?
Keep Safe
Ray

Unfortunately not directly, at least I don't know how. My multimeter doesn't measure current and the friend with the bench PSU isn't that close to bring him over again. However, I do have a super cheap and obviously terrible quality chinese power adapter. It claims to be 5V/1.5A but I bet it can't even deliver that. Well with this adapter the Amstrad turns on and I measure 3.7V at the socket, which means (given the 1.1V drop) it gives 4.8V at no more than 1.5A. I don't know if this is enough to reach any conclusion, with a short it would draw a lot more than 1.5A or is it within possible range?

Quote from: Bryce on 11:35, 23 March 22I don't suppose you own a thermal camera? A 1.1V drop at 1.5A is producing 1.65W of heat which should be easily visible on a thermal camera.
Bryce.

No I don't. And after a bit of googling, getting one doesn't seem probable  :)

You have a multimeter that doesn't measure amps?? Wow, I didn't know such a thing existed!

Bryce.

Quote from: Bryce on 09:45, 24 March 22You have a multimeter that doesn't measure amps?? Wow, I didn't know such a thing existed!

It is easy to recognise this multimeter, it doesn't measure either ohms!

Quote from: Bryce on 09:45, 24 March 22

Quote from: konc on 20:53, 23 March 22

Quote from: Audronic on 00:25, 23 March 22Is it possible to measure the "Current" being drawn by the Motherboard ?
Keep Safe
Ray

Unfortunately not directly, at least I don't know how. My multimeter doesn't measure current and the friend with the bench PSU isn't that close to bring him over again. However, I do have a super cheap and obviously terrible quality chinese power adapter. It claims to be 5V/1.5A but I bet it can't even deliver that. Well with this adapter the Amstrad turns on and I measure 3.7V at the socket, which means (given the 1.1V drop) it gives 4.8V at no more than 1.5A. I don't know if this is enough to reach any conclusion, with a short it would draw a lot more than 1.5A or is it within possible range?

Quote from: Bryce on 11:35, 23 March 22I don't suppose you own a thermal camera? A 1.1V drop at 1.5A is producing 1.65W of heat which should be easily visible on a thermal camera.
Bryce.

No I don't. And after a bit of googling, getting one doesn't seem probable  :)

You have a multimeter that doesn't measure amps?? Wow, I didn't know such a thing existed!

Bryce.

I guess it's not too "multi" :) Well now I have an excuse to get a better multimeter and learn how to measure amps. I only know that it's not as trivial as voltage and you need to intercept the connection and pass it through the meter. 

Hello again,
So I got a new multimeter and learned how to measure current. I found it easier to intercept the molex wire that could be easily removed from the plug

1648659683529.jpg

With the floppy drive disconnected (if it plays any role if it's not used) and only feeding the 5V it's drawing 0.8A, 0.78 to be precise. It seems normal to me but what do you guys think?

That seems normal. What about the 12V rail. Is it at 12V and how many amps is it pulling?

Bryce.

Ah does it matter? It's not connected at all, like the floppy drive. I never thought that a problem in the 12V would affect the 5V, let me measure that as well and I'll post back.

Quote from: Bryce on 15:47, 31 March 22That seems normal. What about the 12V rail. Is it at 12V and how many amps is it pulling?

Bryce.

I'm back, 12V looks good to me. I measure 12.1 at the first pin header and the full 12.1 reach the floppy drive power connector (floppy disconnected). It draws 60mA.

Behold a fully socketed board that's driving me nuts. 

I removed every single chip to power on the computer without it and check the voltage, none made any difference. Still the same 1.1V drop, so no chip is shorted or makes a bad connection. 

Unless anyone has a new hunch I don't see what else I can do with it except keeping it for parts (especially now that I can remove any chip easily!) and placing a local ad for a new one. 

I have the same symptoms as konc (bad command on cat). I checked the voltages and I have 5,176 in the powerjack when cpc is off and 4,873 when cpc is on. I dont have 1.1 drop like konc but a small one. I dont know if this is enough for the same symptoms.
I also checked the voltages that go to the drive itself. i have 12.1 and 4.81. Is 4.81 enough? Should it be at least 5?

Quoting myself

Quote from: konc on 10:57, 22 March 22Brought a friend over with a proper bench PSU, the same. We increased temporarily the voltage to 5.6V so that the Amstrad gets 4.5V (again the same 1.1V drop) and everything is working correctly. I loaded and played 2 games!

So even 4.5 is enough. 4.8-4.9 are very common values and considered normal

So back to square one for me. I have to find (with some help) which chip might be failing

Is the drive doing anything when you type cat, does it spin up?

If I am not mistaken, "Bad Command" can also happen if the head is not moving.

Quote from: eto on 10:47, 30 April 22If I am not mistaken, "Bad Command" can also happen if the head is not moving.

That is correct. The head is not moving. I have to find why. Because i dont want to hijack konc's thread, I have already started a similar topic a couple of months ago. I am here because the voltages was a new undiscovered approach. 

A quick lesson in drive failure diagnosis:

The first thing that "Bad command" tells you is that BASIC hasn't registered the commands that ROM7 offers. The next step is to find out why. Does ROM7 work? Is it getting the voltage required? (probably, because the rest of the computer works). Is it damaged? Can you get ROM7 to react to anything? What gets sent to the 765 when you do a CAT and how does it react to the command?
The head will only move if everything works: ie: ROM7 understood the command and sent it to the disk controller (765). So the next step is to check whether the ROM correctly reacted to the command. If it did, then the next step is to check what the 765 did after the command arrived: Did it activate the correct outputs? If it did, you need to look at how the drive reacted to those outputs. Were all the bits (drive select, MotorOn, Step, etc) set correctly (there are several buffers IC's in between that could have failed)? If they were, why didn't the drive react accordingly? Broken connection, failed buffer? Is the 12V rail missing? Is the drive spinning up? Is the 765 getting a proper index pulse back from the drive?

Start from the source and work towards the mechanical results. Check that all the right signals are getting sent (CPU -> ROM7 -> 765 -> Buffers -> Drive). Then check that the drive has the voltages to react to them and then check that the drive is sending the right signals back (also via buffer IC's).

Bryce.

Quote from: Bryce on 17:33, 30 April 22A quick lesson in drive failure diagnosis:

The first thing that "Bad command" tells you is that BASIC hasn't registered the commands that ROM7 offers. The next step is to find out why. Does ROM7 work? Is it getting the voltage required? (probably, because the rest of the computer works). Is it damaged? Can you get ROM7 to react to anything? What gets sent to the 765 when you do a CAT and how does it react to the command?
The head will only move if everything works: ie: ROM7 understood the command and sent it to the disk controller (765). So the next step is to check whether the ROM correctly reacted to the command. If it did, then the next step is to check what the 765 did after the command arrived: Did it activate the correct outputs? If it did, you need to look at how the drive reacted to those outputs. Were all the bits (drive select, MotorOn, Step, etc) set correctly (there are several buffers IC's in between that could have failed)? If they were, why didn't the drive react accordingly? Broken connection, failed buffer? Is the 12V rail missing? Is the drive spinning up? Is the 765 getting a proper index pulse back from the drive?

Start from the source and work towards the mechanical results. Check that all the right signals are getting sent (CPU -> ROM7 -> 765 -> Buffers -> Drive). Then check that the drive has the voltages to react to them and then check that the drive is sending the right signals back (also via buffer IC's).

Bryce.

The only thing that is tested and work, is the drive itself (Connected in another cpc6128). The other links in the chain I dont have the knowledge and the ability to test them.

Quote from: vasilisk on 17:56, 30 April 22

Quote from: Bryce on 17:33, 30 April 22A quick lesson in drive failure diagnosis:

The first thing that "Bad command" tells you is that BASIC hasn't registered the commands that ROM7 offers. The next step is to find out why. Does ROM7 work? Is it getting the voltage required? (probably, because the rest of the computer works). Is it damaged? Can you get ROM7 to react to anything? What gets sent to the 765 when you do a CAT and how does it react to the command?
The head will only move if everything works: ie: ROM7 understood the command and sent it to the disk controller (765). So the next step is to check whether the ROM correctly reacted to the command. If it did, then the next step is to check what the 765 did after the command arrived: Did it activate the correct outputs? If it did, you need to look at how the drive reacted to those outputs. Were all the bits (drive select, MotorOn, Step, etc) set correctly (there are several buffers IC's in between that could have failed)? If they were, why didn't the drive react accordingly? Broken connection, failed buffer? Is the 12V rail missing? Is the drive spinning up? Is the 765 getting a proper index pulse back from the drive?

Start from the source and work towards the mechanical results. Check that all the right signals are getting sent (CPU -> ROM7 -> 765 -> Buffers -> Drive). Then check that the drive has the voltages to react to them and then check that the drive is sending the right signals back (also via buffer IC's).

Bryce.

The only thing that is tested and work, is the drive itself (Connected in another cpc6128). The other links in the chain I dont have the knowledge and the ability to test them.

Then you need to send your mainboard to someone who can check the first parts of the chain.

Bryce.

Quote from: Bryce on 18:04, 30 April 22

Quote from: vasilisk on 17:56, 30 April 22

Quote from: Bryce on 17:33, 30 April 22A quick lesson in drive failure diagnosis:

The first thing that "Bad command" tells you is that BASIC hasn't registered the commands that ROM7 offers. The next step is to find out why. Does ROM7 work? Is it getting the voltage required? (probably, because the rest of the computer works). Is it damaged? Can you get ROM7 to react to anything? What gets sent to the 765 when you do a CAT and how does it react to the command?
The head will only move if everything works: ie: ROM7 understood the command and sent it to the disk controller (765). So the next step is to check whether the ROM correctly reacted to the command. If it did, then the next step is to check what the 765 did after the command arrived: Did it activate the correct outputs? If it did, you need to look at how the drive reacted to those outputs. Were all the bits (drive select, MotorOn, Step, etc) set correctly (there are several buffers IC's in between that could have failed)? If they were, why didn't the drive react accordingly? Broken connection, failed buffer? Is the 12V rail missing? Is the drive spinning up? Is the 765 getting a proper index pulse back from the drive?

Start from the source and work towards the mechanical results. Check that all the right signals are getting sent (CPU -> ROM7 -> 765 -> Buffers -> Drive). Then check that the drive has the voltages to react to them and then check that the drive is sending the right signals back (also via buffer IC's).

Bryce.

The only thing that is tested and work, is the drive itself (Connected in another cpc6128). The other links in the chain I dont have the knowledge and the ability to test them.

Then you need to send your mainboard to someone who can check the first parts of the chain.

Bryce.

I was about to. I waited to check the voltages like konc's situtation

Quote from: konc on 14:10, 14 April 22Behold a fully socketed board that's driving me nuts.

I removed every single chip to power on the computer without it and check the voltage, none made any difference. Still the same 1.1V drop, so no chip is shorted or makes a bad connection.

Unless anyone has a new hunch I don't see what else I can do with it except keeping it for parts (especially now that I can remove any chip easily!) and placing a local ad for a new one.

Have you check the tape relay?. It's connected to the 5v power line when you take out all ics In socket.

Are you sure you've properly cleaned the power switch and power socket?

Bryce.

Quote from: dragon on 12:34, 24 May 22

Quote from: konc on 14:10, 14 April 22Behold a fully socketed board that's driving me nuts.
I removed every single chip to power on the computer without it and check the voltage, none made any difference. Still the same 1.1V drop, so no chip is shorted or makes a bad connection.
Unless anyone has a new hunch I don't see what else I can do with it except keeping it for parts (especially now that I can remove any chip easily!) and placing a local ad for a new one.

Have you check the tape relay?. It's connected to the 5v power line when you take out all ics In socket.

That's the Omron black or white box on the top left of the motherboard, right? I removed it completely and measured, no change.
When I say I tried removing every IC I mean one by one, removing the IC, soldering a socket and powering on the computer without that chip to see if there was any difference, measuring on the 5V socket directly.

Quote from: Bryce on 13:00, 24 May 22Are you sure you've properly cleaned the power switch and power socket?
Bryce.

Yes, one of the first things just to be sure: The socket has been replaced with a new one and I only measure with the switch overridden (jumper on the pins)

Quote from: konc on 14:00, 24 May 22

Quote from: dragon on 12:34, 24 May 22

Quote from: konc on 14:10, 14 April 22Behold a fully socketed board that's driving me nuts.
I removed every single chip to power on the computer without it and check the voltage, none made any difference. Still the same 1.1V drop, so no chip is shorted or makes a bad connection.
Unless anyone has a new hunch I don't see what else I can do with it except keeping it for parts (especially now that I can remove any chip easily!) and placing a local ad for a new one.

Have you check the tape relay?. It's connected to the 5v power line when you take out all ics In socket.

That's the Omron black or white box on the top left of the motherboard, right? I removed it completely and measured, no change.
When I say I tried removing every IC I mean one by one, removing the IC, soldering a socket and powering on the computer without that chip to see if there was any difference, measuring on the 5V socket directly.

Mmm. And you have try with all ics unmount at same time and was the same i assume.

Well I don't know if this can be done really. But I'm thinking two tests.

1. Follow the 5v rail path,and divide it in zones. The idea was that in some points of board you solder something that cut the voltage to the rest of the path without cut the track phisically of course. Or maybe there is some jump in the board track  that you can use for that. The idea was find the exact zone that is cause the problem.

2. If you don't use the power socket and try insert 5v in other sites?. As the hole of an ic directly?. this can be mixed with one,for example if you find a jump in 5v track and you can interrupt the power to the rest of the board. You can create a isolated zone and enter the 5v in these zone and view if it drop.

I don't Know if the Current has been checked if not Please check how much current is been drawn ?

Keep Safe

Ray

Quote from: Audronic on 23:29, 24 May 22I don't Know if the Current has been checked if not Please check how much current is been drawn ?

Keep Safe

Ray

Yes, it's on the previous page, see here: https://www.cpcwiki.eu/forum/hardware-related/cpc-6128-returns-bad-command-on-cat/msg214255/#msg214255

Ok I missed it
Ray