CPC 6128 returns "Bad command" on cat

[konc]

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.

[Poliander]

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.

So the floppy does not even spin up?

[konc]

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.

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. 

[eto]

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/)

[Bryce]

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

Bryce.

[konc]

Hello again,

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

This 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:

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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.

[eto]

corroded power jack? or corroded power switch?

[konc]

corroded power jack? or corroded power switch?

I 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.

[Bryce]

Are the wires from the PSU too thin?

Bryce.

[eto]

Are 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.

[Bryce]

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.

[konc]

Are 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.

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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?

[Audronic]

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

Keep Safe
Ray

[Bryce]

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.

[konc]

Is 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?

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.

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

[Audronic]

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

Keep Safe

Ray

[konc]

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.

[Bryce]

Is 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?

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.

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.

[TotO]

You 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!

[konc]

Is 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?

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.

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. 

[konc]

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

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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?

[Bryce]

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

Bryce.

[konc]

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.

[konc]

That 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.

[konc]

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.