Author Topic: Short circuit protection on the CPC / monitor  (Read 2880 times)

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Offline 00WReX

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Short circuit protection on the CPC / monitor
« on: 02:44, 04 October 12 »
OK, here goes another stupid question that I just have to ask  :D  sorry in advance  :-[

I have not heard or read of this before, but does the Monitor / CPC power supply have short circuit protection on the 5v (or 12v).
 
And yes, I shorted pins 27 (Vcc 5v) and pin 49 (Gnd) on the expansion connector  :-X .
 
Not intentional, but lack of thought while playing with a mod. (should not have started it after a hard night shift & lack of sleep  :laugh: )
 
Immediately the CPC went off. The CTM (monitor) was still on and turning the CPC on and off it was dead...no power LED, nothing.
 
I turned off the CTM off and then back on, and then the CPC fired up...all appears fine.
 
Also, can this type of short cause any potential issues on the CPC.
 
The reason I am playing with this CPC6128 is that it is a "sacrificial one"...not that I want to kill it, but it is one I set aside to modify, and I picked this one because it has a sound issue that I beleive is the AY chip (issue has been there as long as I have had the CPC).
 
Amstrad CPC 6128 sound problems.
 
Now the weird part...After the CPC fired back up, I tested a heap of things to give it a workout, everything worked fine including software that requires 128k etc, except a strange thing with FutureOS (just happened to be the last thing I ran, before going to bed).
FuturOS on my MegaROM, (and also the MegaFlash) locks up immediately at the main menu screen comes up.
This previously worked, and the ROM boards plugged into another CPC it works fine.
All the other ROMs I have work fine & SYMBOS also works fine.
 
On top of that, it gets stranger...If I hold down an arrow key when I load FutureOS, the cursor moves around the screen, and as long as I dont let go of a cursor key (so keep holding one while filcking over to the next) I cant move the pointer around the screen.
 
As soon as I let go, it locks up. Only happens on the CPC I shorted...
 
Again sorry for the stupid / strange post...but interested in any thoughts (other than your an idiot  :P ).
 
Cheers,
Shane
 
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Offline Bryce

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Re: Short circuit protection on the CPC / monitor
« Reply #1 on: 12:24, 04 October 12 »
The GTxx and CTMxx use a 78LMxx regulators for both the 5V and 12V outputs. This little component is not only unbelievably robust, but it also has its own built in thermal cut-off protection. If you short the outputs the 78 will shut off pretty quickly, you should leave it off for a few minutes to cool down before you turn it back on. It can only take so much though, so shorting the output for more than a few seconds could destroy the monitors output completely.

On the CPC side, it all depends where you shorted the voltage. The expansion port isn't a good place to do this. Depending what tracks the high current flowed through, you may have burnt some of the smaller 5V rail tracks, so certain components may not be getting 5V any more, which might explain why FOS is having issues. But you also mentioned that this CPC was having sound issues, so it might just be that the AY is dying and has nothing to do with your shorting of the 5V. Especially as the AY also looks after the keyboard scanning.

Bryce.

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Re: Short circuit protection on the CPC / monitor
« Reply #2 on: 12:27, 04 October 12 »
The most dangerous thing on CPC is the +12V line.
For users using only 3"1/2 or HxC drives, the best is to remove the external cable to no more input this voltage. (don't lose it)

I doubt that +5v can kill anything on the CPC. I'm wrong ?
"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

Offline Bryce

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Re: Short circuit protection on the CPC / monitor
« Reply #3 on: 12:33, 04 October 12 »
I doubt that +5v can kill anything on the CPC. I'm wrong ?

It's the current that kills, not the voltage, but shorting the 5V would rarely damage the CPC, but it's still not a good idea. It would kill things if it was connected the wrong way around though. I also agree with TotO, if you don't use the 12V, remove the flylead.

Bryce.

Offline 00WReX

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Re: Short circuit protection on the CPC / monitor
« Reply #4 on: 14:07, 04 October 12 »
And I still feel like an idiot for shorting it...normally a lot more careful than that.  :-[
 

Anyway, just pulled the CPC apart again, gave the main board a good check over, nothing obvious blown...and no tracks missing.  ;)

 
Did a continuity check from contact 27 on the expansion connector to every IC's 5v pin which really only takes a minute or two to check.
They all buzzed OK.
 

Like I mentioned the CPC appears to be doing everything right except for the original sound problem, and now FutureOS locks as mentioned above.
 

I have run heaps of different games including the latest R-Type, the Batman demo + a couple of other demos as I thought they would give the CPC a good workout...all fine.
 
All keys are working, joystick working.
 

Thanks for the info regarding the regulator in the monitor.
Also, I did think maybe the AY has died a bit more as it handles the keyboard scan, but all keys are good.
And yes I have the two CPC's on my desk with the 12v fly lead removed...mainly because it is ugly hanging out the back doing nothing.

 
Thanks for the replies, it's just irritating me at the minute why one program won't work...

 
Cheers,
Shane
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Offline Bryce

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Re: Short circuit protection on the CPC / monitor
« Reply #5 on: 14:32, 04 October 12 »
I'd swap the AY anyway, just to see if that solves the problem.

Bryce.

Offline 00WReX

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Re: Short circuit protection on the CPC / monitor
« Reply #6 on: 15:34, 04 October 12 »
Thanks again.
 
Yes, I'll try to get my hands on one.

Might grab a couple from here...
 
1pcs AY-3-8912 Programmable Sound Generator | eBay

Cheers,
Shane
« Last Edit: 15:43, 04 October 12 by 00WReX »
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Offline 00WReX

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Re: Short circuit protection on the CPC / monitor
« Reply #7 on: 14:49, 06 October 12 »
OK ordered a couple of the AY chips...will see how they go when they turn up.
On the other issue with FutureOS...I will move that to FutureOS Corner as I have found the same issue on another CPC I have that has the same revision of motherboard. I initially thought that I had tried FutureOS on this CPC but I had not long swapped my ROM board onto this CPC so It was another CPC that it was working on.
 
Cheers,
Shane
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Offline Bryce

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Re: Short circuit protection on the CPC / monitor
« Reply #8 on: 16:56, 06 October 12 »
I have the same Issue with FOS on a CPC6128+ and on 2x CPC464+, but TFM still seems to think that it's a hardware problem.

Bryce.

Offline robcfg

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Re: Short circuit protection on the CPC / monitor
« Reply #9 on: 19:50, 06 October 12 »
Maybe it's a long shot, but the 20I motherboard and the Plus motherboards all have Microchip instead of General Instruments AY-3-8910's.


I guess they should be identical, but then again, all 6845's should be identical too...

Offline 00WReX

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Re: Short circuit protection on the CPC / monitor
« Reply #10 on: 02:33, 07 October 12 »
Maybe it's a long shot, but the 20I motherboard and the Plus motherboards all have Microchip instead of General Instruments AY-3-8910's.


I guess they should be identical, but then again, all 6845's should be identical too...

 
Wow, that's interesting you mention that...I not long before you mentioned the same thing.

FutureOS corner
 
It will be interesting if the AY chips I have just ordered are indeed "GI" versions when they arrive.
And if they are, then maybe I can see for myself if they behave differently.
 
Cheers,
Shane
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Offline TFM

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Re: Short circuit protection on the CPC / monitor
« Reply #11 on: 03:13, 07 October 12 »
WoW! Great idea! Yes if you would replace the AY and things change their behaviour (run, or not working), then we would know that the AY is the cause :o  Good luck! I'm very curious!
 
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Offline TFM

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Re: Short circuit protection on the CPC / monitor
« Reply #12 on: 03:14, 07 October 12 »
I have the same Issue with FutureOS on a CPC6128+ and on 2x CPC464+, but TFM still seems to think that it's a hardware problem.

Bryce.
Hihi! Yes, indeed! And it is highly likely the AY, soon we will know more... can't wait!
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Offline robcfg

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Re: Short circuit protection on the CPC / monitor
« Reply #13 on: 03:31, 07 October 12 »
That would be very interesting to know.


Maybe it does something weird with port reading...


Great misteries of the XXI century!  :P

Offline 00WReX

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Re: Short circuit protection on the CPC / monitor
« Reply #14 on: 04:50, 07 October 12 »
Just looked over every Main board on the CPC wiki page and found the following used the "microchip" AY, the rest used the GI.
 
CPC464
MC0044D with the 40010 Gate Array (the MC0044D with 4007 Gate Array uses the GI)
MC0099A (the last 464 MB)
 
CPC6128
MC0020I
MC0023G (Schneider with Centronics connectors)
MC0100A (the last 6128 MB)
 
CPC464+ & CPC6128+
MC0122B
MC0122C
MC0122D
 
Cheers,
Shane
The CPC in Australia...
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