Help needed for a broken 6128

[Bignumbas]

I obtained a 6128 with no screen and manage to plug in 12v with the wrong polarity due to some misinformation, damn
The ram was getting hot and it was booting to the gray screen of death so I replaced all the ram.
It now boots to some ROM copyright info, blue screen and yellow writing as it should  but no command prompt appears.
There is no BASIC 1.1 written on screen, no "Ready" prompt and no flashing cursor.
It seems to me that I may have fried the AMSDOS rom a bit.
Anybody got any suggestions as to what needs replacing?

[Audronic]

@Bignumbas
Can you Post a Photo of the Screen ?
What state do you live in, there may be someone near you who can help ?
Ray

[Bryce]

I obtained a 6128 with no screen and manage to plug in 12v with the wrong polarity due to some misinformation, damn
The ram was getting hot and it was booting to the gray screen of death so I replaced all the ram.
It now boots to some ROM copyright info, blue screen and yellow writing as it should  but no command prompt appears.
There is no BASIC 1.1 written on screen, no "Ready" prompt and no flashing cursor.
It seems to me that I may have fried the AMSDOS rom a bit.
Anybody got any suggestions as to what needs replacing?

Yup, you've probably fried AmsDOS. Luckily it can be replaced with a suitably programmed standard 16K EPROM. So get yourself an EPROM and swap it out. You may however discover that you have sound/keyboard issues after you've fixed the boot problems.

Bryce.

[tjjq44]

To be sure nothing else than amsdos is broken, simply remove your amsdos ROM and boot the CPC, it should behave like a 464: type [run"] then if it asks you "press play then any key" you win! Just replace a new amsdos or even better, parados :-)

[Bignumbas]

Soory I posted this reply to the email I was sent, evidently that goes nowhere 
Hi Ray

I live in Bundaberg, QLD

Photo of the screen?

This is what I am getting
http://www.cpcwiki.eu/forum/index.php?action=dlattach;topic=11251.0;attach=16717;image

If I touch pin 3 on the Zilog FDC I get an error message about line 3190 and then a ready prompt, but every command I type after that gets a bad syntax error.

This topic is the closest I have found to my problem
http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/repairs-to-a-cpc6128/10/

cheers

Mark

[Audronic]

@Bignumbas


Bundaberg is a bit far from Victoria for me to pop over and have a look.
Others may like to chip (sic) in if they are local. ?
That looks very similar to my problems (The Link).
It would be very unusual for the same fault ?
Anyway put your Finger/probe/CRO near IC 201 and re Boot ?
Read the link stuff as I had a good time fixing 2 Similar faulty units.
Good luck


Ray

[Bignumbas]

Well i am going in boots and all now.I have ordered some Eproms, 32k and 16k chips and I am going to put in Parados and as this is a French system am going to burn the UK basic as well.I found you can get the key decals online now so I will change the french keys to english. Never got the hang of an azerty keyboard, I used to work on french language windows PC's.I have also ordered a 74hc240 for ic203 as well.When I look at where the reverse polarity goes its basically the whole board.I scored a 3.5 " drive that I can turn on ready so I will be putting in an abba switch plus I want to try out my Gotek drive on the 6128  when I get all this together.I have ordered a new drive belt for the 3" drive but do not have any discs, yet.
I am also looking at a dual rom setup as well.
On an off topic note I will be burning some Amiga 500 3.1 kickstarts as well, just waiting on chipsand I have a Sega mastersystem game cartridge I will be putting in an eprom to play with.
I had a 464 back in 1985 with a DD1 drive & colour monitor, those were the days! 

[Bryce]

Did you just randomly choose IC203 to replace or have you reason to believe that it is fried? If you were really lucky, the ROM took the hit and shorted before anything else had a chance to die, but unfortunately this is only rarely the case. I have repaired CPCs that had the same fate and had to swap almost 70% of the ICs. The most sensitive ICs on the board are the RAM, the AY and the CRTC. The TTLs may have all survived.

Bryce.

[Bignumbas]

Well my only guide I could find is this post http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/repairs-to-a-cpc6128/.Replacing all the ram brought me halfway back to a live CPC.I find the schematic in the Amstrad service manual a little lacking in detail. It misses out some detail and ic's
Do you really live in Wallis Futuna Bryce?I lived in Vanuatu for 15 years.

[Bryce]

No, I live in Germany, but I like to choose some obscure little Islands flag just for fun. I change it on occassions.

Vanuatu?? Wow, that's remote, what were you doing out there? Did you have a CPC there!?? 

Define "halfway back to a live CPC", what's it doing now?

Bryce.

[1024MAK]

I obtained a 6128 with no screen and manage to plug in 12v with the wrong polarity due to some misinformation, damn

@Bignumbas, did you plug a 12V supply into the 5V input? Is that what you think you did? Only all the logic, the CPU, memory etc are all supplied by the +5V rail.

Mark

[Bignumbas]

No I had reverse polarity on the 12 volt supply. I am using an old ide external power supply that had a 4 pin plug with 12v and 5v.When I initially plugged it in it seem to short out the power supply. 
The when I reversed it back and powered on there was only the grey screen and all the ram chips were getting hot.

[Bryce]

Then you effectively put -7V on the 5V rail. Most chips won't have been happy with this.

Bryce. 

[Bignumbas]

I have noticed this Bryce. Just trying to pick my way through the dead ones.

[Audronic]

Then you effectively put -7V on the 5V rail. Most chips won't have been happy with this.
Bryce.

I this the circuit ?


Ray

[Bryce]

Where did you find that? Is it in the Service manual?

That part of the circuit switches the 12V supply on as soon as the 5V rail is ready. If you put 12V backwards on to that circuit, the 5V rail is sharing the same ground circuit, so the 12V will flow into the 5V rail in the wrong direction. However, those transistors will never switch on because the 5V rail is now effectively seeing -7V.

The delay on power up is done here:

[attach=2]

R101 and C135 form an RC delay. C135 is (relatively) slowly charged through R101. When the correct voltage is achieved it will trigger IC117 which triggers a reset several milliseconds after power has been applied. This ensures that all ICs are up and running and all started at exactly the same time.

Bryce.

[Audronic]

Where did you find that? Is it in the Service manual?

Bryce.

Hi Bryce


It is in the :- Amstrad Amendment Service Manual.PDF
On Page 20 ( CPC6128 Chassis Schematic Diagram )

Thanks for the explanation of the Power up Delay (Reset)

Ray

[Bignumbas]

Thanks for that missing info Bryce.I have just received some parts from china on the slow boat.I have put in sockets for both the roms.I have replaced the ls74chip above the FDC at the end of the board.When I put in the 40025 System Rom alone the system boots to the ready prompt but asks for a cassette when a DIR command is given, Whew! I have burnt a UK rom to replace the FR one and it works so I now have a QWERTY KB.If I put in the DOS 40015 ROM it does not get to the ready prompt.I am going to burn your Dual dos rom tonight Bryce and try that.I have some Winbond 256k roms to play with now.Things are looking better 

[Bignumbas]

Well its good news and bad news.Looks like I have a 464 for the moment. CAT command responds insert cassette.Keyboard works fine as a Qwerty with my new UK 6128  40025 rom.When I plug in the Amsdos rom 40015 and type |DISC I get Unknown command.Looks like this rom is not being seen by the system.Has anybody got a suggestion as to why?

[00WReX]

So is that with the original AMSDOS ROM or have you burnt a new one ?
I see you have burnt a 40025 ROM, but not clear about the 40015.

Cheers,
Shane

[Audronic]

@Bignumbas

I have just burned the Parados file to a 27c128 to replace the 40015 in the DDI-1 and this works for my 464,  I know that you want to put it into a 6128.

The zip contains Amsdos.v0.5 and Parados 1.2 Modified 40-40 Track

Ray

[Bignumbas]

I have both he original roms, 40051(french) and 40015[size=78%].[/size][/size]I have made a 40025 (English) and it works. Inserted by itself with no 40015 I get the READY prompt and I can type qwerty but it wants a cassette inserted on the CAT command.The original 40015 when it is installed with the 40025/40051 stops the boot process just before the prompt, so there is no READY prompt. [size=78%][/size]I have experimented with making the 40015 and when it is correct ( I think) I get the same problem as with the original 40015 rom. [size=78%][/size]When I make one and it is not correct I get the READY prompt but it does not recognize the disk commands and says to insert a cassette just like there is no recognized 40015.[size=78%]Clear as mud now? 

[Audronic]

@Bignumbas


Have a look at pin 20 On the 40015 rom it should be LOW to enable the rom.??
This comes from the 40007/10


Good Luck    Ray

[Audronic]

@Bignumbas


Any Progress ?


Ray

[Bignumbas]

A little progress, I have replaced Q302, Q303, IC203, IC207 & IC212 plus the entire 16 ram chips.
I have also put sockets in for the two rom chips 40015 and 40025.
I tested IC206 and IC208 pin voltages and they were both radically different to each other so  I ordered them and am waiting again.
Maybe when I get them it will work.
The keyboard works and the system boots to the ready prompt without the 40015 plugged in.
I have no digital test equipment so that makes it harder, I am left with part replacement.
I bought a mix of stuff on ebay the other day for $74 because I spotted a Dktronics 64k expansion module in among the stuff, I have ordered some 41256 chips for that.
Plus it had an Amstrad game tape so i might try loading that to see if the system works on cassette.
I also had to replace the drive belt but I have no 3" disks as yet but I do have an HXCusb Gotek ready for when I get the 40015 to be recognized.
What would the voltage be for a low signal?