GX4000 To CPC Modification Progress So Far...

[Badstarr]

Hi guys!


Its been a while since a have had any real time to dedicate to this project but the easter break has finally given me the opportunity to make some progress.


Some may remember that my cat attacked my GX4000 mod a while ago which resulted in some damage that first appeared to have bricked the console, then after a little investigation I found a capacitor had twisted round shorting the Video Conversion chip that feeds the SCART socket. If I had only noticed this before I plugged the darned thing into the mains. The lesson here, never leave an important project with enticing cables hanging down near a playful tomcat!


Fortunately though, Amstrad's rather odd dual power supply "design feature" meant that this poor console was granted a reprieve via the +5V socket. So it looked like a little soldering would restore the damaged expansion port connections. Unfortunately the machine was too buggy after its feline encounter. It still worked great as a console but unpredictably as a CPC, no matter what I tried I just couldn't seem to pin down the faults, so it has been relegated to console duties once more :-(


But!!! I have caught back up after starting over again with one of my other GX4000's and have now started to house the machine in a 464 case! What's more, I have managed to get the cassette loading to work with a bit of a caveat, there is a lot of noise/drain when the cassette motor starts up. I'm sure I just need to decouple it, using some caps and diodes to make a pseudo secondary power supply or filter if anyone has any suggestions I'm all ears!


Anyways here is a Youtube video of it working and loading a tape, you will be able to see that the screen jumps about like a jack in the box while loading so I wont be loading from tape again until I'm sure I'm not going to brick the machine! Oh and enjoy watching me get my ass kicked playing Dragon Ninja! 





Here are some pictures too, I'm thinking that that huge hole in the front of the case may be a nice place to fit the HxC screen... (EDIT- Keep getting a Security Error when I try to post more photos so its just the one for now, take my word for it there is a huge hole in the front of the case  )


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Well next up will be for me to complete the Expansion Bus to accommodate a MegaFlash, Disk Drive Interface and hopefully when I'm confident I have a fully dk'Tronics compatible design- a 512k RAM Expansion!


I had originally intended to fit a slimline FDD but I think it may be a bit of a big ask to fit it into the case with everything else but we'll see what happens!

[redbox]

This is fantastic despite all of the problems you're having.

Did you follow a guide or are you doing it yourself?

[Badstarr]

Thanks Redbox!


I followed a guide written by "The Cranium" up to a point. It's a good guide but I suspect fairly old so its not full of colour photos like we are used to these days. It doesn't cover how to connect a cassette for example but its not really that difficult to be fair. Other than that I have used the schematics on the Wiki to build (well 90% build) the disk interface which I intend to install soon. There are a couple of gotchas when following The Cranium's guide eg. Connecting up a 464 keyboard is not quite as straight forward as it would seem, the delete key is routed differently. It's things like this I will be including in my own guide with a few shortcuts, its not really necessary to solder everything directly to the ASIC for example. Hopefully it won't be too much longer before its finished! 

[Bryce]

The wobbling screen looks like the PSU isn't able to cope with everything. You may not need to decouple the cassette circuitry, just use a bigger PSU.

Bryce.

[redbox]

The wobbling screen looks like the PSU isn't able to cope with everything. You may not need to decouple the cassette circuitry, just use a bigger PSU.

Am I right in saying that on the GX4000 you need to use the 12v input if you're outputting to SCART and the 5v input if you're using the monitor DIN?

If you're using 5v and the SCART, that might be the problem.

[Bryce]

The 12V (which I think was actually 10V in reality) is only used to set the SCART pin to tell the TV to switch to RGB. I don't think it powers anything on the PCB.

Bryce.

[Badstarr]

Thanks Bryce ! I had wondered if it might be the PSU but as I'm powering it from an MP2 Modulator I thought it would cope, then again there could be a problem with it might try a different PSU when I have some time. Right now its just the GX4000 Mainboard and the cassette installed so I thought it would cope but it has just occurred to me that there is a small 5v regulator on the gx4000 (unless my memory is playing tricks with me) and I'm wondering if that might be upsetting things a little? The way I have the cassette deck hocked up is before the regulator at the power switch so I'm thinking it may be working it a little harder than it likes to be? Just a "lunch break" theory ;-)

[Bryce]

I just checked the schematics and they do actually use the 11V to power a little more than just the SCART. If you look at the following diagram, you'll see that the 5V input isn't regulated, it goes straight to the board.
The 11V has a 10V output for the SCART switch, but also a 78L05 (1A) regulator that can power the general 5V for the board and a second 7805 (100mA) that only supplies the outputs of the RGB encoder. LK7 would allow the 5V to power both the general and the modulator 5V from a single (regulated) 5V source.
A pretty strange setup and I don't think it's a very good idea to connect both 5V and 11V at the same time, although D183 is there to stop any potential problems, it's putting quite a strain on the regulator.

Are you using the SCART output? If so, I would recommend only using the 11V input, but it will need to supply about 1.5A at least and there's no way the MP2 can do that, it's only designed to supply a disk drive motor.

Bryce.

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[Badstarr]

I see, that makes a bit more sense to me now. I'm running the GX4000 through the 5v input from the MP2 Modulator, I'm not using the SCART, I'm using the RGB output as this gives a crisper picture on my LCD TV. I know people say the SCART should provide a better picture but it always looks blurry, this is the case on all 3 of my current GX4000 consoles and was also the case with the two I have repaired and either sold or passed on.

I have avoided using the 11v adaptor as I have a slight suspicion that this leads eventually to the ominous GX4000 black screen of death. I'm not sure if this may be due to the GX4000s eccentric power scheme or the dodgy Amstrad PSUs, I have had at least two of them just disintegrate while unplugging them from the mains!

I have had a quick look at the back of the GX cases and they specify a voltage but don't specify the current. I had assumed that the 5v power requirements would be roughly the same as a 464 plus. The thought does occur though, that the 464 cassette deck still has the amplifier enabled, I wonder if that maybe tips the balance when the motor is running?

[Bryce]

Depending on the age and condition of the motor, it may require a few mA more than it did when it was new.

Bryce.

[ralferoo]

I know people say the SCART should provide a better picture but it always looks blurry

If it's just blurry horizontally, you might have a poorly made cable. But more likely, and definitely if it's also blurry vertically, it's almost certainly some processing the TV is doing to the signal to "smooth" the image so it doesn't look too blocky. Check all the options in the menus. Typically it's called sharpness and 50 means no fiddling, 0 means maximum blur and 100 means lots of crazy processing that makes the image look awful.

[Badstarr]

I had wondered if the motor might be pulling a few more mA than it should, I had a dodgy starter motor on my car once that would hum like crazy and cause the radio/CD player to loose its presets if it landed on a damaged portion of the armature, needless to say I replaced it pretty quickly. I have a sort of spare cassette unit I could try so my process will be- try a new PSU, try a new cassette deck, and if that doesn't work try some form of decoupling. Doubtless trying to find a new motor would be a fools errand (?).

As for the SCART, now you mention it I don't think I have ever used a different cable at any point with my GX4000s so there is a very good chance you are on to something there! The picture is more fuzzy than blurry I suppose so then again it could be some post processing but its been quite consistent between 3 TVs so I suspect a new SCART cable may be the cure then.

[Gryzor]

Video down? Awwww...

[Badstarr]

Is the video down? I just gave it a go and it loaded ok, can you try again and let me know if its ok? I was in PCWorld today and tried loading it up on one of those Microsoft tablet things, it took a little while to load up but it eventually appeared on the YouTube app so it might just be the YouTube server load.

[Gryzor]

Ah! It works now It used to say "unavailable", so it was not a conneciton issue. Thanks!


How I love those loading screens...

[Badstarr]

The Dragon Ninja loading screen is my all time favourite! It's so beautifully rendered, I remember picking up my copy of the game after my friends and I went out for my 13th birthday party (19 years ago almost to the day incidentally) it cost the princely sum of £1.69 from Toys R Us when they were selling the 8bits software off cheap. When we returned to my house I fired up my 6128 to load it up and the loading picture slowly filled the screen of my GT65 monitor. My mate's jaws all dropped with comments like... "Wow! That's as good as an Atari ST!" Shame the game itself wasn't as well turned out graphically. Not a bad effort...but...the surprises kept on coming with the digitised speech! Which made my Atari ST owning friends a little jealous! Not sure about the accent though, sounds a bit cockney (?) lol!

[Gryzor]

Heheheh,... I actually also enjoy the process of loading the screen. The anticipation, the mesmerising lines...

[Munchausen]

Badstarr - this project is awesome and I'm really looking forward to the guide. I'd like to build a converted GX4000 into the case of my damaged 6128, it would be immense! Do you think it would fit? How tight is fitting it into the 464 case? Does it have a disk controller?


Keep us posted on your progress!

[Bryce]

The GX4000 didn't have any of the disc controller parts or even space on the PCB to fit them. All these needed to be added by Badstarr on a seperate PCB.

Bryce.

[Munchausen]

Yes I know this... perhaps I was unclear. I meant, has he added a disc controller? And now you've answered my question

[Badstarr]

The 464 case is full of empty space, this was the main reason for building this project into a 464 case. The GX4000 main board is actually a perfect fit from front to back, I'm not too sure about the 6128,s case but I think it's dimension would be roughly the same, however, its the height that would pose an issue with the 6128 case. The 12v regulators heatsink would hit the back of the 6128s keyboard and would need to be moved elsewhere. Not really a huge issue but it would be one more bit of hassle in an already quite challenging process (at least for me anyway lol!). I would imagine fitting in a disk interface would pose quite a challenge in the 6128s case, space is certainly at a premium!

I have treated the heatsink and other attached components, such as the joystick ports the same way nature treats vestigial organs, if they don't do any harm let them stay where they are, this cuts down on bugs later down the line.

As for the disk interface...well, I etched a PCB, the first I have ever made. It was perhaps a little poorly laid out but the free version of EagleCAD and its Board size restrictions made this a little inevitable. I soldered everything in place and commenced checking the board for shorts and found that the +5 and Ground were shorted somewhere, to cut a long story short (no pun intended) I couldn't track down the error, everything seemed correct, go figure....Anyhow, I made a good old fashioned point to point version, apart from accidentally wiring the logic IC sockets backwards (dyslexia lures KO!) and having to deadbug them the interface is kinda working. The GX4000 happily defaults to 6128 mode and it will access the disk (HxC) as its supposed to but apart from the first time I used it, it mainly just makes the the ominous, "da da deerrr duh, duh, duh duh der..." Drive A Read Fail... I can load the the HxC software manager maybe 2 times out of 10 which will then report "Write ERROR..." When the program starts.

It's really odd as it seems when the machine is first switched on it will usually happily CAT the disk, but then it gradually starts to unravel??? I'm wondering if this may be due to the slightly lower clock rate of the GX4000 causing the Data Separator to have difficulty latching on to the MFM signal or if its just power related. I have tried a bigger PSU, which solves the screen jitters that occurred while loading from cassette (spot on Bryce as usual!!).

It's a weird one but I'm sure I will figure it out in the fullness of time, I have checked all my connections on the board and going to the GX4000 (shall we call it a CPC from now on???) and there appear to be no errors. I have tried 3 FDC ICs and 3 Data Separators all behave the exact same way so its not those at least. The saga continues....

[Badstarr]

Eureka!!!! It's working! After all the messing about, checking and testing thinking out of the box, inside the box and both at the same time. I pushed the data separater hard into the ic socket and Boom! 30 seconds later I'm watching batman forever!!! Video to follow soon! I'm off to do a victory boogie ! :-D

[TFM]

Great!!! Well done!!! Congratulations!!!