Amstrad Mega PC / Amstrad Mega PC Plus

I have an orange wire on the back side of the board (near the 315-5487).
If possible could you share the service manual? I have a PDF but the quality is very low (cannot read the text)

 

Sorry I misunderstood that you done it following Ronnie's advice.
So.. help me Obi-W... ehmm @Ronnie, you are my only hope

 

Kuush Puhhrrrr!......I'm your Father....Oh wait wrong message .

To fully make the Mega PC ISA Sega Card NTSC (the easiest/proper way) : First, you have to replace the main Master Clock Oscillator (NTSC divider), then there is a jumper cable on the solder side of the card (this could be different colors Orange/Black/Yellow. etc) . Remove the cable (leaving the trace/pin floating) or attach to Vcc . If you want to do a more complex region mod (switch) you will need additional parts.

The Sega ASIC found in the Mega PC Sega card is a bit different than others found in any other Sega units. If you pay close attention to the 315-5487 chip itself, you will notice it has a dot after the actual number. I've never seen this ASIC (with the dot) in any other Sega unit and believe me, I've pretty much seen all of the revisions out there. Obviously there is the more common 315-5487 ASIC but is not the same without the dot.

 

Thank you very much for your feedback. I'm sorry if I gave wrong info on my other post (I'll edit it later and put the right picture), as this wasn't the proper wire. I mistook them because they were the same colour on my board. The proper wire to dissolder is indeed on the lower part of the board and it's actually a bridge between R75 and C43. The following picture I just took after removing the board shows where the wire is located at (already dissoldered here and poorly insulated by myself):

http://dioxaz.free.fr/download/megapc/pics/DSC_0291.JPG

If we trace back to the 315-5487 chip we reach its pin 46 which controls the 50/60Hz frequency (named "NTSC", page 15 of the service manual). Curiously, I didn't have to perform any additional modification on my board to convert it to "NTSC" as it's still using its original master clock oscillator. I don't know if I could harm my system this way as it appears to work normally. I thought it was similar to the ones found on the regular consoles (same pinout, AKAIK) but it seems not to be the case. On regular European consoles, simply disconnecting or cutting bridges is enough for frequency mods.

Shane McRetro should have a copy of the service manual somewhere in his Mega PC Zip package (posted somewhere on this thread) but I'm reposting a link to it here: http://dioxaz.free.fr/download/megapc/mega.pdf

Unfortunately, no better quality copy of it exists outside this one which is really infortunate.

 

Thanks @Ronnie and @Dioxaz, operation completed!
I'm planning to add a switch to select 50/60, I'm just wondering where to place it
In the meantime I've added an IBM Microdrive (384Mb), 4Mb Ram and replaced the battery with a Barrel Varta 3.6.
I've used the free slot to connect a SB16 + and external Roland CM500.
Quite funny that the PSU is Seasonic like in my actual PC (a good brand IMHO)
A couple of pictures here:
https://www.dropbox.com/sh/macduyqv465x5nk/AABHmpHmd-04_xdQUjvplhuxa?dl=0
Thanks again, hope to expand it better in the near future

 

You may not need to replace the OSC but that's the proper way to do it. I don't know if it may affect anything in the long run but I personally wouldn't chance it. What's really happening is that you are getting PAL/60Hz instead of NTSC/60Hz so it may not work properly if you are planning to connect a Sega Cd or 32X to it for example (these systems are extremely picky about clocking signals and timing). The Mega Pc Sega card doesn't have a video encoder therefore any external converter or monitor are directly getting RGB from it. RGB doesn't care about region/color decoding so that's the main reason its displaying no problem.

 

Speaking of Sega CD, would it be possible to use one with a Mega PC? I know that people have attempted to use a Sega CD unit with a TeraDrive through the covered expansion connector on the top, but I don't know how successful they were.

Here's a crazy idea: connecting a Mega PC or TeraDrive to a Pioneer LaserActive to use it as a Sega CD. Probably not possible, but man would that be cool.

 

Before beginning this post, I'd like to point out an eBay auction for a brand new PC14DSM monitor!!! It's located in England so Mega PC owners from there who aren't in possession with the monitor, this may be for you. I'm reluctant to bid on it, as the price is already getting high and as I finally found a working solution. But some of you from this forum might already be on it or aware of that auction.

Edit: The monitor finally went away for €285.25. I hope this is at least someone who owns a Mega PC (probably from here, who knows?).

@DeChief: The Mega PC was originally designed to be used with a Mega/Sega CD. Some special devices (control cartridge and ribbon cable) had to be ordered from Amstrad. This is without saying that I'm sure nobody have seen what those devices were supposed to look like. However, you can make your own ribbon cable and blanking one pin of an existing game cartridge would make the Mega PC boot into Mega CD mode. But I can't really tell as I never tested myself.

I think that's the most tangible explanation. I remember people attempting modding NTSC consoles to 50Hz and getting strange video artefacts afterwards because the video encoder was getting confused. Also, it looks like there's some slight speed differences too when not using the proper oscillator (but it's almost unnoticeable, I found more in this Sega-16 thread).

Some more good news regarding region modding of the Mega PC. I think it's safe to say it's the easiest machine to region mod so far! Okay, looking at the original design on the service manual, both pin 46 and 107 were supposed to be connected to VCC with a 1K resistor in-between for both (R74 and R75). But the folks at Amstrad seem to have made a design mistake because on the actual board we end up with a jumper wire going from pin 46 to a 0V point (right pin of C43) and R75 is absent, as some of you have noticed. If you're familiar to region modding, you know what it means. Pin 46 connected to 0V means 50Hz (low potential). And pin 107 connected to VCC (or nothing) means EUR/US (high potential). Earlier, I removed the jumper wire and it gave me 60Hz. This means that the 315-5487* chip also defaults to high potential when those region pins are unconnected. That's right, you guessed it. The next step was removing R74 which doesn't change the country option yet (defaulting to high potential, as I said).

So what did I do next? Finding a better spot for connecting on-off swicthes (a dip switch will also work, and that's what I used) to a 0V point. Using a multimeter, I was able to find several spots that were connected to that same 0V point (also beware, 0V isn't ground!). This is the one I picked up, right below C43:

http://dioxaz.free.fr/download/megapc/pics/DSC_0300.JPG

On the picture above, the black wire is connected to 0V and the 2 other ones go to pin 46 and pin 107 (their closest points). When soldering to the points matching pin 46 and pin 107, I first put some adhesive tape on top of neighbouring connections so I minimize the chance of shorting them because of too much tin melting. This is important! Those connections are so close you never know what will be the consequences if you have tin getting over them! I naturally used a fine tip soldering iron for the job indeed. I wouldn't recommend using a bigger tip one as it would be way too big for those tiny components. Also, if you plan to perform a region mod as well as the frequency mod, be sure to also remove R74 before doing anything, like I did myself and like you see in the picture! Otherwise you'll short VCC (+5V) and 0V which may not be a good thing for your board.

And this is the other side of my wires, with the switch attached to it: http://dioxaz.free.fr/download/megapc/pics/DSC_0301.JPG

Apologizes for the blurry picture. Dip switches 1 and 2 upper pins go respectively to pin 46 and 107. I bridged the lower pins and soldered that 0V black wire to them. At this point I also ensured all connections were properly made with my multimeter (I had a problem where country switch wasn't working because my bridge wasn't strong enough). This is what my early tests looked like:

http://dioxaz.free.fr/download/megapc/pics/DSC_0293.JPG
http://dioxaz.free.fr/download/megapc/pics/DSC_0294.JPG

I almost made it into a Teradrive!

I also took the opportunity to test the ACV-011 in the Mega PC's original 50Hz. Well, things are a bit different. When using setting 14 (OOXO), picture is truncated at the bottom (not enough active lines it seems). So I have to use setting 15 (OOOX) instead. Setting 15 works for both 50 and 60Hz as it has enough active lines for 50Hz but too much for 60Hz. As a result, garbage appears at the bottom in 60Hz but it looks almost perfect in 50Hz. Unfortunately, setting 15 yields random picture drop-outs which can be annoying during gameplay. So I'm keeping my Mega PC in US-60Hz configuration as it works perfectly with no drop-outs in setting 14 (with now a possibility to restore its original 50Hz for my Euro-locked games). The Mega PC syncs still need readjusting in order to make them work better with some scalers, monitors and line-doublers.

 

Thank you for this! I reupped over on GameSX and elsewhere for future repairs

 

any idea what value xtal we need for full/correct ntsc modding? I can see the PAL one is a 53.20342mhz one , what about ntsc?
Thanks!

 

The actual frequency is 53.693181MHz - but the crystals are normally marked with less precision than this (I.E. something like 53.6932MHz)

 

thanks TriMesh
I see on this pic here that a 53.6931 xtal is used: https://upload.wikimedia.org/wikipedia/commons/e/ef/Genesis-Model2-Motherboard.jpg
So I am going to search for one , thanks!

edit:
wow it seems a really uncommon value...!

 

Yep, they are impossible to find. The only way to get one is use a new circuit (i.e. programmable silicon oscillator) or pull one out of an ntsc Sega Genesis

 

found them on aliexpress but it's not something I would buy from there
harvest from from some old and battered console seems the only way...

 

They are now - but they used to be fairly common back when everything used standard definition video. It's actually 15x the NTSC color subcarrier frequency. The original PlayStation used the same frequency for the same reason (unfortunately, the one in a PlayStation is 3.3V and not 5V).

 

I recently did a quick test: Mega PC with an Asus VE248H monitor. It works for PC and MD modes alike. But since the Mega PC also outputs audio thru its VGA connector, this monitor probably isn't a viable solution... yet.

 

^ good to know

btw, has anyone found a way to disable the onboard AdLib card? (I assume the YM3812 it has is only used from the PC side and the sega custom has integrated the megadrive fm chip - or am I wrong on this?)

 

anyone happens to know with the megadrive card has this IRQ jumper block? What can use IRQ 2,3 or 5 ? AdLib does not need IRQs, is it for the pc gameport ?

 

I put a Sound Blaster CT-1350 in my Mega PC, and used its jumper to disable FM. However you should note this doesn't actually stop programs & games from detecting the YM3812 chip on the Sound Blaster. It only stops the Sound Blaster from outputting Ad Lib.

As for disabling the onboard Ad Lib of Mega PC, I would venture a guess it's not possible without removing the chip. I'm pretty sure software just "looks" for the YM3812 or compatible chip, changing IRQ shouldn't affect that at all.

I'm curious as to why you want to disable built-in Ad Lib, though. I wish I could do it too, but gave up.

 

Well if you put the CT1350 and disabled the onboard YM3812 the games will still detect one because they will get a response from the megapc card. They don't care if they are on separate cards, to them it's all the same.
I would assume that yes, one needs to remove the megapc opl2 and even better remove as well the bus glue logic driving it. A couple of 74xxx bus drivers - it would be nice if the service manual was readable , as-is it's a blurry mess.
AdLib or gameports don't use IRQs so it's a mystery what that jumper block controls. Technically 3812 can use an IRQ but no adlib ever did - the jumpers are there but non-connected or even populated.

I have some plans to install a soundcard and it would be nice to completely disable the megapc adlib - the CT1350 is a good choice and I have a couple so it's a good backup plan. What jumper controls that? https://stason.org/TULARC/pc/sound-...ABS-INC-Sound-card-SOUNDBLASTER-2-0-CT13.html