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Upgrading the Amstrad Mega PC

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limboy777

Experienced Member
Joined
Apr 21, 2012
Messages
64
Hi All

Ive got two Amstrad Mega PC's although im always on the lookout for spares and documentation etc.

These came with a 386 SX 25 processor as standard and 1mb RAM

One of mine has 8mb RAM (2 X 4MB simms) as well as the cyrix 486 drx2 50 upgrade. I know these are 30 pin simms and that the mega pc can only take 4mb max per free memory slot.

Can you please tell me if I just bought 4mb 30 pin simms to fill the other two slots this would be ok. For example do I have to worry about parity, 60ns/70ns and all the ratings?

I want to upgrade the computer but dont want to buy stuff I cant use.

Some pictures are here of the simms currently installed - ive taken the pictures at an angle as I havent taken apart the computer fully yet.

I am also wondering if someone can point me in the direction of where I can get the original cryix 3.5 floppies or files so that I can install them should I replace my hard drive

Thank you in advance

http://s1269.photobucket.com/albums/jj587/limboy777/Amstrad Mega PC/
 
A problem that you need to deal with is the 'barrel' batteries. At least one has started to leak (due to age no doubt) - the light blue one. How do I know? Photo #16 shows a green deposit on the solder joint of the battery, a classic sign.
 
I don't know about parity, but I bought a couple of 30 pin SIMMs at random, hoping them to be at least 1 MB each. When I installed them into my Amstrad MegaPC, they turned out to be 4 MB modules so now it got a whopping 16 MB RAM!

It should be mentioned there is a bit of stuff to remove to reach the memory slots, but I'm sure you already have located it and figured it out. Basically I had to take away the front, and in order to remove the Megadrive card, I had to lift and pull it backwards in a way that I was unsure if I'd get everything back in place again. Basically you only want to upgrade the memory once.

I have no idea about CPU upgrades though, if that is your unasked question if you can get an upgrade 486 into the machine.

My biggest issue is that I should replace or at least remove the CMOS battery, since it is flat. I kind of remember getting into that but for some reason I didn't finish the task. Then I don't have any original parts, so I'm using a generic AT keyboard and regular monitors. Yes, plural since one monitor can handle the DOS/VGA resolution and one monitor can handle the Sega RGB mode. I'm on a hunt for a dual or multi scan monitor but those are hard to find. I even tried a RGB to VGA adapter board from China. While it happily converted the 15 kHz RGB to VGA, it was less successful at putting through the 31 kHz VGA signal without upscaling, despite the reseller spec suggests the board should be able to.
 
modem7 said:
I think it is best that leaking batteries be dealt with as soon as possible, before damage starts to occur to the motherboard.
An example of damage is shown in the following photo. Copper tracks have been 'eaten away'.
http://www.minuszerodegrees.net/images2/batt_int_ext.jpg
Click to expand...

Ive taken the battery out today. I have a replacement NICAD but also a cr2032 holder.but dont know if i need a rechargable 2032 or standard one. Would be grateful for any help. Im uploading some more pics to the album where I have taken it apart. Would be grateful for help on the memory front please.
 
carlsson said:
I don't know about parity, but I bought a couple of 30 pin SIMMs at random, hoping them to be at least 1 MB each. When I installed them into my Amstrad MegaPC, they turned out to be 4 MB modules so now it got a whopping 16 MB RAM!

It should be mentioned there is a bit of stuff to remove to reach the memory slots, but I'm sure you already have located it and figured it out. Basically I had to take away the front, and in order to remove the Megadrive card, I had to lift and pull it backwards in a way that I was unsure if I'd get everything back in place again. Basically you only want to upgrade the memory once.

I have no idea about CPU upgrades though, if that is your unasked question if you can get an upgrade 486 into the machine.

My biggest issue is that I should replace or at least remove the CMOS battery, since it is flat. I kind of remember getting into that but for some reason I didn't finish the task. Then I don't have any original parts, so I'm using a generic AT keyboard and regular monitors. Yes, plural since one monitor can handle the DOS/VGA resolution and one monitor can handle the Sega RGB mode. I'm on a hunt for a dual or multi scan monitor but those are hard to find. I even tried a RGB to VGA adapter board from China. While it happily converted the 15 kHz RGB to VGA, it was less successful at putting through the 31 kHz VGA signal without upscaling, despite the reseller spec suggests the board should be able to.
Click to expand...

Amiga monitors should work. Ive got the origina mega pc monitor but it has some issues I need to fix. Opening up the mega pc is a pain in the rear but I took lots of pics while I was doing it. Do you have pics of your 4mb simms?

you can upgrade the mega pc to 486 using the clip on upgrades. see my pics
 
limboy777 said:
Ive taken the battery out today. I have a replacement NICAD but also a cr2032 holder.but dont know if i need a rechargable 2032 or standard one.
Click to expand...
I presume that you intend for the 2032 holder to be soldered in where the leaking Ni-CD was desoldered from. Whatever battery (unmodified) is placed there is going to be charged by the motherboard. Different battery technologies (Ni-CD, Li-ion, Ni-MH, ...) require different charging methods. The motherboard believes that a 3.6V Ni-CD is in place and will charge accordingly.

Option: Ni-MH

For this application, replacing a Ni-CD with Ni-MH should work (reference: 'Replacing NiCds with NiMH cells:' section of http://www.ka7oei.com/nicds.html).
So you would need to locate a 3.6V Ni-MH battery.

Option: Non-rechargeable

A non-rechargeable battery should work if a blocking diode is placed in series with it. The blocking diode prevents the battery charging circuitry from charging the battery (because charging a non-rechargeable battery can result in damage).

blocking_diode.jpg
 
modem7 said:
I presume that you intend for the 2032 holder to be soldered in where the leaking Ni-CD was desoldered from. Whatever battery (unmodified) is placed there is going to be charged by the motherboard. Different battery technologies (Ni-CD, Li-ion, Ni-MH, ...) require different charging methods. The motherboard believes that a 3.6V Ni-CD is in place and will charge accordingly.

Option: Ni-MH

For this application, replacing a Ni-CD with Ni-MH should work (reference: 'Replacing NiCds with NiMH cells:' section of http://www.ka7oei.com/nicds.html).
So you would need to locate a 3.6V Ni-MH battery.

Option: Non-rechargeable

A non-rechargeable battery should work if a blocking diode is placed in series with it. The blocking diode prevents the battery charging circuitry from charging the battery (because charging a non-rechargeable battery can result in damage).

blocking_diode.jpg
Click to expand...


Thank you for replying.

Option A -Can i replace a NICD with ANIMH with any other modifications i.e blocking diode

Option B - What does a blocking diode look like? Do I need one if I put in a rechargable cr2032?
 
No, I didn't take any pictures during my memory upgrade. As for monitors, you would need a 1940, 1942, 1950 (?) or 1960 to do dual or multi sync. Obviously I'm not lucky enough to have either of those. My trusty old 1084's would only be good for 15 kHz modes.
 
limboy777 said:
Option A -Can i replace a NICD with ANIMH with any other modifications i.e blocking diode
Click to expand...
In this application, it is a straight replacement - remove 3.6V Ni-CD then replace with 3.6V Ni-MH.

limboy777 said:
Option B - What does a blocking diode look like?
Click to expand...
The "blocking" is only describing what function the diode is being used for.
One of the following cheap/common diodes will do: 1N4001 or 1N4002 or 1N4003 or 1N4004.

limboy777 said:
Do I need one if I put in a rechargable cr2032?
Click to expand...
No.
 
Thank you for responding to my question once again

modem7 said:
In this application, it is a straight replacement - remove 3.6V Ni-CD then replace with 3.6V Ni-MH.


The "blocking" is only describing what function the diode is being used for.
One of the following cheap/common diodes will do: 1N4001 or 1N4002 or 1N4003 or 1N4004.


No.
Click to expand...
 
I have installed the cr2032 battery holder with a diode and it works perfect. Thanks for your help.
 
limboy777 said:
Thank you for replying.

Option A -Can i replace a NICD with ANIMH with any other modifications i.e blocking diode
Click to expand...
I assume you meant "without" instead of "with"? If so, yes, as Modem7 said.

Option B - What does a blocking diode look like? Do I need one if I put in a rechargable cr2032?
Click to expand...
It won't be a CR2032 if it's rechargeable.

I use 3.6V NiCd or NiMH cordless phone batteries to replace rechargeable CMOS batteries; $2.00 in any dollar store around here, and easy to mount off-board just in case they do leak some day.
 
MikeS said:
I assume you meant "without" instead of "with"? If so, yes, as Modem7 said.

It won't be a CR2032 if it's rechargeable.

I use 3.6V NiCd or NiMH cordless phone batteries to replace rechargeable CMOS batteries; $2.00 in any dollar store around here, and easy to mount off-board just in case they do leak some day.
Click to expand...

Thanks Mike

I installed the cr2032 with a diode and have also found some custom made cr2032 holders with built in diodes for old comuters that I use for my other two vintage computers.
 
Perhaps I should have a go at my battery as well. Only that I expect it to be an evening worth just to disassemble and get the motherboard out to more easily work on it.
 
Hello

I recently purchased a Mega PC as well and I'd love to upgrade it, to have the 16MB of Ram, a 486 processor and even the SVGA graphic memory chips.
- About the ram simms, Ive been searching ebay and found these: 161019153517, can I use them? They say with parity (or should I use without parity?);
- The Cyrix processor can be this one on ebay: 271804876675? (Does it require soldering?)
- And which SVGA memory chip should I acquire?

Thanks in advance! And long lives the Mega PC!!!!
 
Let's resurrect this thread for a 2nd time:

I just picked up a Mega PC, am in the process of getting it configured to my liking. Unlike many people, I had no problem with the CMOS battery: no explosion, no leaking - not even a drop. Of course I immediately snipped out the long-dead battery. I've read about quite a few Mega PC owners soldering in either a new barrel battery or a CR2032 holder w/anti-recharging diode. I can't understand why someone would go to all that trouble, when I just lazily attached an AA-battery "kit" to the external battery input on the motherboard. Specifically, I'm using this one from Ultimateapple2 which they market for the Apple IIgs but obviously it has wider applications...

https://www.ultimateapple2.com/catalogzen155a/index.php?main_page=product_info&products_id=21

I've also installed a 50MHz Cyrix 486 SRx2 "snap-in" upgrade (as I have the need for speed), Intel 387SX 16-25MHz coprocessor (just for kicks), 16MB system RAM and the video RAM upgrade. Before I put the machine into (hyper-)active use I'm going to install a little fan over the CPU, because that sucker runs HOT and the included heat sink won't fit (there's no clearance space under the FDD+HDD cage).

Thing came complete w/original monitor, keyboard, mousey and manuals. Whoopiee!!! Like some other owners have reported, my monitor is finicky about turning on: the power button doesn't always stay locked in when pressed. Luckily once it does come on, it stays on. Other than that everything works like a charm. What a neat, quaint little machine. I'm glad I finally picked one up.

All I really need now is a 60Hz mod for the Mega Drive side. Why? Because I'm a USA-ican who doesn't appreciate his games running ~17% slower than he's used to!

I'm going to start doing some "hard core" [!] testing of the onboard Ad Lib support, to see if anything's different from the usual fare. I've already noticed that PCM playback thru the YM3812 either doesn't work at all, or works with hiccups: at least in the case of Super Fighter and Sango Fighter which are the only (of the few!) games I've thus far tested that actually bother to support this "feature." Using a real, authentic Ad Lib sound card, the PCM plays back 100% with no issues, 100% of the time. So... a curiousity at this point.

Aaaaaaaaand that's about it for now. Where did all you other Mega PC owners scoot off to? In the words of Sebastian Krist, "Come back here."
 
You will find a very short battery life with the type of diode you have used.
Signal diodes typical have a forward voltage drop of .6V, that brings your battery voltage down to 3.0v at the RTC chip. RTC chips typically require a minimum of 2.8v to operate. So you have a .2v margin before you battery becomes useless.

I would suggest you use a fr107 schottky barrier diode, they have a forward voltage drop of about .24v giving you a margin of over .5v on your battery life.
 
Update: My Mega PC's ISA Mega Drive card is now 60Hz modded thanks to Ronnie @ assemblergames.com, so I can now play games at the proper USA speed as God intended. ;) I can confirm that the 60Hz mode displays fine on the bundled Amstrad monitor.

I ended up installing a Sound Blaster CT-1350B with C/MS upgrade, as I didn't want to rob the PC side of DAC sound. I disabled the Sound Blaster's onboard Ad Lib chip and game port by way of jumpers, so as not to interfere with the Mega PC's native support for these.

Not a bad system setup, don'cha think?
 
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