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Amstrad Mega PC

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First post, by Zalog

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So thought it was probably time to start a thread on my current little project.....

The origins of this one go back a fair way so best start at the beginning. Back when I was a teen I was fascinated by the Amstrad Mega-PC they had on display at the Model shop in Gloucester, it was very cool and played all the games I wanted, plus it would be great for word processing. After much nagging my dad did get a PC, however it wasn’t a mega-pc much to my disappointment. I did get to have a go on one however as a friend of mine was given a Mega-PC Plus (yes the 486 version did exist !) as a Christmas present and I occasionally got to play with it when I visited – the mix of megadrive and basic 486 games was pretty cool – unfortunately he threw it away for a Pentium a few years later ☹.

So the Mega-PC always stuck in my memory – after doing a bit of research, about 8 years ago when I came across one with its monitor (rare!) keyboard, joystick and gamepad, that had been stored in in a shed, I brought it for the princely sun of £40. But that said the base unit was a state, it was covered in mud, the motherboard had suffered a battery leak and damaged the HDD controller and the 3.5” floppy drive had been replaced with a standard unit by cutting into the face plate (ouch!), also some idiot had cut the integrated monitor power cable in half ! Still with some careful cleaning and a repair to the power cable – amazingly it worked – abet without a HDD on the PC side due to the battery leak. After playing on the Mega-drive side for a couple of years and using the PC via the floppy as a word processor for locoscript (!), life eventually got in the way and it was stored safely away. Moving up to the present, recently my daughter started doing A level computer science at school, so I fished out the Mega-PC as a demonstrator system– rather than let her take apart my modern system for a look see.

This got me thinking, could I get it fully working again….. Well a quick search on the internet told me the motherboard could be replaced and as a plus I could in effect get a Mega-PC plus – and thus a plan was hatched and I waited 6 months for a 7486SLC to pop up. Due to Covid and people having time to clean their attics, one popped up recently that I was able to get hold of. It was missing its HDD and a few other strange things (such as the video ram oddity which I’ll talk about another day…..) – but they are as rare as Hen’s teeth these days, so despite these things, as it ran I brought it.

After looking at the faceplate of the 7486, I realised that I didn’t have the skill to transplant the 7486 drive cover into the missing bit on the Mega-PC faceplate and thus I decided as it wasn’t going to be a museum piece, I might as well go on the upgrade warpath and do some upgrades with some of the kit I have in store. So far the motherboard is in, the clock chip replaced with an Alt12887 and I’ve fitted a Gotek into the floppy bay (see photo) !

The current upgrade plan is:
Fitted – 786SLC Motherboard, replaced Dallas Clockchip with Alt12887, Gotek Flash Floppy
Awaiting to add - Cyrix FasMath CX-83S87-33-JP Co-processor (who knows I may need it 😊 ), 16MB RAM, Quantum LPS 400MB HDD (Master), CF IDE Adaptor (Slave), Soundblaster AWE64 (for now as I have it to hand – will hopefully replace it with Keropi‘s special soundcard for the Mega-PC when its ready). A heatsink to add to the CPU (as it gets hot playing DOOM !), and perhaps a suitable flatscreen.....

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Reply 1 of 35, by Zalog

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******BREAKING MONITOR NEWS*******

Last night I made a MAJOR discovery - an LCD screen that works on both the PC and Megadrive sides using ONLY a VGA cable and WITHOUT a scan doubler or any other modification !!
Since a lack of monitor is a major reason why MegaPC's often get discarded, and as the MegaPC is a picky beast and doesn't like many 15Khz dual sync monitors, the find of a modern monitor that works and can easily be purchased is a big deal !
Photos of the big discovery are below...

So the monitor I brought is a Benq BL-702A which cost £90 in the UK inc. P&P

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With PC Side

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No this isn't the new Microsoft Load Screen......

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SUCCESS !!! - Displaying Tanglewood brilliantly from the Megadrive side

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Just in case you don't believe there isn't a scan doubler hidden round the back !!

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So folks if you have a Amstrad MegaPC, this may be the modern monitor you've been waiting for. Certainly a must buy if you haven't got a original megadrive monitor, and perhaps a good spare if you already have.
If only they came in 90's PC beige !!

Reply 2 of 35, by keropi

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great story and good job finding that monitor - these specific ones are known to amiga world as well
I bought a nec one some months ago that is also 15k compatible but although amiga would display ok the megapc could not for whatever reason

I also have 1 unit upgraded with the 486slc33 mobo - it gives a nice boost for DOS 2D gaming. You do need at least a heatsink on the cpu - for ram and cache you can get some from Tiido I have mine fitted with 12ns cache and 16mb 60ns ram sticks

wQ7NqHzl.png

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 3 of 35, by Zalog

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keropi wrote on 2020-07-08, 09:48:
great story and good job finding that monitor - these specific ones are known to amiga world as well I bought a nec one some mon […]
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great story and good job finding that monitor - these specific ones are known to amiga world as well
I bought a nec one some months ago that is also 15k compatible but although amiga would display ok the megapc could not for whatever reason

I also have 1 unit upgraded with the 486slc33 mobo - it gives a nice boost for DOS 2D gaming. You do need at least a heatsink on the cpu - for ram and cache you can get some from Tiido I have mine fitted with 12ns cache and 16mb 60ns ram sticks

wQ7NqHzl.png

Good to hear that the monitor works on other 15k systems - hopefully Benq will keep making them for a good while yet if there is a good market for them.

That's a good inspirational pic ! - certainly something I'll be aiming for ! The board has 64Kb of cache already so I should be ok with cache, although will check the speeds of that and my RAM as it'll be useful to have faster ones if they are avaliable.
On the subject of RAM, the wierdness that is this boards video memory will give you a laugh - I'll post it up here when I can as it really is interesting.
I have to admit I plagiarised your heatsink choice and have got the same type - just have to fit it now !

Reply 4 of 35, by keropi

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here it is
p8hKqRwl.png

mine came with adhesive thermal pads installed... I highly recommend a 40x40x10mm fan - it fits nicely but you have to use a little hotglue to fix it into place

DgoCCvel.png

fGpABVvl.png

there are also some mods that can be done to improve the megadrive audio quality, with Tiido's help we went from
this: https://soundcloud.com/user-919839499/stock-sonic2-0b
to this: https://soundcloud.com/user-919839499/modded- … inal1-sonic2-0b

need to read the vram story now 🤣

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 5 of 35, by Zalog

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keropi wrote on 2020-07-08, 10:37:

need to read the vram story now 🤣

😀 Since you asked for it !

So when I brought the 7486 it had the 512KB video ram sticker on - however the seller told me that the ram had been upgraded to 1MB years ago. When I looked under the hood I found some AAA1M304P-07 chips in the sockets so that sounded reasonable......

However when I tried to run a couple of games, they complained of a lack of Video memory - odd I thought, I checked and re-seated the chips and then ran a test...

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Which told me that I had only 256KB video ram... odd so pulled the extra chips and still had 256KB RAM - which was due to the 4x 256k ram chips soldered onto the board.

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A check of the 1M chips showed they were working, So that got me off into reserch mode - and I looked into the video card spec sheets, which told me this.....

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So for those ram sockets to work they need to be filled with 4x 256k chips to give me (inc the soldered ones) 512kb total ram
If I want 1MB then I would need to replace both the socketed and soldered chips with 8x 512k chips.
The guy had brought 1M chips and installed them thinking he'd upgraded the video ram to 1MB - but they had never worked due to the video adapters requirements !!! 😂😂😂

However in addition to having a good laugh, I did learn a bit more about the 7486SLC's video adapter and its "Local Bus mode".
This is enabled by closing 2,3 on JP18 and JP19 on the motherboard - and I'd already enabled it at that time.
If you look back at the diagnostics image above you'll see it reporting the bus as "MCA" or IBM's Microchannel Bus - odd...

So looked into the specs and....

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the 5420 definitely doesn't have VL-BUS which is a bit odd - but a few dozen pages down I came across this...

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Which confirms that the CF-7 on the 5420 enables MCA mode !

So the question is why is the MCA Bus setting labelled "Local Bus" on the board and why is the default video set to "ISA" on the board ??
My suspicion is that Amstrad was trying to avoid IBM's patents again (the PC1512 is a classic example !), and that as the patents for PS/2 and MCA were notorious, this was done to try and get round them.
I suspect they hoped calling it "Local Bus" would mean IBM wouldn't notice - and if they did, then they had the defence that it was set to "ISA" as default anyway and if the user changed it that was their fault !!! 😂

EDIT - That sound improvement is good - how was it done ??

Reply 6 of 35, by keropi

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ah you also researched the onboard 5420!
I can tell you about "local bus" since it's one of the first things I tried. If you enable it you get a BIOS message on boot that "WINBUS" is enabled.
So this is something that works in conjunction with the WINBOND chipset and indeed it's the MCA pins 😁
it's thanks to winbond we have this nice speed boost - I made this pic ages ago in hopes that one day I will document all that stuff (fps is from wolf3d)

5mWZ2V2l.jpg
full size: https://i.imgur.com/5mWZ2V2.jpg

The jumpers connect to pins #14 (JP18) and #13 (JP19) of GD5420 and on "local bus" setting they connect directly to the W83C311 chipset (no datasheet for it sadly - ISA setting connects the 5420 to a different set of pins on winbond chipset)

That thing about VRAM I did not really check, I also put 4x256k chips in mine and I thought I had 1mb - I missed the 512K x8 requirement 🤣 speedsys pics did not help either as they showed 1mb
I did some research though - it is possible in theory IMHO to replace the GD5420 to something better up to 5428 - the pins that connect to the 2nd ram bank to allow 32bit access are there - I have not checked about the MCA pins though - are they consistent on later chips as well?

here is a pic of a mobo I attempted to repair - I also removed the 5420 in case it was hogging the bus - it did not but I got this nice pic:

x53zT9Il.jpg
full size: https://i.imgur.com/x53zT9I.jpg

it is nice to find another one interested on the 486slc mobo!!!!!

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 7 of 35, by Zalog

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keropi wrote on 2020-07-08, 12:02:

ah you also researched the onboard 5420!

I did some research though - it is possible in theory IMHO to replace the GD5420 to something better up to 5428 - the pins that connect to the 2nd ram bank to allow 32bit access are there - I have not checked about the MCA pins though - are they consistent on later chips as well?

Looking at the diagram below it would indeed suggest that the MCA config is the same as the 5420 as for the 5428 (although the 5420 has a couple of reserved pins, which I suspect relate to the 32bit VRAM access) - I suspect your right and you could stick in a 5428 and replaced the fixed ram with sockets so you could reach the giddy heights of 2MB - although the soldering involved is way, way beyond my very limited skills to try out !

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it is nice to find another one interested on the 486slc mobo!!!!!

I suspect folk like us are so rare as the boards are so rare these days 😁 certainly everyone I've shown it to has found the board a really interesting design, and in some ways historic since these were the last unusal PC's Amstrad made (the later series were just standard parts PC's 🙁 ) . Definitely worth a bit more study as there is so little out there about them 😃.

Reply 8 of 35, by keropi

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Yes those reserved pins are for 32bit access and lead to the empty vram sockets - but they are all connected on the board so this is why it will be possible to just replace with a better cirus chip
Going to 2 mb will require too much soldering but a chip replace and 1mb would be easily achievable IMHO

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 9 of 35, by keropi

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OK I did more testing with the whole 512kb/1mb deal
first of all univbe also detects 1mb of vram:

Z90llCrl.png

so naturally we want to test the largest mode available - it is 1024x768x8bits since 5420 is only 8bit color
that doesn't look good:

tlTTfaql.png

maybe it's interlaced? I tried the original system monitor (that ofcourse cannot support 1024x768 resolution)

ymsrnoXl.png

looks like a crt trying to display an unsupported resolution but the corruption is still the same .

800x600x8bit seems to be working fine, and 2 pages modes are all ok

ybOmUX3l.png

but what about 640x480x8bits with 3 pages?

oldI0YUl.png

hd7ChtLl.png

VMCSi9Fl.png

so far so good but when the time comes for the horizontal bank switching mode I get this:

Tom5hVxl.png

vertical is fine though

CLlm0uBl.png

Is the horizontal test running out of vram to store 3 pages? forcing univbe to 512kb of vram makes the 1024x768 modes disappear and the 640x480 mode become 1 page instead of 3.
So yes it seems something is off with vram usage! It is not something anyone would notice back then since with the original monitor you'd just use it's max 800x600 resolution (and tbh I'd say 640x480 is more readable on that monitor so that probably would be the resolution of choice) and there were no VBE2.0 extensions to worry about pages etc...
Maybe the board was meant to have something better than the absolute lowest 5420 of the series , I mean "reserved AKA non-connected" pins on the pcb are actually connected and going places.

Here is a side-by-side comparison, the only thing that is not connected is the OVRW pin that is used in overlays - I assume with decoders or video processors or something like that

ioFsSe5l.png
full size: https://i.imgur.com/ioFsSe5.png

soooo I am itching to replace the 5420 with a 5428 (might as well go for the best of the bunch) 🤣

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 10 of 35, by Zalog

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keropi wrote on 2020-07-08, 16:03:

Yes those reserved pins are for 32bit access and lead to the empty vram sockets - but they are all connected on the board so this is why it will be possible to just replace with a better cirus chip
Going to 2 mb will require too much soldering but a chip replace and 1mb would be easily achievable IMHO

That is useful and thanks for doing all those tests, that is very interesting - something is definitely up !
I've been back and looked at my board and there is 512KB soldered on there - I wonder therefore by Amstrad using the 16bit VRAM bus, only half of the installed RAM becomes accessible, even though the full VRAM is visible (?).

I looked out the scanned copy of the 7486SLC service manual and found this , which might be useful ...

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Looks like Amstrad didn't read the bit in the Cirrus Logic manual I found about 512k x 8 and feel that 256k x8 gives 1MB, which it does, although it may not be all usable with the 5420 (!!)........ Although your comment wondering about if this board was ment for a better graphics processor does make me think that with the 5428 graphics chip, a VRAM change might not be needed and you should be able to access the full 1MB??!

the only thing that is not connected is the OVRW pin that is used in overlays

I think that's for the 5425 which had a TV card option (?) so probably not needed on the megapc 😉

soooo I am itching to replace the 5420 with a 5428 (might as well go for the best of the bunch) 🤣

Wow that is brave - definitely worth trying if you have the skills (I definitely don't !) and can take the risk, would definitely love to see that working ! 😃

Reply 11 of 35, by keropi

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wait , what 7486SLC service manual you are talking about? share share share! 😁
I can indeed easily replace the 5420 with a 5428 (I asked a friend to send me a 5428 card to steal the chip) the most annoying thing is to take apart the system once more 🤣
and yes I think that by only changing the cirus chip we will get full 1mb vram - tbh I don't think more is needed for this system - even the 5420 is performing great on it

btw, I have the original megapc service manual but with high-res pictures a person on fb took for me - they are not scans but they are way better than the low-res pdf that is available: http://www.mediafire.com/file/qiwaqx8uhvg9g5g … ans_HQ.rar/file

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 12 of 35, by Zalog

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keropi wrote on 2020-07-08, 18:08:

wait , what 7486SLC service manual you are talking about? share share share! 😁

Unfortunately I only have a printed out copy of the service manual

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Will have a look into scanning it, although it may take a little while to sort.....

To help folk, here are a couple of quick phone snaps of a few interesting bits....

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Don't think this was the final revision though !!

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Diagram Lettering - 486 SX (!) Rev. B - suspect they were relabeling a 386SX design !
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And jumper settings for those that need them....

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Reply 13 of 35, by keropi

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Ah even photos like these are great! thanks! I see there are missing details anyways due to photocopy
It makes sense they modified a 386SX design since the 486SLC will just work in place of a 386sx all it needs is bios to enable that 1kb of L1 🤣
Thanks for the jumpers, I was wondering what JP12 was for 😁

could it be that 2020 is a MEGA year? 😁

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 14 of 35, by Jo22

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Wonderful thread! Thanks a lot for all the information and the pictures, too! 🙂👍

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 15 of 35, by Zalog

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Jo22 wrote on 2020-07-10, 17:22:

Wonderful thread! Thanks a lot for all the information and the pictures, too! 🙂👍

Glad your enjoying it - the MegaPC is a great machine to work on and good fun to play on too !

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Reply 16 of 35, by Zalog

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keropi wrote on 2020-07-09, 10:47:
Ah even photos like these are great! thanks! I see there are missing details anyways due to photocopy It makes sense they modifi […]
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Ah even photos like these are great! thanks! I see there are missing details anyways due to photocopy
It makes sense they modified a 386SX design since the 486SLC will just work in place of a 386sx all it needs is bios to enable that 1kb of L1 🤣
Thanks for the jumpers, I was wondering what JP12 was for 😁

could it be that 2020 is a MEGA year? 😁

Have scanned the service manual and sent you a copy 😀 - it does confirm its basically a 386sx design, but as you'll see on diagram 1 of the fold out scans I sent, Amstrad added several specific circuits to the motherboard to support additional 486SLC features (Flush, A20M, KEN) in addition to the BIOS L1 cache support - which should help performance a bit.

Definitely hoping for a MEGA 2020 too 😁

Reply 17 of 35, by Zalog

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So have been busy over the weekend.......

Dropped in a Cyrix FasMath 33Mhz Co-processor - unfortunately the Cyrix logo is a bit dull so it doesn't photo well 🙁

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Added on a nice gold coloured heatsink onto the 486SLC - things are running much cooler now and it does seem to make things run smoother under high work loads

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Dropped in my old AWE64 card, so now we have soundfx on the pc side - this isn't the final card, but it'll do for now until Keropi creates a special megapc one !

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Then link the Megadrive/Adlib card to the Line in of the AWE card to pass through the sound and mix them together - the mix of sfx and OPL2 music is pleasingly retro !

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All in all a very productive weekend !

Reply 18 of 35, by keropi

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heheh upgrades!!!! nice!
I also noticed on schematic the vga chip is marked as "CL-GD542x" and not GD5420 specifically like all other ICs... so they just went for the cheaper option apparently!

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 19 of 35, by Zalog

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keropi wrote on 2020-07-13, 11:18:

I also noticed on schematic the vga chip is marked as "CL-GD542x" and not GD5420 specifically like all other ICs... so they just went for the cheaper option apparently!

That's good, looks like there should be no problem adding a 5428 then 😁

BTW have you noticed the artwork on the reverse of the ISA riser card ?

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It looks like there was planned space for another 16bit ISA slot on the reverse of the riser (locations are marked J5/J6) - and probably left out to save costs and a backplate re-design too.
I am wondering though if this feasible to add an 8bit slot to J6 so that a small internal ISA card (maybe an XTIDE) could be added... as there does look to be just enough room for a small card in there 😀