SEGA Megadrive test kit [homebrew MOD]

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paulpsomiadis
Posts: 88
Joined: Wed Dec 29, 2004 5:16 pm
Location: U.K.

SEGA Megadrive test kit [homebrew MOD]

Post by paulpsomiadis » Tue Jan 30, 2018 10:33 am

Hello once again folks – after a VERY long time, my next mod is ready!
Some people will remember me from my Radica mods HERE:
http://benheck.com/forums/viewtopic.php?f=58&t=17720
http://benheck.com/forums/viewtopic.php?f=58&t=1517
Anyways onto the meat-and-potatoes of this article...

The SEGA Megadrive [Genesis] Test kit

First some Q&A

1) Q) Why do this? A) It opens the way for experimentation and testing of components in the console that have highest fail rate – also future proofing so that IF they fail, they can be replaced. Also I figured that SEGA must have had some sort of retail hardware test kit at some point - to help with simulating faults and repairing faulty units.

2) Q) What actually inspired this whole MOD? A) The Ex-Soviet Z80 clone :-)

3) Q) What was the [original] plan? A) To add a socket for EVERY chip, bar the custom ASICs.

4) Q) What was the [revised] plan? A) in practice, the above was not possible as there is a lack of real-estate on the mainboard in certain areas for one and also some chips do not have drop in replacements that improve over the original or have any positive benefit.

5) Q) What was most challenging A) Re-installing the factory “VDP freeze fix” resistor on pin 6 of the CPU for the VA4 mainboard. This required back-tracing the path of the via for the pin to find the corresponding pads underneath. Removing the through hole solder on this CPU pin [drilled the pin hole with a small drill bit] on the board and then using the pads underneath to re-connect the pin to the board going via the resistor, and pads underneath to it’s original point on the top of the board.

6) Q) What was the weirdest thing? A) The VA4 board REQUIRES a Motorola 68000 to work correctly with the 32X...more on this later when I talk directly about the VA4.

7) Q)How long did this take? A) I’ve been workin
g on this for about 6 months.

8.) Q) What is your best guess for total man-hours? A) About a month total if you worked on it every day and didn’t have to work around a full time job.

9) Q) Would you be motivated to do this again for [random sum of cash]? A) NO, and NO in that order.

10) Q) Would you be motivated to do this again for a rare hardware trade? A) YES – only for a Dynacom Megavision [Early Brazilian Megadrive clone – before the M.D. was officially licensed by SEGA Brazil]

http://saturnoz.blogspot.co.uk/2009/02/ ... nacom.html

Ingredients of the MOD
For this recipe you will need many spoonfuls of coffee/sugar and late nights...
A soldering iron, de-soldering braid [Servisol soldamop - green top] a de-soldering tool, solder
Many, many, many ZIF sockets (I will not list these, just look at the pictures)
Pliers / drill / drill bits / screw drivers / tweezers / wire cutters
Toggle switches – or equivalent
Possible other replacement parts depending on the age of the SEGA Megadrive
(replacement reset button / headphone jack / power jack / electrolytic capacitors)
At least one SEGA Megadrive [three were used here – main / backup / parts]
Possible replacement parts [where and when necessary]
LOTS of patience and time

For the IMAGES – go HERE: https://imgur.com/a/YmDcE

What was added
Motorola MC68000 10MHz [VA4] – from an old Amiga A500 (it has an expansion card with a built-in 68000 and still works fine)
Hitachi 68000 12MHz [VA6] – bought from eBay
80A-CPU Soviet Z80 Clone [VA4 and VA6] – gotta’ love these obscure clone chips!
New Z80 work RAM (thin profile) [VA4 and VA6]
New 68000 work RAM [VA4]
Upgrade to op-amp – replaced the LM358 with a MC34071P...less noisy :-) [VA4 and VA6]
CSYNC fix - this involves cutting the trace leading to the CSYNC pin on the A/V jack then soldering from the (not connected) pin 11 of the video encoder chip via a 220Uf electrolytic cap and 470 ohm resistor onto pin 6 of the A/V jack – giving correct CSYNC output. [VA4 and VA6]
RGB force – the green wire on the video encoder that forces pure RGB output. [VA4 and VA6]
32X compatibility fix alterations – see 32X fixes section... [VA4 and VA6]
Toggle switches – for region switching and CSYNC enable/disable[VA4 and VA6]
The serial port and accompanying components from the parts board VA6 [onto working VA6]

What was salvaged
Three Z80 CPUs
An electrolytic cap / ferrite coil and bead cap

What was lost
Motorola MC68000 8MHz – pin snapped [VA4]
68000 work RAM – static shock (top chip) pin snapped (bottom chip) [VA4]
Top metal shield on both consoles [useless and gets in the way]

The VA6
Now onto specific challenges with the VA6...
The actual easy part was doing the 32X fix – just remove a cap and add a jumper wire (see the section below for the 32X fix information and technical docs download)
Obviously, getting all of the sockets to fit was the overall challenge – a bigger headache was actually getting all of the surrounding components to fit on the board and still allow the sockets to fit.
So as you folks can see, there are a lot of electrolytic caps that have been moved on their side to fit under the sockets on the main board, and despite the orientation of some sockets needing to be backwards – each has pin 1 marked with a red stripe.
Also, the plastic case had to be altered in several places (also the heat sink) by either removing screw holes entirely or sanding them down a little to allow everything to fit. The open/close lever for the 68000 socket also had to be trimmed to allow access to the corner screw [still a bit tight].
Another challenge came in trying to mitigate the dreaded “jail bars” effect that the original Megadrive is famous for. As you can see on the underside of the board – there are two traces that have clearly been repaired – these were initially cut to try and separate the VDP from pin 6 of the video encoder chip. Sadly this just made the VA6 non functional – so I restored these and opted instead to link pin 6 of the video encoder to ground.
This helps reduce the jail bars – but obviously, the only way to eliminate them is with an RGB Bypass board...a project for a later date.
The usual language and region/language switches were added along with a CSYNC on/off switch to simulate the original “faulty” behaviour of the megadrive where this pin is incorrectly wired.
The headphone jack, power jack and reset button needed to be replaced, as this was a “rescue” console – I also re-tensioned the cart slot and re-soldered it underneath.
Finally the A/V jack was replaced with a standard DIN socket, and cart slot cleaned.

The VA4
Now onto specific challenges with the VA4...
Various similar things were done here, so we’ll cover them first.
Region/language switches and CSYNC switch, CSYNC fix and RGB force wire, standard DIN socket, op-amp replacement, 32X fix [again, more info below], re-tension and re-solder cart slot, moving of electrolytic caps, trace repair near VDP [again, a result of early jail bar experiments], altered plastic in the case and heat sink.
Now for the different stuff:
The socket for the Z80 work RAM was put in backwards to avoid having to sand the screw hole and also avoid the nearby resistor pack. Sadly, this meant the lever had to be shortened as well – as it covered the reset button.
The power jack, headphone jack and reset button were all in reasonable condition and merely needed re-soldering – but the cart slot and Mega-CD edge connector were both filthy and needed a good clean, which was done with a bit of WD40 and cotton buds [q-tips] and a polishing block for the edge connector respectively.
This board was a LOT older, which made de soldering much tougher [alas, more dead chips and damaged through-hole solder points]. To repair the through-hole solder I use an easy technique as follows:
Take some normal multi strand wire and separate the strands. Take two of these individual strands and tin them with solder. Place on either side of the damaged solder hole and create a loop. Solder to the top of the board but leave the hole OPEN. Put the socket through the board and loop the strands around the leg of the socket. Solder all in place making sure the strands and new solder link to the track on the underside of the board. You have now fixed the through-hole solder. :-)
Oh and in-case people ask, the heat sinks for the Ex-Soviet Z80 clone chips are REQUIRED – whereas the one I added to the 68000 on the VA6 above was just because I initially intended to over clock [this didn’t happen in the long run].

32X Fixes
In this section I will talk about the minor fixes for making both Megadrives fully 32X compatible...
This was the easiest and hardest part of each MOD respectively – for the technical docs from SEGA used to perform this MOD, looky HERE:
http://www.filedropper.com/sega32xservicebulletins32x

The VA6 was easy, as it requires only to remove C78 and attach a jumper wire from the top pad to pin 15 on the 68000 CPU.
The VA4 on the other hand is a whole different story – people who are Megadrive fans will know how notoriously fiddly the VA4 main board is...
You begin by removing C78 and C100 on the underside of the board.
Then remove C112, again from the underside and add a THICK link wire from the top pad of C10 to the middle of EM16.
Finally remove C97 and FB7 from the top of the board – replace FB7 with a link wire and C97 with the C112 you removed in the previous step.
You can see most of this clearly enough on the images of the VA4 with the voltage regulator heat sink removed (sorry that the switch wires are in the way a bit...)
Once I did this, I thought I was done...yet the 32X was STILL unstable – after scratching my head for HOURS...I remembered that this board used a Motorola 68000.
Once this was changed to the correct type of 68000 and I gave the 32X contacts a REALLY good clean [they were filthy...like BLACK filthy] everything worked just fine!

The Final Analysis
All in all, this came out better than expected and I think I will keep the VA4 as my main console.
The VA6 will be sent to Ben as a gift to say a VERY big thank you for choosing me as one of the winners for the ZX Spectrum Portable competition over HERE:
https://www.element14.com/community/doc ... m-portable
(I will contact the Benheck forum moderators for a P.O. Box or other address, so that I can send this to Ben)
Any more Q&A – feel free to ask, and if anyone has a Dynacom Megavision for trade...let me know... ;-)
-=MOD or DIE=-

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