The All Aussie Sega CD Model 1 Repair Adventure

I actually undid the jumper and tried it out. Obviously the thing wouldn't work properly without it. After the circuit diagram got released along with the service manual I was able to see the circuit and was able to draw that conclusion from the circuit. SEGA has lots of erratas. Just read the bunch of changes you need to do on the mega drive for the 32X to work properly on it. Tell me if isn't that frustrating....

 

Above and beyond!

I love the way the VA4 PAL Mega Drive and 32X worked so well together... You wouldn't want to play Doom while drunk! Any ideas if this affected NTSC units at all?

Shane McRetro stops derailing thread

 

My Genesis 1 VA3 unit was crashing with Space Harrier until I applied the changes. It used to play like a chump but it's playing like a champ now.

 

Thanks for the explanation I_oliveira, you are a wealth of knowledge sir!

 

I never knew that some revisions had issues with the 32x, interesting....

 

Hey Ben,

Just found this... awesome job so far. This write up will surely help others who repair the mega cd. It can be a temperamental beast that's for sure.

$17 bucks for that new cd mech is good value!

So did you end up replacing all the caps?

Darren

 

Thanks Darren, much appreciate it! I believe the caps were replaced before I got it as they are all within tolerance. There are some broken traces which are being fixed at the moment, hopefully the end is in sight

 

UPDATE

Hey everyone, I thought I would give an update on what is happening. I am getting some traces repaired from the SMD corrosion and if it sill has issues I will take the drive to a Sony specialist who can let me know if the drive PCB is broken. I am certain that the power unit and main board are all OK and it is something to do with the drive. Fingers crossed it is something simple like a motor, but we shall see what happens.

 

I just received my CD drive.

 

Awesome, let me know how you go

 

Hello fellow Assemblers,

Dan has done some great work and we think he has narrowed it down to the Sony CD PCB board. He discovered that continuity on the caps on the main board stops when the CD drive is disconnected and thinks there is a ground to short on the CD board. Can someone perform the same experiment on their Sega CD Model 1 so we can compare results?

If someone already knows this is normal, we would be grateful to know as well.

Thank you

 

Update

The issue has now been located to a faulty Sony Controller on the KSK-1200A PCB board (CXD2500AQ), a big thank you to Doug and Gabe from Nerds Warehouse for helping to diagnose the issue. The new part has been ordered and I will update shortly.

 

Nice topic !
I have two defects Mega-CD boards.

- The first comes from an eropean Mega-CD 1 and has 2 issues.
--> The tansistors TR1 and TR2 are broken. But I can't identify them. have you the type and the value of these ones ?
--> It boot up, but it stuck on Mega-CD logo without music (I don't know if it's because of defects transistors or not).

- The second is early model of Japanese Mega-CD (with daugtherboard).

It does not boot. Black screen and no activity on LED. I change all SMD capacitor who have leaked, I try with another power board, another connector between Genesis and Mega-CD (but this one is specific to this first japanese model). No luck, still not boot up.

Simple question : can you fix them ?

 

G'day

Thanks for stopping by, I will try and do my best to help you.

The transistors for T1 and T2 are 2SC1623 L5 which I found on this forum, information provided by TheRealPhoenix http://assemblergames.com/l/threads...ck-from-the-dead-but-has-a-small-issue.55836/ post 4. They are involved with the audio output and can be purchased from here https://www.aliexpress.com/item/100-new-original-2SC1623-T1B-2SC1623-Code-L5/32739683105.html (for $0.08 each)

In regards to your second problem, I'll have a look around and see what I can find.

Hopefully I can get you on the right track with your first problem.

 

That behavior happens when the BIOS starts the sub CPU BIOS and the SUB CPU stalls due to not being able to reach the CD drive hardware (basically it tries to talk to the CDC 4 bit CPU, which controls the drive, to tell it to seek for a DISC TOC) and since the sub CPU does all the control for the RICOH sound chip (which plays music samples) and the zoom/rotation chip you won't get any SEGA/MEGA-CD logos if it stalls.

If you want confirmation, just take a working SEGA/MEGA-CD1 and disconnect the flat cable between the mainboard and the drive. Then power it on.

 

So good news and bad news, the good news is the laser is focusing and reading the TOC and will play audio tracks. The bad news is that there is no audio output via the rca output, the mega drive audio output and there is no levels of audio being displayed on the screen when the track is played, and it won't read data either. After a suggestion from l_oliveira I will check out the schematics and trace the mute signal on the board. Keep watching this space

 

Thanks ! Transistors purshased. I'll give you a feedback when I will have managed to replace them.

EDIT :

Tested. When drive is disconnected, I see the BIOS splash screen, no music and no rotating logo. But I have an image, the Mega-CD's background image. This test is made with a working Mega-CD.

But with my Japanese Mega-CD I have nothing. Sometime, red and green LED briefly flash or keep on, but nothing on screen. As I said, power board and drive are OK.

The issue may come from Mega-CD's mainboard or connector board between Megadrive and Mega-CD. The problem with early japanse Mega-CD boards is the traces on PCB. On, these models we do not see the trace, pretty difficult to diagnose the issue.

here is a photo od this board.
The connector board :
front
back

 

I had the exact same fault on my MEGA-CD (which is one step newer than your unit as mine has the connector board logic ICs condensed into a SEGA custom IC).
The fault was caused by capacitors C10, C12 and C13 which leaked and corroded traces making the circuit between the IC1 (68000), IC2 (CD ASIC) and ICs 5, 6, 7 and 8 (68000 RAM) making access to the RAM impossible. The MD side uploads the CD BIOS (after unpacking it from IC4 EPROM which is actually connected to the MEGA DRIVE side of the system) into that DRAM and init the CD CPU (SUB CPU) which in turn turns on the front leds. If that fails the MD side 68000 will stall on a infinite loop.

So have the connection between ICs 5, 6, 7 and 8 and the rest of the CD CPU (SUB CPU) checked for broken traces. Once I mended the broken trace on my unit it started working properly again.

 

Thanks I_Oliveira, I checked all RAM lines between :

- 68k and IC10 : A1 to A13 and D0 to D7 lines (all 68k side)--> good
- 68k and IC5 : D0 to D3 (68k side) --> good
- 68k and IC5 : D4 to D7 (68k side) --> good
- 68k and IC5 : D8 to D11 (68k side) --> good
- 68k and IC5 : D12 to D15 (68k side) --> found bad trace at D15 between pin 4 of IC8 and 68k's pin 58.

I also test A lines (0 to 19) between 68k and ASIC, no problem. Tested connections between ASIC and IC11/12 all good. Tested PA lines 0 to 8 between IC5/6/7/8 and ASIC, all good.

Others tests

- Sound CPU (315-5476) / sound RAM (IC13 & 14) --> OK
- CDC CPU / RAM (IC17 & 18) --> OK
- CDC CPU / ASIC on PA (0 to 8) and D lines (0 to 15) --> OK
- all fuses OK
- power board OK (tested with another Mega-CD)

Result --> black screen no led activity !!!

At this point, I despair to revive this unit. There is no trace of burn on components. The only point that I have not tested yet is this little resistance on patch board. I cant' measure his rate. I do not even know if it's realy a resistance. It give no conductivity. This component is pink, with three circles : red, red, white and gives with a calculator 22k MΩ !

Any idea ?

 

The component is a capacitor. It's there to help by delaying a signal.
Check DRAM /CAS /RAS A0-A9 (addresses are multiplexed) and /WE signal. My unit failed with black screen with /WE line being broken.