Mega Drive Repair

how do you come up with 2 degrees? have you worked out the thermal mass of the heatsink? :S

 

I have this IR thermometer here and I've used it in the thing ... lol

Really, it just irks me when people mod the consoles internally to do something which could be made externally in a very smart way.

You now have made me interested in trying maybe underpowering the thing a little bit (a PS2 slim power supply with 8.5v/ 6.5Amps) to see if the CD drive can work with 0.5v less but having a decent current supply.

I think almost everyone can get a broken PS2 slim or can get a PS2 slim power supply easily. I for example use it straight on my Game Gear and it works great (the power plug is identical). I bet it works great for the MD2, too.

 

The only reason I wanted to do the mod internally is because it didn't look too difficult. I have a (admittedly cheap) hot air rework station, used foil to protect the other components, and removed the 7805 quite easily. Of all the things I thought would go wrong, not snipping legs off a cap was definitely not one of them. Seriously, who doesn't check a board before powering it on? I guess mistakes like this are the only way to learn

I've always wondered why the MD has a 10v power brick, but drops it to 5v internally. Why not just use a 5v power brick? Is that down to then planning to power devices through the expansion slot?

Anyway, I've found a few sellers of replacement VDPs on Alibaba, but I'm a bit suspicious of the fact that they are supposedly original parts. Not sure whether to try and find a dodgy board (which might also turn out to have VDP problems) or go for one of these.

 

The reason why SEGA went for 10V power brick is they already had a 10V power brick and a 8 pin DIN connector as A/V output on the previous console (Master System).

And probably because originally/historically the barrel plug is used as a 12v (positive center) power plug they also added a pair of rectifier diodes so if someone plug a NES(it's 9VAC) or another power brick with reversed polarity it doesn't fry the system...

When I modded the replacement Mega Drive (from the accident I mentioned above) I kept the original design (keep in mind that a system with a MEGA-CD attached will have three power rails, not two) for power from fears of frying stuff again.

Did the capacitor you forgot to remove was the capacitor placed over the region change jumper ?

 

The cap is the one circled, haven't done any other mods to the system (and if I get it working again, I definitely wont be doing any more modding). That's a different model to the one I have (PAL VA6) but the cap looks the same. The reason I first thought of the CXA1145 was because of where the cap is on the board.

I think I'll order a VDP from Alibaba, will take a while to come but I'll be at Uni anyway so I won't have the console with me for a while.

 

First replace the CXA1145 to see if you can get picture.

Do you have an oscilloscope ? You could use it to verify if the composite sync is coming out from the VDP.

 

Unfortunately I don't. Anything I can test with a multimeter?

I can get a CXA1145 cheap on eBay, I'm just not sure how good these presumably non-genuine parts would be. Might be easier to install a socket in case I have to remove it again.

 

It's just that I feel you should check everything you can before putting the blame on the VDP chip.

The CXA1145 has a clamp circuit. The clamp circuit does short the RGB signals to GND (that's the act of clamping) to discharge the coupling capacitors on the cable (dc-level restoration) at a fixed interval which is defined by the sync signal. Basically it means that if the sync signal is dead/missing, the image will be very, very distorted and very, very dark or even it could completely mute all video related outputs of the CXA1145 chip.

 

Would it affect the RGB inputs of the chip? If there is a voltage on the inputs would that show that the VDP is generating a video signal?

Another thought - if the clamp has shorted RGB outputs to ground, I should get continuity between ground and each pin?

 

You just won't get any signal at the outputs.
as you can see it's the inputs which are clamped.

 

Back at Uni now so I won't have the console with me, possibly for a few weeks. I know the Uni has oscilloscopes, I might ask if I can use one. The RGB output pins all tested at 0v with a multimeter.

If I can't gain access to an oscilloscope, is there any thing else I can test that would indicate which is the problem? I might order both parts anyway just in case, but I don't want to mess with the board any more than I have to.

 

My suggestion is you buy another identical system and use the parts to repair your system (only because you mentioned it has sentimental value I am suggesting this).

The Mega Drive console I blew was my original one I bought with my first job money ... lol I felt horrible. I replaced the whole board (I kept the one I blew so eventually I managed to get it repaired) Now I have three working mega drive consoles of that kind and one slightly newer Japanese console (the one with the white license text before the cartridge starts)

 

That is probably the best option. Once I get this one working, I could always buy the replacement parts for the donor console as I wouldn't care too much if they didn't work. I might even just swap the board over, though a replacement board might need recapping in the near future.

My MD was the first home console I had (technically it was my dad's, the manufacture date printed on the board is over 2 years before I was born), have lots of fun memories playing Sonic 2 with him when I was younger. Fair to say it has a few battle scars as a result

Anyway thanks for all of your help. I'll update the thread when I have got a console to use for parts, or gained access to an oscilloscope.

 

Very interesting know the story about your Mega Drive. I honestly hope you get it running again. I'll try my best to help out.

 

A japanese genesis w/ TMSS!? Never seen one before! Know what board revision it is? Mines a model 1 w/ a VA6.

 

What ? A LOT of Mega Drive consoles were sold in Japan with TMSS, particularly after 1991.

http://www.sega-16.com/forum/showthread.php?11322-Model-1-Japanese-Mega-Drive <- original 1988 MD (non TMSS)

http://www.innovative.net.my/pvt/MDVA4-2.jpg (VA4 non TMSS)

VA5 has no TMSS when set to Japan but has when set to USA. It is identical to VA6 but has the 160 pin 315-5402 instead of 315-5433.

 

Well thats what I heard anyway.

 

But a lot have TMSS too. lol

All that were made during 1991 had.

 

i know. my va6 has it.

 

Also BTW, it seems that the VA5 and the VA6 are the same board but it just has a different chip at IC4 as indicated by the legend that is under the row of pins starting with pin 41 and ending with pin 80. A SEGA chip (manufactured by NEC) that has 315-5433 means its a VA6 w/TMSS as it was in my case. A 315-5402 means its a VA5.