PAL Megadrive Black screen problems (another one)

Hi guys

I have a few Megadrives (IC BD M5 PAL No rev number) that are giving me the common black screen issue.. I am not getting any sound from the headphone port either.

What i have done so far is check both the Voltage regulators (L7805CV) im getting 10.2V on the in and 5.1V on the out. I checked a few pins on the MC68000P8, the Z80, and the IC2 & IC3, they all get a 5V reading on several pins.

Does anybody know where i can go from here,? and possible ways to rectify any problems. And if anybody in the UK knows where to get replacement parts to that would be great. If searched a lot for info on repairs but it all seems to be quite vague.

Thanks in advance.

 

Nobody knows where to go next in troubleshooting?

 

Have u checked the video encoder (I think it's a cx1645) for voltages. It's tricky to identify on the MD as there could be numerous reasons for black screen, is the cartridge working, are the cartridge port pins making good contact? Without an oscilloscope it's hard to tell if the 5V is actually healthy. It may need a recap.

 

Hi thanks for replying.

Yes the game cartridge does work on other consoles and i have cleaned and re-soldered the cartridge slot.

I think the video encoder is IC 13 underneath the heatsheild. I have checked that an get voltages on some of the pins but am unsure on what each of the pins do, and if the VCC pin is getting anything.

 

Check for shorts/low resistance between ground and output on the two regulators. I have had ceramic caps go shorts circuit on mega drives. Kind of hard to track down, but doable.

 

Thanks Oakleaf.. Im a bit of a noob at this stuff. But the Voltage Regulator closest to the graphics ouptut i dont get any reading 0L on continuity mode on multi-meter, but the V Regultor more to the right on the 3rd leg (output) i get about 193ohms.

If that means anything.

 

Then you may have a short. 193 is way to low. Just hope its none of the ics. Try messuring across caps around the board, and try to find the one with the lowest resistance. If you find one wit real low recistance that might be it. But it might also be the ic or something else closest to that cap.

 

O it might be the regulator to. Swapping it might be a first step if you have one. But my bet is its something else

 

Thanks for taking the time to help. What readings should i be looking for if im using the multimeter on continuity mode on the regulators? would it be 0L on all.?

Ive tested both the regulators voltage and they both take 10.2V and give out 5.1V.

 

Not open line but at least in the kiloohm range. Good that there both at around 5v. That means that there not that loaded down that much. I guess you could take the output of the suspect reg and bend the leg, solder two wires and messure current draw, amps. Might be a small short or a bad cap. Or you might be chasing a ghost down this path might be something compleatly diffrent wrong.

 

yeh, im not really clued up on this to be honest. im trying to learn. if you have any good reference material i can read to help it would be great.

The board was pretty bad, but i thought if i can get this one working , then others shouldnt be much of a problem.. Ive attached some pics of the board if you could possibly see anything of interest.

 

Seems like its been wet at some time. You can take away the heatsink while testing. Just dont leave it on for to long. Easyer to diagnose. Just a longshot but it might be salts percipitated on the board that causes it to malfunction. But i guess you have already run down the board with IPA?

 

yes, i think it must of been stored in a shed or outdoor garage for a while, but it still receives power (just get the black screen when switched on).

Yeh, i gave it a clean with alcohol and tried to remove as much rust/corosion as possible from the PCB.

I have DLed the official SEGA schematics that are floating about online, but a lot of the flowcharts in there are quite vague unless you really know what you are doing.

 

Wow. That board needs some TLC. It may be beyond repair, given the amount of corrosion. Some of the smd caps on the underside look corroded as well.

To save that board, u would have to confirm the continuity of practically everything, remove a lot more corrosion and check every solder joint. It may be more effort than it's worth given the availability and cheapness of Mega Drives.

 

How are you connecting it by the way? RF coposite or RGB. I would also open the RF box to check for corrosion. Im sure youl get it working mega drives die hard

 

Haha yeh it is a pretty haggard board. my thought was if i can have a good go at this and learn all i can, most other boards wont be as difficult.

Do you have a noob guide to checking the continuity for most parts.. I have a lot of boards so it was mainly a learning exercise.


"Oakleaf" im connecting through the composite port not RF.

 

Dont give up! Never surrender! Yeah but if its been wet/real moist that rf modulator box is a real moisture trap. And the corrosion in there might load down on the composite video. As the composite is muxed with audio somewhere around there.

 

yeh for sure man, i can be quite persistent lol.

So even if the RF is not being used it can still effect the rest of the board if damaged enough? would it be possible to just remove the whole RF unit if it is badly corroded? if im not planning on using RF again.

would it be a case of just checking each trace from the voltage regulator to each IC chip in beep mode to check continuity?

 

Yes, remove the RF box. With a lot of luck, thats it.

And no, you cant check between 5v an ic with continuity cause there will be resistors inductors and wath not inbetween. You can check for ground that way thou.
To check the ics look up genesis on console5s techwiki and lool at the pinouts. Check resistance between ground an vcc on each ic. You can also check for 5v on them when turned on if carefull.

It might be a bum adress buss tho. You might be beeping well in to the new year

 

nice one, i will try those things out.

thanks for the help. although i might be back at some point..

have a good one.