SEGA Mega Drive (Genesis) II - Black Screen Repair 

Glad it helped!, those master crystal are SEGA custom crystals you'll only find them in a PAL Master System or Mega Drive.

I'm working on a device that is mainly operating at TTL levels (5v Logic) so my scope is set to DC Coupled. The SEGA Mega drive also has analog audio. If I was hunting down a problem on the analog audio side I would have my scope set to AC Coupled.

@@Four_X You say the Sega mega drive has analog audio, again I’m a NOOB so, Isn’t audio always observed on AC coupled, because it behaves like a sinewave? And what other kind of audio is there, digital? Thank you so much for your reply, btw. I like your approach to electronic repairs, you don’t just spew-out a bunch of electronic jargon, you take your time and go about it logically, I like that, I think you’re a good teacher and that’s why I’m now subscribed to your channel. Anyhoo, kudos sir! And thanks again.

​@@maxwarfield6699 "so, Isn’t audio always observed on AC coupled, because it behaves like a sinewave". Correct most of the time, However the are occasion when looking at and Audio Signal with a Oscilloscope set to DC Coupled will be advantageous. One of those occasions would be to see if there was any DC offset to the Audio signal. This would at minimum cause distortion to the Audio and indicate there is something wrong somewhere. It's never good practice to couple two devices (I.E. CD Player to a Amplifier) with a DC offset, especially a device that is going to be amplified. A device outputting a high DC offset could also damage the Amplifier. This is also the same with Analog Video. If you watch my “Sinclair ZX Spectrum+ (Plus) 48K - Repair, Mod & Restore” video here ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-cE8cCvQOt-k.html at 17:30 you see me perform a Composite video mod and I explain why you have to use a Capacitor to eliminate the DC offset from the Composite video signal when connecting the Composite video signal to a TV.

The reset button doesn’t work like that in a sega Mega Drive Ant. The (315-5660) custom sega chip inside the Mega Drive II only triggers a system reset on a falling edge of a low from the reset button. Therefore it wouldn't matter if the reset button was held down it would only invoke a reset once. The Mega Drive II would only reset again once you let go and pressed the reset button again and the (315-5660) custom sega chip recognized a falling edge from the reset button.

I think it's marked FB1, it's a ferrite bead. There used to suppress and dissipate high frequency noise. if you look at the SEGA Mega Drive II Schematic here gamesx.com/wiki/lib/exe/fetch.php?media=schematics:genesis-2-md-2-schematic-1_of_4.png You'll see it in series with the Crystal Oscillator (OSC1).

@@Four_X No, no, on my NTSC board it’s located right next to the voltage regulator but on your PAL board, the component I’m referring too is sitting right next to the power jack, that you glued and reflowed. It’s a small black device with four pins, it looks like an opto-coupler [opto-isolator] and even yours, is marked CF1- you can clearly see it at 1:30 min of your video, just right of your thumb - what is that thing? Please let me know, I’ve looked all over the web, I’ve found nothing and I think I’m going crazy! Thank you kindly for replying. Cheers!

​@@maxwarfield6699 Sorry I don't know why the hell I was looking at the Crystal Oscillator (OSC1). That component labelled CF1 is an Inductor, Chock. you can see it on the SEGA Mega Drive II Schematic here gamesx.com/wiki/lib/exe/fetch.php?media=schematics:genesis-2-md-2-schematic-4_of_4.png, It's used to block higher-frequency (noise) while allowing direct current (DC) it can also block lower-frequencies of alternating current (AC). It's basically acting as a noise filter just in case your input power supply is noisy.

@@Four_X So, the CF could stand for “choke-filter”? And it’s replacing a notch-filter [inductor+capacitor]? Do you know how one might test this device? Please let me know, if you do. And again, thank you for replying - I’m learning a lot. Cheers!

It's still a great day when you can save a retro system.

Thanks Jamie. Always a good day when you save a retro system from the scrap pile.

It was dead as a doornail. Ha.

Thanks, so glad you liked the video.

Good luck with the repair, let me know it you get it working.

I cleaned out the cartridge slot according your demonstration and the console came alive. I had no audio, but that turned out the PAL I/G switch in the RF modulator was broken and stuck on PAL I, unless I pressed really hard on it.

Did you fix it?

Sounds like a shorted component to me.

@@Four_X do you do a repair service? Though probably not cost effective... I'm more curious than anything

It's going to be one of two things. 1) dirty DIN pins on the video output connector. 2) dry joint's (solder) on the DIN's video output connection to the PCB.

@Four X Update! I looked at my DIN pins as you advised, they were sparkling clean, but I still blew H20 in it as hard as I could :D and wiped the outer metal ring with a cloth and it seem that the problem is gone :D. Phew, hopefully I dont ever have to work the (solder) :D. thanks for your help!

Get yourself a Logic Probe, there not expensive and check signals like clock, reset, interrupts ect, check the RAM and look for any stuck (High, Low or Floating) signals.

It going to be difficult if you're just using a multimeter and don't have a oscilloscope to check the signal, you could get a logic probe and do it that way. Most chips will have a pinout online just google the chip ID number.

I think i can understand, Yes it was a bad cristal.