Early Genesis model 1 problems?

powerofrecall · Post by **powerofrecall** » Thu Feb 07, 2013 4:59 am

Hi all.

Not too long ago I snagged an early Genesis model 1 off ebay, HDG, no TMSS, ext port intact. The seller sold it alone and it had a couple cosmetic problems with the case plastic (that I've since fixed), but other than that he assured it to be in working order. It seems to be, but I've noticed weirdness with it. I do plan on recapping it as soon as the parts I've ordered come, the original caps are the poor quality Chhsi that get spotted so often on the older boards. It's a VA3.

The most noticeable problems it has:
- Rainbow banding in the picture that starts out very noticeable, but as the machine stays on, gradually fades a bit--to the point where it is barely noticeable
- Random lockups with certain carts, doesn't always happen, some carts seem worse or more prone to it than others. Games I flash to my Megacart seem most likely to crash in funny ways.
- FM sound problems. Not of the analog/distortion variety, but stranger; dropped or delayed notes, sound effects, sometimes an instrument or sound effect will play back wrong. While playing a few games of NBA Jam TE, I specifically noticed some of the background music going out of sync. Are these known issues with certain discrete YM2612s, or is this something else?

So ultimately, should I worry, or are these kind of intermittent weird issues likely to go away when I replace the nearly 25 year old electrolytics with new, quality Nichicons?

Gigasoft · Post by **Gigasoft** » Thu Feb 07, 2013 11:26 am

There could be a problem with the cart interface or with accessing the main bus from Z80. Try going to the sound test in Sonic the Hedgehog 2 and let one of the songs loop a few times. If there is no problem (except maybe the drums being scratchy), then the Z80 to 68000 interface is faulty. Otherwise, it might be the Z80 RAM or its connections.

powerofrecall · Post by **powerofrecall** » Thu Feb 07, 2013 8:09 pm

No issues with music playback in Sonic 2. I let a few tunes go for 5 minutes or so, sounded fine.

You mention the z80 to 68000, and I noticed something I thought I was only imagining previously--this genesis seems to run into slowdown more than my others. It is a signetics 68k, but I doubt that has anything to do with it. I'm no EE and my understanding of this electrically is minimal, but if it's a bus issue, is it possible that a malfunctioning bus connection could do that? Z80 reading the cart ROM area takes the 68k off the bus and halts it, DMA takes the 68k off the bus and halts it, if I'm not mistaken. So, if there's weirdness in the bus lines, maybe that's resulting in the slowdowns and spurious problems. Stuff like DMA taking longer than usual to complete, problems with the z80 trying to read in FM patches or pcm out of the cart ROM. If that was the case though, I probably would have heard those problems in Sonic 2, though... It's intermittent enough I might have to let the machine run for a while and observe what happens.

powerofrecall · Post by **powerofrecall** » Mon Feb 18, 2013 1:26 am

Recapped everything, picture problems are basically gone (yes!), haven't had any lockups or crashes since doing it (but I've yet to put it through the paces for an extended length of time). Still seems to have intermittent problems with NBA Jam TE at least. Tried a few other games and it's pretty much 99% fine other than weird things I can't replicate on my other Genesis'.

One thing I am was able to replicate consistently: With NFL '95, during the title screen music, a few seconds in, there will be an audible silent gap in the music and then it will continue on as normal. It also does this in the sound test, so it's not an issue of game slowdown or anything. None of my other machines do this, no model 2's do it, and the other model 1 va6 I have doesn't do it either.

I'm willing to live with these minor problems, but I'm curious as hell as to what makes this stuff happen.