Never seen this NTSC board revision before

[marauder666]

Received this from a forum member for a video upgrade this week, despite the couriers best attempt to get someone to steal it.

 

Admittedly we haven't had many NTSC 7800s through our hands, but this one is different to all the others.

It's got the bodge cap on the Rockwell CPU on the back side of the pcb, C64 installed (now lifted), but an additional resistor R70, 220 ohm on the RAM write enable pins.

 

Not a bodged on resistor, but a space for it next to CR3, 4.  It's also hand soldered.

 

It's on the Sobola schematic as RA, so I'm assuming it was a bodge at one time, and then later integrated.

 

 

[marauder666]

R70 is the same resistor as the one which is bodged onto boards with Sony RAM, just a different value.

 

[juansolo]

So this machine was fun and has brought about an interesting revalation that @-^CrossBow^- might be interested in. It really didn't like having C64 removed. It caused wierd issues with the Dragonfly and lot of games on it. Getting them to start when cold, crashing, and just general all round instability. We've spent over a week trying things (including swapping out the Rockwell CPU for a UMC, that didn't work well at all...) before dropping C64 back in, and suddenly stability returned to the Dragonfly. Ballblazer still crashes after about 30 mins, but just about everything else now appears to work just fine.

Not to mention this machine had some of the most bizarre solder/flux reaction going on. Touch a pin with the iron and the solder jumped off the pin onto the iron. It's like the think is coated in antiflux. Because we found that touching the Maria when running could crash it, or the cart, we ended up looking at all the pads and they were iffy as hell. Ended up pulling and cleaning the pins on every IC, and also drowning the PCB in IPA to clean this crusty flux out of the holes. Then using a ton of good flux just to get something looking like a decent joint on them. To be fair that certainly helped the stability of this thing!

 

Anyhow, have a picture of it done, because this one was a LOT of work!

 

[+-^CrossBow^-]

It is more strange than most in many ways. It is using what looks to be an older board revision that was more common in earlier '87 made units I've seen. It has a brand of RAM I've never seen before in any of the 7800s I've had in my hands. Does it have or did you try and add the small cap off the A15 line from the CPU from pin 25 to ground? I've added this on a few rockwell CPU 7800s I've come across that didn't have it when they were showing issues with BallBlazer but in some instances, adding in that cap would make things worse. So it really seems like each 7800 had to have the components picked and chosen specifically for that 7800 to work. 

 

They are not tolerant of their timings. 

 

BTW, I documented another 7800 last year that was giving issues with BallBlazer and the fix was replacing the main crystal itself with another from a donor. But I had to try like 3 crystals before I found one it was happy with. Also that is a bother with it not wanting to work without c64 attached. In my experience, that has only effected 2600 games and didn't impact function on the 7800 side at all. I just disabled c64 from the last 7800 I did last week and was using the DF to test things and even played a round of E.X.O. on it without issues. (Good thing since it had a rockwell CPU in it...).

 

[juansolo]

27 minutes ago, -^CrossBow^- said:

It is more strange than most in many ways. It is using what looks to be an older board revision that was more common in earlier '87 made units I've seen. It has a brand of RAM I've never seen before in any of the 7800s I've had in my hands. Does it have or did you try and add the small cap off the A15 line from the CPU from pin 25 to ground? I've added this on a few rockwell CPU 7800s I've come across that didn't have it when they were showing issues with BallBlazer but in some instances, adding in that cap would make things worse. So it really seems like each 7800 had to have the components picked and chosen specifically for that 7800 to work.

It had it, we've tried it on and off, ended up leaving it on as it seemed more stable with it.

 

Interestingly this was another one like mine in that it had two stickers on the back, an A1 that was then covered with an A3. So it may have been back.

 

27 minutes ago, -^CrossBow^- said:

BTW, I documented another 7800 last year that was giving issues with BallBlazer and the fix was replacing the main crystal itself with another from a donor. But I had to try like 3 crystals before I found one it was happy with. Also that is a bother with it not wanting to work without c64 attached. In my experience, that has only effected 2600 games and didn't impact function on the 7800 side at all. I just disabled c64 from the last 7800 I did last week and was using the DF to test things and even played a round of E.X.O. on it without issues. (Good thing since it had a rockwell CPU in it...).

The Ballblazer thing is just odd. With everything else seemingly working ok now, we decided to just leave it as is for now. It was certainly an interesting journey!

 

@Muddyfunster's (another Rockwell machine) is coming back so we can fix a mistake we made (my fault...) and as it is also having probs with the DF. Gonna drop C64 back in that and see if that sorts that one too. Will be interesting if it does.

 

Also I've bought another US one cheap that looks nice and scruffy. That'll be here in a couple of weeks. Sadly I didn't win the smashed up one as people started bidding on that one.

Edited January 26, 2022 by juansolo

[marauder666]

I think Jack's Atari were just throwing parts at 7800s to get them to work.

 

This one also has all 74HCT logic ICs.

Edited January 26, 2022 by marauder666