derkom's attempt

[derkom]

Okay, I have good progress to report. Not success yet, but it seems within reach.

First today I reflowed all the PLCC chip sockets, MFP, and all the RAM, and what do you know, the bus errors went away. I could see on the logic analyser that the system appeared to be running properly. One bus error on startup, followed by zero bus errors and plenty of activity on all address lines. Plugged in a keyboard and sure enough, a floppy LED! However, still just white screen video and no diagnostic cartridge output on serial.

So I reflowed pretty much everything else on the board, hoping that my video and serial issues might go away, but to no avail. Still white screen, still no diagnostic cart on serial. I should note that the serial chip I'm using is also pulled from Atarian Computing's Alpha board and is not known to work, although there is no reason to believe it wouldn't. When I ordered all my items from Mouser, they didn't have any of the MAX238s in stock. I ordered one on fleabay, but it has yet to arrive.

So, what is one to do with an ST with a floppy LED and no video output to confirm things are running? Why, hook up a floppy drive and try out an autobooting game of course! I soldered on the floppy connector, attached a floppy drive, and popped in a copy of Lotus III and turned it on, and sure enough, it runs the game. Still no video of course, but this certainly confirms that the system is indeed running.

Suggestions for diagnosing the video and/or serial issues are welcome. I figure I can look at the appropriate YM2149 pins to see if anything is being sent over to the MAX238. Don't have a clue about tracking down the video stuff. I sure hope I haven't cooked any of the SMT stuff, because I do not look forward to doing that again.

[exxos]

Getting there

Can you send a close up shot of the video DAC circuit ?

Also do you get sound from the game ?

[derkom]

Can you send a close up shot of the video DAC circuit ?

It ain't pretty (SMT is not my forte), but here you go. I've left this image quite large.

IMG_20191203_210725.jpg (476.84 KiB) Viewed 2728 times

Also do you get sound from the game ?

I knew you were gonna ask that. Right now my monitor hookup is video only. I'll get something audio capable set up in a bit and let you know.

[exxos]

I knew you were gonna ask that. Right now my monitor hookup is video only. I'll get something audio capable set up in a bit and let you know.

You can prove it with your scope as well

I was suggest you try measuring with your scope on the 75R resistors and see if you see something which resembles a square wave on the output.. Something like this...

DS1Z_QuickPrint10.png (40.79 KiB) Viewed 2726 times

#stolenfromicky

[derkom]

Was easy enough to hook up my speakers. Turns out Lotus III doesn't make any sound at whatever it shows first (I don't even remember), but AV-8B Harrier Assault has lovely chip music in its title screen and I confirm I've got sound.

[derkom]

It occurred to me that since I've got floppies booting, I could see if it'll still boot a floppy with the diag cart in, and it does not. So it probably is running the diagnostic software, just not getting the output anywhere useful.

[exxos]

It occurred to me that since I've got floppies booting, I could see if it'll still boot a floppy with the diag cart in, and it does not. So it probably is running the diagnostic software, just not getting the output anywhere useful.

Did you see my post above about testing the video RGB on your scope ?

[derkom]

Did you see my post above about testing the video RGB on your scope ?

Oh, yeah, sorry about that. I was thinking (not thinking, really) you were talking about audio there. Seems like there's basically nothing on those resistors.

EDIT: Err, scratch that. I was doing something (else) dumb. Gimme a sec.

[exxos]

Oh, yeah, sorry about that. I was thinking (not thinking, really) you were talking about audio there. Seems like there's basically nothing on those resistors.

I would trace it back then..

These are the 3 RED inputs to the DAC.

1.jpg (59.8 KiB) Viewed 2696 times

3 RED outputs from shifter..

2.JPG (42.48 KiB) Viewed 2696 times

Also check you have 32MHz on this pin.

3.JPG (26.56 KiB) Viewed 2693 times

[derkom]

Okay, there are square waves on all of them.

R36 and R37 look like this:

R37.png (103.91 KiB) Viewed 2692 times

But R14 looks a bit wonky:

R14.png (103.74 KiB) Viewed 2692 times