exxos's Atari, Amiga & retro forum

Having watched a couple of YouTube videos of fixing the Atari 8 bits, a brown (red) screen seems to mean that the machine isn't really starting to boot at all. Do you see any activity on the address/data lines? (And I see the CAS goes through that delay line chip.)

stephen_usher wrote: 13 Feb 2022 19:15 Having watched a couple of YouTube videos of fixing the Atari 8 bits, a brown (red) screen seems to mean that the machine isn't really starting to boot at all. Do you see any activity on the address/data lines? (And I see the CAS goes through that delay line chip.)

Have watched a few myself, and found some decent posts on AA.

In short, yes, but it isn't very clean on the data lines, you can see a signal, but its kind of flattened out as though it's fighting against something. Both CAS and RAS are present on boot up, but CAS stops after a while, maybe 10-15s in.

SYNC on the CPU is also indicating activity and all the clocks are good. I have a replacement modern delay line and it makes no difference. Delay line failures are pretty common 600XL killers I read.

I know it isn't the GTIA, ANTIC, SALLY, OS ROM or POKEY as they have either tested ok in my 800XL with SuperSalt or been replaced with known working parts, same for all the other ICs on the board, mostly LS logic.

Also, the fact that the 600XL does give some sort of screen generally means that the display side is working. I get the same brown screen on RF and Composite.

I'm going to start scoping out my 800XL this week, and map out what a working machine shows on each pin compared to the dead one. The Service Manual and schematic are pretty damn poor :(

I also need to figure out the pinout of the BASIC ROM as the schematic shows a 28 pin IC, but there is a 24 pin IC fitted, and I'd like to see what's occurring on the CS lines.

All part of the fun and games of retroing!!

Well that's annoying. Very annoying.

I bought a Western Design Center W65C21S6TPG-14 PIA last weekend as a direct replacement for the R6520. It showed in eBay as due yesterday, but it's only just been posted today!!

So no more messing around with the 600XL just yet!!

I do dislike slow posting eBay sellers who give (for them) an impossible delivery date.

stephen_usher wrote: 19 Feb 2022 13:42 I do dislike slow posting eBay sellers who give (for them) an impossible delivery date.

Quite, and the slow posting is going to be reflected in the feedback!!

rubber_jonnie wrote: 29 Jan 2022 15:53 Unfortunately, the PIA is soldered in on both my 800XL & 65XE, so that's no good. It's a 6520 PIA, so available at least, though looking at the service manual I'm not convinced that it is in a position to break the machine in the way it does, neither the block diagram nor the schematic seem to show that it plays a major part other than I/O for things like the joysticks etc, and removing it makes no change either.

In the 800 (and 400) the PIA controls "only" one I/O signal and the game controllers. But on the XL/XE it control the memory banking, both RAM & ROM. I don't know if that's the problem in your case. But the PIA is definitely critical in newer computers.

Ok, so it seems that the Western Design Center W65C21S6TPG-14 (CMD / Rockwell) PIA is a direct replacement for the R6520 in the XL.

I wouldn't be so sure. Yes, it might be a pin compatible replacement. That doesn't mean it is exactly the same thing. You are replacing an NMOS part with a CMOS one. The specs and analog characteristics would certainly be very different. Possibly, and even probably if you want, it might not matter. After all it is a newer, faster and less power hungry part. But, at least conceivable, it might create problems in some situations.

ijor wrote: 20 Feb 2022 20:27
In the 800 (and 400) the PIA controls "only" one I/O signal and the game controllers. But on the XL/XE it control the memory banking, both RAM & ROM. I don't know if that's the problem in your case. But the PIA is definitely critical in newer computers.

Good to know, thanks, I'm still getting the hang of XL troubleshooting so any info is helpful. Unlike the service manual :lol:

ijor wrote: 20 Feb 2022 20:27
I wouldn't be so sure. Yes, it might be a pin compatible replacement. That doesn't mean it is exactly the same thing. You are replacing an NMOS part with a CMOS one. The specs and analog characteristics would certainly be very different. Possibly, and even probably if you want, it might not matter. After all it is a newer, faster and less power hungry part. But, at least conceivable, it might create problems in some situations.

Yeah, I am aware, thanks, but I couldn't find any reasonably priced 6520's at the time, so I thought it would be worth a punt based on the datasheet stating it was a replacement like for like. You never know I may get lucky :)

If not I'll just have to buy a normal 6520.

Annoyingly I have some BBC Bs and thought perhaps they had a 6520, but it turned out to be 6522s, so out of luck.

@rubber_jonnie I might have a couple of 6520s, so let me know if you need one.

PhilC wrote: 21 Feb 2022 12:51 @rubber_jonnie I might have a couple of 6520s, so let me know if you need one.

Ooooh, that sounds good! How many do you have? I'll definitely take one please, two if possible, nice to have spares in stock :)

I'll PM you, cheers.

rubber_jonnie wrote: 21 Feb 2022 13:04

PhilC wrote: 21 Feb 2022 12:51 @rubber_jonnie I might have a couple of 6520s, so let me know if you need one.

Ooooh, that sounds good! How many do you have? I'l definitely take one please, two if possible, nice to have spares in stock :)

I'll PM you, cheers.

Big thanks to @PhilC who is sending me 2 6520s :)