800XL loads software OK on SIO2PC, yet Multicarts glitch... Any thoughts?

[Beeblebrox]

I acquired a PAL 800XL. Taiwanese made so majority not socketed. The gtia, antic, cpu and Pokey were socketed,presumably by a previous owner or perhaps at the factory. 

 

I hooked up chroma for S-Video colour which was fine.  Boots to blue ready prompt no problem. Self test memory test was OK. Keys all register including the console keys. 

 

However I soon found with Uno cart plugged in and also Side3 it just didn't like them, with the following issues:

 

Uno cart in the cart bay sees a black screen only on boot up. 

 

Side3 in loader mode sees the same. 

 

Side3 switched to SpartaDosX mode gets interesting, with initially a blue screen but no polling and random ascii chars on screen and a range of on screen glitching. See screenshots at the end of this post. (I am typing this on my phone so not easy to place images where I want to). 

 

Having had past experience with fixing these things and thinking it might be one of the main 40pin chips I first removed and tested the gtia, antic, cpu and pokey in another machine one at a time. All worked.

 

Next I socketed the 74ls08 and replaced it with a 74f08. No change. 

 

I then socketed all the ram and also tested all the Samsung branded chips whilst out in a ram tester.. All were OK. I also put mostek ram chips in. Same behavior. No issue without a multcart but glitching with them. 

 

Incidentally using a Star Raiders cart is fine. (I know that bypasses the likes of os rom, etc etc so I wasn't surprised it worked.) 

 

I then socketed and replaced the OS rom and MMU. No change. 

 

I then proceeded to socket the basic, pia, 74ls138, and 375 chips, testing them each time and putting them in another XL. All worked in the other XL. 

 

Next I swapped in my U1MB from another machine. With no carts plugged in I booted to SpartaDosX OK, but in bios it is totally unstable and screen glitches and freezes. 

 

I removed u1mb and popped the os rom and MMU back. 

 

Here's the weird thing. Oddly on the stock setup (ie no u1mb or carts present) if I load xexs from SIO2PC on respect I have no issues and software runs fine.  So this presumably proves the main 40pin ICs are OK and also their sockets would also presumably be OK? 

 

If however with u1mb in place if I try to load uflash software to flash the U1MB firmware, it starts to load the firmware to memory but freezes. 

 

So at this stage I am left with the following possible causes:

 

-There are some 74### logic chips somehow causing issues when using carts and/or u1mb. 

-the same assumption but some passive components are to blame (which I think is unlikely). 

 

Incidentally I've used the original brick psu, a USB power cable and also a modern replacement psu. No difference. 

 

The fact that with uno cart and side3 in the cart bay, or u1mb installed with no carts plugged in all glitch, but when using SIO2PC by itself the system is stable I would assume should tell us something. 

 

Anyone have any ideas? 

 

I don't think there is any point removing the old double wipe sockets for the gtia, antic, cpu and Pokey  to replace them with new sockets. Also, before I go ahead and socket and replace all the other 74### logic chips I thought I'd see what others thought was a likely cause for all this. 

 

Bottom line:

 

-It seems with a stock setup (ie no u1mb and no carts plugged in) and just using SIO2PC to load software there are no issues, no graphical glitches or system crashing/freezing. 

 

-With u1mb installed and no carts plugged in, the system is unstable in bios and also if I try to load the likes of uflash. 

 

-With no u1mb installed and either uno cart or side3 cart plugged in nothing works or, as in the case of side3 in SDX mode, there is loads of on screen glitching, freezing, etc. 

 

-and yes, to clarify I have the old candle bios installed on the U1MB. Not had a chance to update it. 

 

 

Edited March 17, 2023 by Beeblebrox

[_The Doctor__]

make sure cartridge port leafs are not damaged and they are soldered nicely, clean, check under the cart stabilizer/tab assy for junk etc.

make sure the MMU and delay line are fine and use the better SHARP branded PAL MMU chip.

make sure PIA is good

the timing could be off so make sure capacitor in the crystal oscillator and or freddy(800xlf etc) areas are good.

make sure no passive components are cracked, shorted, open etc. 

sometime CPU is marginal and will work elsewhere but not in a specific machine, so no mexico cpu's

Edited March 17, 2023 by _The Doctor__

[Beeblebrox]

Quick few points of further clarification

 

So I'd already tried 2 other MMU chips, a different pia and different cpu as well as a new delay line. Same behavior. 

 

No Freddie as this isn't an xlf

 

Again with SIO2PC (with no U1MB or carts) it's all running fine.  So assume all 40pin ICs and crystals etc are OK. Issues only exibited on stock set up with mutil carts or when u1mb is in place. 

 

I'll check the following tmrw

 

The cart socket 

Passisve components, traces and vias for cracks, leaks, etc 

 

If stable performance on SIO2PC wasn't present then I'd understand. But because it works OK with SIO2PC it's less clear. 

 

Thanks 

Edited March 17, 2023 by Beeblebrox

[_The Doctor__]

No you are falling prey to execution vs timing differences, sio2pc has nothing to do with cartridge timing, it's not less clear. You have a timing issue or connection issue with the cartridge port, that leaves capacitance, resistance, and actual timing edges. This is why I look to the parts mentioned with and emphasis on timing circuit. You can swap components till the cows come home, but you won't have a cartridge act properly if timing is off. It is not like standard memory and execution in that sense. So I look to oscillator, frequency matters may be at play so checking the phases is at hand. checking that normally yields a bad cap, broken component or failing crystal if all other avenues have been exhausted. The only other issue at play would be the reset circuit and that's basically the cd4050, another chip, and a cap to check.

 

Loading via SIO has nothing to do with your cartridge issue. This is evidenced by your stating that software loads and runs, but cartridges do not.

I've seen this before so that's why I am giving said guidance.

Edited March 17, 2023 by _The Doctor__

[Beeblebrox]

@_The Doctor__Thanks, and this presumably would tie in with u1mb instability on the same machine, where I don't have any carts plugged in but u1mb bios glitches as per screenshot? 

 

I'll focus on the areas you mention and report back. 

[HiassofT]

Give the "stabilize XL" mod a try, the simple one-wire connection from pin 4 to pin 13 on the LS08 is easy enough to do

http://nleaudio.com/css/stabiliz.htm

 

It sounds a bit like you are suffering from timing issues and swapping out the ls08 for a f08 will only help a little bit - the "stabilizing" mod will shift PHI2 timing a lot more.

 

so long,

 

Hias