Beeb behaving badly

[KenLowe]

Hi folks,

I've got a beeb that's behaving rather badly. The beeb has a few internal addons, specifically:

• IntegraB ROM / RAM expansion board
• VideoNula
• Retroclinic 1770 disc controller
• Retroclinic tube level shifter and associated Pi3A
• Retroclinic ZIF ROM socket - located in ashtray

Additionally, I see that the original crystals have been replaced with modern low profile ones.

I've currently got the RGB output wired through a standard SCART / HDMI box, but I've also had it wired through a RGBtoHDMI.

Everything works fine until the beeb's been on for a couple of hours. Then it starts to randomly crash / hang. In mode 7, some fairly serious noise also starts to appear on the screen. Basically a bunch pixels / sets of pixels randomly flashing up on the screen.

Firstly, looking at the random crash / hang issue
Unplugging the tube level shifter seemed to fix things, but eventually the beeb started to hang again. I've moved the IntegraB into another machine, and it's working fine there. The same issue occurs if I put a different IntegraB into this machine. Removing the IntegraB seemed to solve the issue, but I never left the machine running for too long in this configuration, so it's possible it might have started crashing if I had left it long enough.
PSU voltages seem to be steady throughout.
After much investigation, I decided to replace the SY6502A with a R6502AP, and that seems to have solved the issue. I've had the machine running on and off for a few days now, with this replacement CPU, and it's not been crashing at all.

I think this is the first machine I've seen with a SY6502A CPU, and it feels like a marginal timing issue I'm dealing with. Are there known timing issues with this CPU? Particularly when everything's warmed up? The CPU seems to work fine in one of my other beebs with an IntegraB.

Secondly, the Mode 7 noise
I'll post up a short video, once the beeb has warmed up sufficiently (might take a couple of hours). All other modes are fine, so I'm guessing this is either a 6MHz clock or SA5050 issue. I've checked IC40, and it does have the faster 74S00, so I'm assuming that's not the issue.
This has prompted me to revisit my 48MHz to 6Mhz & 8Mhz clock circuit, and I've scratched up a basic design. I'll post more about this circuit on that thread. However, in the short term, I'm also going to see if it's possible to get a Pi Pico to generate those signals for me. Has anyone ever used a Pico for this before?

Ahead of me posting up the video showing the noise, is there anything else that could be causing this and that I should look at?

Thanks
Ken.

[KenLowe]

Video of noise in Mode 7:

https://youtu.be/qc_LSVSe1Rc

[hoglet]

Was the old CPU a SY6502 (1MHz) or a SY6502A (2MHz)?

[maniacminer]

Ah, yeah, the Synertek 6502A is famous for thermal instability. Put a heatsink on it and the problem will go away. You can easily test it with a puff of freezer spray and watch the issue magically disappear. On the LA I saw the processor shoving in random, very short pulses into the data and address buses, obviously causing chaos with everything else. I'd like to see a new VIDPROC with a PiPico that would be v.cool.

[shifters74]

Thanks Ken,

that's useful to know about the CPU swap fixing the issue (you might want to add it to the fault finding index too for us newbies)

Useful to know a heatsink will help with it too!

cheers

Shifters

[KenLowe]

Was the old CPU a SY6502 (1MHz) or a SY6502A (2MHz)?

Ah, yes, it was a SY6502A. I'll update my post to reflect that...

[KenLowe]

Ah, yeah, the Synertek 6502A is famous for thermal instability. Put a heatsink on it and the problem will go away. You can easily test it with a puff of freezer spray and watch the issue magically disappear.

Thanks for confirming. I'm pretty sure I've got a suitable heatsink somewhere. I'll try that and see what difference it makes. And, yeah, I really need to get myself some freezer spray to help with these types of investigations.

On the LA I saw the processor shoving in random, very short pulses into the data and address buses, obviously causing chaos with everything else. I'd like to see a new VIDPROC with a PiPico that would be v.cool.

That's well beyond what I was planning to do. I'm just wanting to get a nice clean 6MHz and 8MHz clock!

[maniacminer]

For freezer spray get this https://www.rapidonline.com/plastikote- ... ml-96-8005 £3.50+VAT for a large can, it's a good deal.

I've got a SY6502A Beeb on the bench now that is exhibiting the opposite symptoms to yours. Mine improves as it warms up, it's never perfect though. Random corruption is present in most things, the intro to Opus Software's "Dogfight" scrolls a MODE 1 screen whilst playing music. This Beeb can't manage more than a few seconds from cold without going wild. Once it's thoroughly warmed up, popping the lid off and giving it a couple of short bursts of the freezer spray makes it, umm, freeze