[CPC 464] Weird sync failure after 10 minutes of running

baralgin1003 · 06:19, 26 June 23

Hello! My 464 is acting weird. In BASIC, it can work for hours, but when you start Exolon, the screen disappears very soon. After the picture disappears, the computer is still working, you can press the keys and, judging by the sounds, the gameplay continues. Personally, I have not yet encountered this in other games, but the previous owner said that this behavior happens in many games. I looked with an oscilloscope, in the synchronization signal for RGB (from GA), VSync disappears at this moment. Hsync and Vsync from CRTC comes correct. The voltage on the logic board is 5v. What could be wrong? This behavior appears after 10-15 minutes after a cold start, and after the start, upon reset and the next boot, the screen disappears already on the 2nd Exolon game screen. I suspect it's a software bug since I've only seen it with Exolon, but in this case it's odd that the previous owner was talking about it happening more often. I don't want to think that the problem is in GA, because after touching GND to the RST Z80 pin, the computer is reset to BASIC and synchronization (with the picture) appears as after a cold start. I ran demos and did not observe a similar problem.

McArti0 · 22:54, 26 June 23

Exolon on the boot screen runs on one non-returning interrupt and disabled interrupts.

Check that after writing CALL &BFFC,&76F3 the cpc will freeze and the image will disappear.

The screen should remain.

Check that after writing POKE &B93A,&76 the cpc will freeze and the image will disappear.

The screen should remain too.

pelrun · 01:46, 27 June 23

The very first thing I'd check for would be a dry joint on either the CRTC Vsync output pin or the GA vsync input, or a tiny crack in it's trace. Are you probing the signal directly at the GA pin, or just back at the CRTC?

McArti0 · 08:03, 27 June 23

Quote from: pelrun on 01:46, 27 June 23... a tiny crack in it's trace. ...

or socket GA ...

baralgin1003 · 09:02, 27 June 23

Quote from: McArti0 on 22:54, 26 June 23after writing CALL &BFFC,&76F3 the cpc will freeze and the image will disappear.
after writing POKE &B93A,&76 the cpc will freeze and the image will disappear.
The screen should remain too.

in both cases, the CPC freezes and the picture on the screen is present. behavior is the same

Quote from: pelrun on 01:46, 27 June 23The very first thing I'd check for would be a dry joint on either the CRTC Vsync output pin or the GA vsync input, or a tiny crack in it's trace. Are you probing the signal directly at the GA pin, or just back at the CRTC?

at the time of the problem, vsync disappears only from SYNC signal at GA output (only hsync with a frequency of 15 kHz remains in it), at the input (14, 15 pins) vsync and hsync are present. Everything should be fine with the contact, I have already replaced the GA socket and soldered the CRTC pins

I can also exclude overheating of GA, for the duration of the tests I installed a large heatsink on it. GA temperature around 30-40 degrees Celsius

Found another game where the problem repeats - Baby YO: https://www.cpc-power.com/index.php?page=detail&num=341 is there any possibility that these games use the same screen mode in any way?
I also noticed oddities: in the games Baba`s palace and Bitume there is noise instead of music
I'm sorry if my questions are stupid, I'm a new cpc owner

McArti0 · 10:40, 27 June 23

Check PIN10 GA, INT signal 300Hz.

Does it disappear exactly when the image disappears?

baralgin1003 · 10:47, 27 June 23

in BASIC and when reading from disk, this signal is present, mainly at 1. When Exolon loaded it changes to 0 and remains in that state during the entire game. At the moment the image disappears, it is also at 0

McArti0 · 11:15, 27 June 23

After POKE &B93A,&76 too?

baralgin1003 · 11:19, 27 June 23

Yes, after POKEY INT is false

McArti0 · 17:47, 27 June 23

In BABY YO interrupts permanently disabled. Mayby this is clue? Mayby no.

Try this setting (same like Exolon)

1 ink 0,0:border 0
10 ra=&bc00:da=&bd00
20 data &3F, &20, &2B, &8E, &26, &00, &18, &1D, &00, &07
30 for r=0 to 9
40 read d:out ra,r : out da,d
50 next

save

and maybe POKE &B93A,&76

baralgin1003 · 20:11, 27 June 23

I launched exolon and waited until the picture disappeared. Then I compiled and downloaded your program. Here is the result of her work (click to enlarge). The computer did not freeze and the picture did not disappear

After POKE &B93A,&76 computer freezes without the disappearance of the picture

baralgin1003 · 20:18, 27 June 23

Quote from: McArti0 on 17:47, 27 June 23In BABY YO interrupts permanently disabled. Mayby this is clue?

Judging by the fact that in Exolon the INT signal is also always at 0, this may also be disabled.

baralgin1003 · 13:34, 30 June 23

Is there a possibility of a malfunction of GA?

McArti0 · 06:41, 01 July 23

Unfortunately yes.
We have not been able to reproduce the error in any other way, but it is very likely that GA is corrupted.

baralgin1003 · 11:06, 01 July 23

Thanks for your help. I ordered a new GA and I hope it will arrive to me in a month. I'll definitely let you know the result.

baralgin1003 · 12:40, 29 August 23

hi, I'm with a report. GA replacement helped, the computer works properly

Bryce · 13:00, 29 August 23

Does your GA have a heatsink? If not, that maybe the reason for the crashes.

Bryce.

baralgin1003 · 13:05, 29 August 23

Yes, it has a factory heatsink. before me, even the thermal paste someone changed. I installed a new GA and it feels even less heated.

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[CPC 464] Weird sync failure after 10 minutes of running