exxos's Atari, Amiga & retro forum

Please, I need help troubleshooting an Amiga 600 which gives me a black screen. Here's what I did so far:

• Recaped the whole motherboard. It had heavy corrosion damage, so some PCB tracks had to be rebuilt. I tested already all affected areas, there's continuity on every track I replaced, and I completely cleaned the corroded solder/tracks.

• U7 (odd CIA) was getting hotter than normal, so I removed it until I know whats happening. Since a missing odd CIA gives a light green screen on boot (at least on my Amiga 500), I removed it until I fix the black screen and it goes to light green.

• Halt pin on the 68000 goes high at power on, and then low about 1 second after. While HALT is high, I can see data on address/data lines on 68000 and KS ROM, but it all ceases when it goes low (obviously).

• Reset circuitry seems to be ok: pin 3 on 555 starts high and goes low after half a second.

Any sugestion about what to test next, please? It seems Gayle is halting the 68000, but I have no idea on what can cause that. Any help is greatly appreciated, thanks!

Eduardo

Get diagrom amd a serial cable.. otherwise you are guessing.

you have CSYNC on Agnus pin 80 and at pin 10 on the CXA1145 Encoder chip ?

I'd agree with @terriblefire, you need something like diagrom/serial output as without any concrete data to go on about what is failing, you are going to spend an age going round in circles.

HALT, like RESET on the 68000 is active low, however it is also bi-directional, in other words another device can pull HALT low and cease activity, or if the CPU gets garbage data, it can pull HALT low itself and stop processing.

What you will find a struggle to do is work out which case you have, therefore you need a good plan to capture the problem in flight and that includes accurate data about what the system is actually doing, hence diagrom etc.

Sorry, I forgot to say, CSYNC, HSYNC, VSYNC are all present, and with correct timings. So, there is video signal, but all black.

Eduardo L M wrote: 22 Apr 2021 21:53 Sorry, I forgot to say, CSYNC, HSYNC, VSYNC are all present, and with correct timings. So, there is video signal, but all black.

If a CIA is missing then it is probably not getting interrupts needed to boot.

Get DiagROM in there. Even with a full logic analyser hooked up to the bus i'd never attack a repair like this without DiagROM

Thanks for the tip, I'm waiting a DiagROM to arrive in the mail. And I also ordered an adapter for the programmer so I can burn diagroms by myself!

But to make it output to serial port, I'll have to resolder the CIA I removed! Or perhaps put the even CIA in the place of the odd one, since the one I removed was getting hot... I'll do that and report back, thanks a lot!

Just to add more info: I had a KS 1.2 in my parts box, so I tried it.

The HALT signal remains high after power on! And I get activity on address/data lines. Still black screen, tough.

I think it's safe to assume KS 2.x has code that may HALT everything if something goes wrong during early boot stages. The same seems not to occur on KS 1.2.

HALT happens on a double bus fault. If you have a data or address bus fault it could be entirely luck that one KS HALTs vs another.

Problem solved!!

For the record, I'll explain what was happening:

1) I didn't say because I thought it was not important, but when I recaped the amiga, I removed CXA1145! But studying the schematics, I noticed RGB goes thru it and then to the DB23 connector. So, CXA1145 is not for composite/RF only, A600 won't have image if it fails! I was testing with an A520 connected to the DB23 video output. So, I had sync signals, but no RGB!

2) When I put CXA1145 back, colors started to show: blue screen at first. Then I removed the KS ROM and tested it in another good amiga, and to my surprise, it was not a 2.x KS, it was 1.3! I changed the kickstart ROM with a known good 2.05 I had.

3) Machine tried to boot, I saw white and gray screens flashing, but black after that. I decided to resolder the odd CIA. Machine boots!! I can see the floppy animation!!

4) Now the even CIA, I soldered the CIA that overheated before. But bad news: floppy doesn't boot and CIA gets very hot soon after boot..

So, the good news is that I now have a booting A600! But the bad news is that I need to source a new CIA. By the way, does anyone have a spare one to sell??

Thank you very much guys!

Eduardo