A500+ green screen

General hardware issues and troubleshooting etc

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BennehBoy
Posts: 261
Joined: Sat Apr 24, 2021 12:35 pm

A500+ green screen

Post by BennehBoy »

I'm attempting to revive a Varta damaged A500+

So far I've repaired one broken trace, a faulty via, and replaced U12. All traces in the vacinity of the leak (which was very minor) tone out correctly according to Amiga PCB Explorer. I've no idea if there was a fault prior to the leak.

There's no output from the serial port with diagrom - think one of the line drivers may be dead, but I do get output from Paula that I have captured with an FTDI (see attached).

All the main chips on the board work fine in another A500+ except the even CIA (which I don't think should impact this).

Any suggestions on where to check next? I have a scope, logic analyser (16 channel), multimeter etc to hand - just not that familiar with Amiga troubleshooting.
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a500plus_diagrom.txt
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BennehBoy
Posts: 261
Joined: Sat Apr 24, 2021 12:35 pm

Re: A500+ green screen

Post by BennehBoy »

Some further info...

Stuck a logic probe on Gary and nothing is floating...

Logic probe on the chipram shows the address lines are always high.

Could the latch U34 be bad?

Agnus works in another machine.
BennehBoy
Posts: 261
Joined: Sat Apr 24, 2021 12:35 pm

Re: A500+ green screen

Post by BennehBoy »

Brain engaged somewhat... I guess this is me going down a rabbit hole since the ram is addressed via RAS & CAS....
BennehBoy
Posts: 261
Joined: Sat Apr 24, 2021 12:35 pm

Re: A500+ green screen

Post by BennehBoy »

U13 Pin 8 is stuck high - suspect that's failure due to the battery leak.
Danoo
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Location: Queensland, Australia

Re: A500+ green screen

Post by Danoo »

BennehBoy wrote: Tue Jul 09, 2024 10:27 pm
Agnus works in another machine.
A possible path if nothing else is solving the issue. What does the Agnus socket look like, had a green screen issue on a 500 a few years back. Agnus worked fine in other machines, but still a green screen when put back in the faulty machine. Cleaned the socket up nicely, still faulty, replaced the socket and the problem went away.
Life is really simple, but we insist on making it complicated - Confucius
BennehBoy
Posts: 261
Joined: Sat Apr 24, 2021 12:35 pm

Re: A500+ green screen

Post by BennehBoy »

Danoo wrote: Wed Jul 10, 2024 1:30 am A possible path if nothing else is solving the issue. What does the Agnus socket look like, had a green screen issue on a 500 a few years back. Agnus worked fine in other machines, but still a green screen when put back in the faulty machine. Cleaned the socket up nicely, still faulty, replaced the socket and the problem went away.
Hi,

Agnus socket is super clean.

I think I've found the issue now though, I pulled U13 and it fails test in my TL866. The PCB trace also now pulses at that pin, so I guess it was being held high by the failed chip. I'm waiting on some spare IC's to arrive before I can test further. Assuming this fixes the issue I then need to figure out why serial is not making it out of the Serial port although Paula is transmitting it correctly - probably an issue with the DS1488N's - U38/U39... (U38 specifically - continuity is good from Paula to it, and from the Serial port via the filter to it.)
BennehBoy
Posts: 261
Joined: Sat Apr 24, 2021 12:35 pm

Re: A500+ green screen

Post by BennehBoy »

Machine is booting now that U13 is replaced.
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JezC
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Re: A500+ green screen

Post by JezC »

BennehBoy wrote: Fri Jul 12, 2024 9:05 pm Machine is booting now that U13 is replaced.
:yay: :yay2: :dualthumbup: :cheer: :cheers:
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