Trying to repair rev 6A Amiga 500...

Bren McGuire · 04 June 2020, 06:04

I got hold of a relative's A500, a rev 6A board, but I am having trouble understanding what's wrong with it.

The machine boots to a grey screen and stops there forever.
Things I have done:
- Tested most of the custom chips on a good known working A500 motherboard, they all worked. I didn't test CPU, Gary or Agnus, though.
- Fixed two resistors, E502 and E503, which were literally blown off, seems like someone did something stupid with the serial port (and also why I suspected the CIAs)
- Reseated all that could be reseated.
- Using a known-working A500 PSU, too.

At this point I am not sure where else to look. What else could be faulty besides those two resistors? Amiga CB Explorer only has a rev 8 board, and the numbers are different so I cannot really trace where this is going.
Any help appreciated.

solarmon · 04 June 2020, 08:29

You need to test and eliminate the CPU, Gary and Agnus.

What is the behaviour of the Power LED? Does it change brightness on boot?

Are you able to perform a keyboard reset when on the grey screen? Check the reset signals.

Does the floppy disk drive do anything and make the usual click/clack sounds?

Daedalus · 04 June 2020, 10:02

My suspicion is Paula. To go through the normal boot process but hang on a grey screen without giving a guru or a different colour just before the insert floppy screen would appear means the OS is partially running, but waiting on something like an interrupt. If Paula is partially working but some traces are damaged or parts of the power supply circuitry are damaged, Paula won't be able to handle interrupts correctly, and the OS will sit waiting forever at a grey screen. I've seen it on a couple of A600s, so that's where I would start.

Bren McGuire · 05 June 2020, 15:57

Mine was too after the CIAs, but as I wrote above, I tested Paula on another A500, and it worked just fine. So where do I go from here?

Quote:

Originally Posted by Daedalus

My suspicion is Paula.

Ugh, I hate pulling chips, but I just might. No, the LED did not change in brightness.
I am testing a bare board, so I don't have a keyboard or floppy connected, but I can try.

Quote:

Originally Posted by solarmon

You need to test and eliminate the CPU, Gary and Agnus.
What is the behaviour of the Power LED? Does it change brightness on boot?
Are you able to perform a keyboard reset when on the grey screen? Check the reset signals.
Does the floppy disk drive do anything and make the usual click/clack sounds?

Bren McGuire · 07 June 2020, 17:46

Hi Daedalus, could you help me out here mate?

solarmon · 07 June 2020, 18:59

Have you checked the interrupt signals and traces?

See my logical diagram at:

https://sites.google.com/one-n.co.uk...h.x1x6lbtkpzya

and use the schematcis and board layout at:

https://www.amigawiki.org/doku.php?i...ice:schematics
https://www.amigapcb.org/

Bren McGuire · 08 June 2020, 18:34

Thank you I will do that. Just checking for continuity, right? Or are there components between those lines?

Quote:

Originally Posted by solarmon

Have you checked the interrupt signals and traces?

See my logical diagram at:

https://sites.google.com/one-n.co.uk...h.x1x6lbtkpzya

solarmon · 08 June 2020, 19:48

Checking for continuity of those traces is the first thing to do. Then test that each is not shorted to anything else - i.e. to the pin/trace next to it; or to ground or to VCC.

There are some signals that are pulled up to VCC via pull up resistors - so those need to be checked that they have not shorted or have gone out of spec.

Bren McGuire · 11 June 2020, 19:23

This has become quite the headache, i thank your help a lot! Will see when I find some time to do this probing.
Your diagram is very helpful. I printed it along with a copy of the rev6 schematics

Quote:

Originally Posted by solarmon

Checking for continuity of those traces is the first thing to do. Then test that each is not shorted to anything else - i.e. to the pin/trace next to it; or to ground or to VCC.

There are some signals that are pulled up to VCC via pull up resistors - so those need to be checked that they have not shorted or have gone out of spec.

Bren McGuire · 28 August 2020, 22:36

This is breaking my head, maybe I don't have the right tools? I can't find what's wrong with my Amiga.
I have just burned a DiagROM and it gets stuck with ab lack screen after doing some RAM checks successfully. I don't know where else to go.

robinsonb5 · 28 August 2020, 22:58

Can you get some serial output from DiagROM?

04 June 2020, 06:04	#1
Bren McGuire Registered User Join Date: Nov 2019 Location: Croydon Posts: 594	Trying to repair rev 6A Amiga 500... I got hold of a relative's A500, a rev 6A board, but I am having trouble understanding what's wrong with it. The machine boots to a grey screen and stops there forever. Things I have done: - Tested most of the custom chips on a good known working A500 motherboard, they all worked. I didn't test CPU, Gary or Agnus, though. - Fixed two resistors, E502 and E503, which were literally blown off, seems like someone did something stupid with the serial port (and also why I suspected the CIAs) - Reseated all that could be reseated. - Using a known-working A500 PSU, too. At this point I am not sure where else to look. What else could be faulty besides those two resistors? Amiga CB Explorer only has a rev 8 board, and the numbers are different so I cannot really trace where this is going. Any help appreciated.

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04 June 2020, 08:29	#2
solarmon Registered User Join Date: Dec 2018 Location: UK Posts: 1,716	You need to test and eliminate the CPU, Gary and Agnus. What is the behaviour of the Power LED? Does it change brightness on boot? Are you able to perform a keyboard reset when on the grey screen? Check the reset signals. Does the floppy disk drive do anything and make the usual click/clack sounds?

04 June 2020, 10:02	#3
Daedalus Registered User Join Date: Jun 2009 Location: Dublin, then Glasgow Posts: 6,377	My suspicion is Paula. To go through the normal boot process but hang on a grey screen without giving a guru or a different colour just before the insert floppy screen would appear means the OS is partially running, but waiting on something like an interrupt. If Paula is partially working but some traces are damaged or parts of the power supply circuitry are damaged, Paula won't be able to handle interrupts correctly, and the OS will sit waiting forever at a grey screen. I've seen it on a couple of A600s, so that's where I would start.

07 June 2020, 17:46	#5
Bren McGuire Registered User Join Date: Nov 2019 Location: Croydon Posts: 594	Hi Daedalus, could you help me out here mate?

07 June 2020, 18:59	#6
solarmon Registered User Join Date: Dec 2018 Location: UK Posts: 1,716	Have you checked the interrupt signals and traces? See my logical diagram at: https://sites.google.com/one-n.co.uk...h.x1x6lbtkpzya and use the schematcis and board layout at: https://www.amigawiki.org/doku.php?i...ice:schematics https://www.amigapcb.org/

08 June 2020, 19:48	#8
solarmon Registered User Join Date: Dec 2018 Location: UK Posts: 1,716	Checking for continuity of those traces is the first thing to do. Then test that each is not shorted to anything else - i.e. to the pin/trace next to it; or to ground or to VCC. There are some signals that are pulled up to VCC via pull up resistors - so those need to be checked that they have not shorted or have gone out of spec.

28 August 2020, 22:36	#10
Bren McGuire Registered User Join Date: Nov 2019 Location: Croydon Posts: 594	This is breaking my head, maybe I don't have the right tools? I can't find what's wrong with my Amiga. I have just burned a DiagROM and it gets stuck with ab lack screen after doing some RAM checks successfully. I don't know where else to go.

28 August 2020, 22:58	#11
robinsonb5 Registered User Join Date: Mar 2012 Location: Norfolk, UK Posts: 1,157	Can you get some serial output from DiagROM?

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