A1200 repair guide. - Page 2

solarmon · 14 October 2019, 10:12

Quote:

Originally Posted by Marchie

Working!

You were right Solarmon - the reset line was being held down. There is a transistor at U49, which (apparently) holds the system in rest until the voltage stabilises, under which there is a resistor, once we removed it, the system works perfectly (I think R629).

So much fun bringing dead systems to life again.

It's a great feeling see that boot up screen, isn't it?!

Do you mean R629 or C629? If R629 failed I don't know why that would keep the reset line low since it is connected to VCC. However, if C629 failed open then, since it is connected to GND, then it would keep the reset line low.

Are you looking to replace the failed component?

Marchie · 15 October 2019, 04:50

Yeah sorry - C629.

Shorting pin1 of U49 got the machine to come to life which is what tipped us off. We removed U49, which didn't help, then tried removing the resistor underneath it and she came to life.

I suppose I'll replace it in time for thoroughness, but I'm continuing with the rebuild for the now. The guy who pulled it (who's an electrical engineer and knows Amigas) said I'm not risking harm to the board by leaving them off.

Now for the keyboard ...

What's the best way to fix dodgy keys (of which it has several)? Is there a way (other than trial and error) to know when you can just replace the plunger and when the membrane itself is damaged?

solarmon · 15 October 2019, 09:04

Quote:

Originally Posted by Marchie

Yeah sorry - C629.

Now for the keyboard ...

What's the best way to fix dodgy keys (of which it has several)? Is there a way (other than trial and error) to know when you can just replace the plunger and when the membrane itself is damaged?

You'll need to open the keyboard up to inspect the membrane and key plungers - they may need cleaning up from dust/grime, or they might be damaged.

Whatever you do, DO NOT USE ISOPROPYL ALCOHOL to clean the keyboard membrane, as it seems to be corrosive to it, as I found out the hard way!

In fact, don't use isopropyl alcohol to clean the keyboard altogether, as you risk it getting on to the membrane through any gaps.

Also, get a key caps puller to make it easier and less prone to damage.

Marchie · 17 October 2019, 23:57

Wow, quite a lot of bad keys, I'm wondering if it's less effort just to connect an A500 keyboard to the keyboard extension header?

Marchie · 19 November 2019, 00:00

Update: rust from the frame was all over the mebrane, when I tried to gently wipe it off with warm water the membrane delaminated: fail.

A500 keyboard connected to the keyboard test header works a treat though. (and no more crappy membrane ribbon cable to undo).

Marchie · 19 November 2019, 00:04

Just when I thought it was all working...

Audio is wrong - I'm getting at least some sound from both speakers, but not getting all the audio channels.

What's the best way to troubleshoot this? Open Protracker and identify exactly which channels are working or not? Or just jump straight into looking for bad caps around the audio circuit? (board has already been recapped so I'm guessing it's damage courtesy of whoever did it).

solarmon · 19 November 2019, 01:12

You can use Amiga Test Kit to check the audio channels.

But yes, use amigapcb.org and the schematics (https://www.amigawiki.org/dnl/schematics/A1200_R2.pdf) to check traces and components related to the audio channels and filters.

I would also inspect and clean the audio sockets. They have physical mixing but gets disconnected when you plug something in - so you can actually get both channels mixed going down on audio socket, if only one is plugged in - i.e you can use one audio cable but get both channels being mixed down it.

Marchie · 11 December 2019, 01:55

Ok, so the audio problem is kind of intermittent -

I was happily playing mods for a while there and thought all was well, but sadly no...

For example, while playing Golden Axe the music will drop-out and come back again on and off during the game. The strange thing is, when the music comes back, it starts at the beginning of the track again. Like a CD that gets halfway through the track and jumps back to the start again. It's not always the same channels that drop out either - sometimes I just lose half the music.

What kind of fault could be causing the music to restart from the beginning mid-game like this?

solarmon · 11 December 2019, 09:30

Are you playing these games using WHDLoad or using floppies or ADF (via a Gotek)?

If playing from floppies or ADF, make sure it is not an OCS/ECS game incompatibility with an AGA A1200 issue. You can try using Relokick or TUDE to degrade your A1200, or use the Early Boot Menu (hold both mouse buttons down on boot) to set the graphics mode to Original and also set Disable CPU caches.

Otherwise, I would recommend using Amiga Tesk Kit and/or getting a Diagrom to do a health check. Especially for CIA timers and IRQ interrupts. Sound and interrupts are both handled by Paula - so that could be the source of your issues.

Marchie · 15 December 2019, 23:38

Amiga Test Kit didn't reveal any issues - mem, CIAs, etc. all good. For sound test it just puts a sine wave out each channel (all good).

I get sound problems from adf but not when running the same game from WHDload so perhaps it's a timing issue? My other machine (which doesn't give any probs with adfs) just has fastRAM but this one has an 030, so that could well be it...

I'll try your suggestion of disabling CPU caches in early boot menu (really don't want to start pulling axcel in and out to test!)

I'm using relokick, not familiar with TUDE, what is it?

solarmon · 16 December 2019, 10:35

Quote:

Originally Posted by Marchie

Amiga Test Kit didn't reveal any issues - mem, CIAs, etc. all good. For sound test it just puts a sine wave out each channel (all good).

I get sound problems from adf but not when running the same game from WHDload so perhaps it's a timing issue? My other machine (which doesn't give any probs with adfs) just has fastRAM but this one has an 030, so that could well be it...

I'll try your suggestion of disabling CPU caches in early boot menu (really don't want to start pulling axcel in and out to test!)

I'm using relokick, not familiar with TUDE, what is it?

TUDE (The Ultimate Degrader and Enhancer) is just another degrader:

http://aminet.net/package/util/misc/TUDE

Yes, if you have an accelerator, check that it is not impacted by timing issues - here is a good resource regarding timing fixes:

http://www.ianstedman.co.uk/Amiga/am..._mobo_fix.html

Marchie · 16 December 2019, 13:07

Disable CPU caches has fixed it completely!

Is there any automatic way to disable CPU caches? (ie: can I build it into the boot sequence of disks that need it for example?)

Glad you suggested it - I would never have thought to trawl the boot option for a solution...

solarmon · 16 December 2019, 13:33

Glad that option worked for you!

This is actually a common issue and resolution on an A1200 (with an 680EC20 processor) for floppy/ADF based games (although, this trick may not work for all games). Games that were developed before the A1200 were designed for the 68000 CPU. Caching in the 680020 and above CPUs just messes with the graphics because of it.

The WHDLoad versions of the games mainly has the fixes for such issues. Some games may have had official new releases/versions that supported the A1200, and possibly some cracked games may have had these fixes too.

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15 October 2019, 04:50	#22
Marchie Registered User Join Date: Jul 2016 Location: Sydney / London Posts: 589	Yeah sorry - C629. Shorting pin1 of U49 got the machine to come to life which is what tipped us off. We removed U49, which didn't help, then tried removing the resistor underneath it and she came to life. I suppose I'll replace it in time for thoroughness, but I'm continuing with the rebuild for the now. The guy who pulled it (who's an electrical engineer and knows Amigas) said I'm not risking harm to the board by leaving them off. Now for the keyboard ... What's the best way to fix dodgy keys (of which it has several)? Is there a way (other than trial and error) to know when you can just replace the plunger and when the membrane itself is damaged?

17 October 2019, 23:57	#24
Marchie Registered User Join Date: Jul 2016 Location: Sydney / London Posts: 589	Wow, quite a lot of bad keys, I'm wondering if it's less effort just to connect an A500 keyboard to the keyboard extension header?

19 November 2019, 00:00	#25
Marchie Registered User Join Date: Jul 2016 Location: Sydney / London Posts: 589	Update: rust from the frame was all over the mebrane, when I tried to gently wipe it off with warm water the membrane delaminated: fail. A500 keyboard connected to the keyboard test header works a treat though. (and no more crappy membrane ribbon cable to undo).

19 November 2019, 00:04	#26
Marchie Registered User Join Date: Jul 2016 Location: Sydney / London Posts: 589	Just when I thought it was all working... Audio is wrong - I'm getting at least some sound from both speakers, but not getting all the audio channels. What's the best way to troubleshoot this? Open Protracker and identify exactly which channels are working or not? Or just jump straight into looking for bad caps around the audio circuit? (board has already been recapped so I'm guessing it's damage courtesy of whoever did it).

19 November 2019, 01:12	#27
solarmon Registered User Join Date: Dec 2018 Location: UK Posts: 1,716	You can use Amiga Test Kit to check the audio channels. But yes, use amigapcb.org and the schematics (https://www.amigawiki.org/dnl/schematics/A1200_R2.pdf) to check traces and components related to the audio channels and filters. I would also inspect and clean the audio sockets. They have physical mixing but gets disconnected when you plug something in - so you can actually get both channels mixed going down on audio socket, if only one is plugged in - i.e you can use one audio cable but get both channels being mixed down it.

11 December 2019, 01:55	#28
Marchie Registered User Join Date: Jul 2016 Location: Sydney / London Posts: 589	Ok, so the audio problem is kind of intermittent - I was happily playing mods for a while there and thought all was well, but sadly no... For example, while playing Golden Axe the music will drop-out and come back again on and off during the game. The strange thing is, when the music comes back, it starts at the beginning of the track again. Like a CD that gets halfway through the track and jumps back to the start again. It's not always the same channels that drop out either - sometimes I just lose half the music. What kind of fault could be causing the music to restart from the beginning mid-game like this?

11 December 2019, 09:30	#29
solarmon Registered User Join Date: Dec 2018 Location: UK Posts: 1,716	Are you playing these games using WHDLoad or using floppies or ADF (via a Gotek)? If playing from floppies or ADF, make sure it is not an OCS/ECS game incompatibility with an AGA A1200 issue. You can try using Relokick or TUDE to degrade your A1200, or use the Early Boot Menu (hold both mouse buttons down on boot) to set the graphics mode to Original and also set Disable CPU caches. Otherwise, I would recommend using Amiga Tesk Kit and/or getting a Diagrom to do a health check. Especially for CIA timers and IRQ interrupts. Sound and interrupts are both handled by Paula - so that could be the source of your issues.

15 December 2019, 23:38	#30
Marchie Registered User Join Date: Jul 2016 Location: Sydney / London Posts: 589	Amiga Test Kit didn't reveal any issues - mem, CIAs, etc. all good. For sound test it just puts a sine wave out each channel (all good). I get sound problems from adf but not when running the same game from WHDload so perhaps it's a timing issue? My other machine (which doesn't give any probs with adfs) just has fastRAM but this one has an 030, so that could well be it... I'll try your suggestion of disabling CPU caches in early boot menu (really don't want to start pulling axcel in and out to test!) I'm using relokick, not familiar with TUDE, what is it?

16 December 2019, 13:07	#32
Marchie Registered User Join Date: Jul 2016 Location: Sydney / London Posts: 589	Disable CPU caches has fixed it completely! Is there any automatic way to disable CPU caches? (ie: can I build it into the boot sequence of disks that need it for example?) Glad you suggested it - I would never have thought to trawl the boot option for a solution...

16 December 2019, 13:33	#33
solarmon Registered User Join Date: Dec 2018 Location: UK Posts: 1,716	Glad that option worked for you! This is actually a common issue and resolution on an A1200 (with an 680EC20 processor) for floppy/ADF based games (although, this trick may not work for all games). Games that were developed before the A1200 were designed for the 68000 CPU. Caching in the 680020 and above CPUs just messes with the graphics because of it. The WHDLoad versions of the games mainly has the fixes for such issues. Some games may have had official new releases/versions that supported the A1200, and possibly some cracked games may have had these fixes too.

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