Fixed: A3000 black screen on power on

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Nutter
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Fixed: A3000 black screen on power on

Post by Nutter »

Hi all,

I am almost finished repairing an A3000 after some very severe battery leakage. I am 95% sure I've found all PCB damage and repaired it but I believe the RTC /CMOS chip is not working. I can measure 32.768kHz from the oscillator but nothing coming out of the PCF8583. I've ordered some from eBay (China) so no idea what I'll get really. Are there any good sources for this chip?

Anyway, I tried a power-on to see where I stood but get nothing on the screen. Just black, and no beeps from the speaker. I suppose I should say that I know very little about these computers so I'm relying on the service manual for guidance. That also means I could be making some elementary mistakes which I hope someone here can point out.

I have removed the RAM expansion and any other plug in cards, so I'm working with only the bare board and power connected. I plugged a cable into the composite port and I am comparing it to another working A3000 that I have here.

If you have the service manual handy, I have gotten to page 46 step 8 (checking the data bus). This is showing some missing signals when probing R50 - R65. However before I investigate further I thought I'd better stop and ask for help. I don't want to be going down a rabbit hole if someone can point me in the right direction.

The attached pics show D2 missing but it's actually installed now. I'm missing the cap near the keyboard connectors (C16 or C116).

Any help or advice appreciated. :wink:
Attachments
batt1.jpg
batt2.jpg
batt3.jpg
batt4.jpg
batt5.jpg
Last edited by Nutter on Thu Sep 27, 2018 10:26 pm, edited 3 times in total.
Nutter
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Re: A3000 black screen on power on

Post by Nutter »

After finding 2 broken traces on the address lines of the ROMs I am getting a blank "grey" screen. As I'm using composite it's a monochrome picture. I need to try to make up a proper lead to get a colour picture. I'll continue tomorrow evening maybe.
JonC
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Re: A3000 black screen on power on

Post by JonC »

Try plugging in the floppy drive. :D

The drive will flash diagnostic fault codes via LED which may help you track down issues.
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Nutter
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Re: A3000 black screen on power on

Post by Nutter »

No life from the drive at all. No sounds nor flashing lights. Not good I guess.
mattdy4
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Re: A3000 black screen on power on

Post by mattdy4 »

Have any spare ROMs to try?
Nutter
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Re: A3000 black screen on power on

Post by Nutter »

I have a working A3000 but I'm reluctant to use anything from it unless I can easily replace it. Regarding the ROMs, can I dump them using an EPROM reader and burn them to a blank EPROM? If so, I would try them in the faulty A3000.
Nutter
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Re: A3000 black screen on power on

Post by Nutter »

What type of EPROMs do I need to use when replacing the 4 ROMs? Will 1megabit (27C010) do ? I want to dump my good ROMs and burn replacements.

I moved the suspect ROMs from my known good A3000 into the other computer. That now fails to boot just like the other. I don't want to put the good ROMs into the faulty A3000 without first dumping them.
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IanS
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Re: A3000 black screen on power on

Post by IanS »

Nutter wrote:
Tue Sep 25, 2018 11:26 pm
What type of EPROMs do I need to use when replacing the 4 ROMs? Will 1megabit (27C010) do ? I want to dump my good ROMs and burn replacements.

I moved the suspect ROMs from my known good A3000 into the other computer. That now fails to boot just like the other. I don't want to put the good ROMs into the faulty A3000 without first dumping them.
Need to be 4Mbit for RISC OS 3.11, which can be downloaded from https://www.4corn.co.uk/archimedes.php no need to dump roms. (Be aware that the 4 files in the download don't go directly into 4 Eproms, they need to be interleaved)
27C4001 Should be what you need.
Nutter
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Re: A3000 black screen on power on

Post by Nutter »

Thanks IanS

I put my good ROMs into the faulty computer and I get some kind of a rolling display but it isn't sync'ed. It looks like a black screen with white writing (using composite). I think it might need booting holding Delete but the keyboard doesn't seem to be working properly. The power led comes on but none of the others. I'm guessing there might be still some PCB issues from the battery leak or with the 8051AH.

How do I go about interleaving those ROMs?

The investigating continues...
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1024MAK
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Re: A3000 black screen on power on

Post by 1024MAK »

You need a program that de-interleaves the data . They exist (the same problem exists on 680XX systems), but at the moment I can’t point you to one.

Mark
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IanS
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Re: A3000 black screen on power on

Post by IanS »

See here for details - viewtopic.php?f=16&t=15457&p=210127#p210109

Once you've concatenated the files into one big file, you may find your programmer has options for the interleaving when opening the file. (My wellon does)
Nutter
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Re: A3000 black screen on power on

Post by Nutter »

Thanks again guys. I'll look into that later I guess. Trying to get the display working first which (I think) means getting the keyboard working.

Is the 8051AH readily available in case it is faulty, or are they hens teeth?
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Re: A3000 black screen on power on

Post by JonC »

Might be worth trying to clean the keyboard connector first, probably just dirty?
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Nutter
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Re: A3000 black screen on power on

Post by Nutter »

They are new connectors. Not exactly the same but the cable fits

https://www.mouser.co.uk/ProductDetail/571-7-520355-0
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Re: A3000 black screen on power on

Post by JonC »

Do any of the keyboard lights work? Caps Lock etc? Do they flash on power up?

If not, it suggests there's an issue with the connection somewhere along the way. Maybe double check the traces from the connectors as they are pretty close to the horrible green gunk! :)
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Eirikur Blodax
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Re: A3000 black screen on power on

Post by Eirikur Blodax »

Hi 'Nutter',

When I revived my A5000 the keyboard didn't work either: the plastic membranes with contact tracks were stuck together in several places such that constant 'keypresses' were generated.
One test you can do immediately is disconnecting the keyboard completely and start up the computer - even with a non-sync'd display you'd be able to see it boot into the desktop (or not...).

Looks like the A3000 keyboard has a similar membrane (https://www.retro-kit.co.uk/page.cfm/co ... d-service/); it might be worth the effort to thoroughly clean it and unstick the membrane - took me less than an hour.

Regards, Erik
With kind regards, Erik
Nutter
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Re: Fixed: A3000 black screen on power on

Post by Nutter »

In my first post I said "I am 95% sure I've found all PCB damage and repaired it".

Well this was almost accurate. More like 70% sure.

I repaired some more tracks on the keyboard circuit and that allowed me to press DEL while booting. That sorted the rolling screen and booted into RISC OS. Then some more track repairs to get the mouse working and it seems like that's it done. I'll leave testing it till tomorrow.

Thank you to everyone who posted in this thread. Your input is very much appreciated.
Nutter
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Re: Fixed: A3000 black screen on power on

Post by Nutter »

This is how the underside looks. Like a rat's nest!
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1024MAK
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Re: Fixed: A3000 black screen on power on

Post by 1024MAK »

Nutter wrote:
Thu Sep 27, 2018 10:35 pm
This is how the underside looks. Like a rat's nest!
Looks rather tidy to me. In fact, looks better than what I manage (mainly because I use the nearest wire to hand, which is nearly always 0.63mm PVC insulated copper), and I've been bodging repairing and hacking modifying PCBs for years... :P

So well done =D>

Mark
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