A5000 Gone for good? :0

[nicf82]

Hi!

I have an A5000 here which I am going to have a go at fixing, managed to get my A4000 working again 100% due to this forum, so thought it might be fun! I have forgotten the frustration by now

This one powers on, but no beep and no flashing floppy drive LED. I have already pulled the old battery out, which did not appear to have leaked to me, and replaced with a 1.2V AA. I also gave the area a clean with 50/50 vinegar and water then IPA.

I have tried leaving it a while and a Del-power on but it has not made any difference.

Has anyone maybe got any pointers on what to look at first? I do have an oscilloscope to work with.

I was trying to find the board schematics also, but not able to find any?

I have attached a couple of photos of the 'worst' looking components, not sure if they are anything to investigate further?

Cheers!
Nic

IC42 a little bit green

C82 Not sure what this white stuff is?

The battery itself looked intact

[steve3000]

Oh, a 33MHz A5000 alpha, very very nice

Maybe a photo from a bit further back will help us see the PCB condition. And the schematics are all available, try some searches for A5000 TRM (technical ref. manual), you should find lots of A5000 repair threads on here too.

[nicf82]

Ah TRM! I will try that search now


Thanks Steve!
Nic

A photo of the entire board:

[iainfm]

Not sure about the alpha, but my non-alpha A5000 doesn't do much if I forget to refit the extra memory (without swapping back the appropriate jumper(s) anyway). I can't remember how little it does, but I'm fairly sure it's next to nothing.

Edit: Going by this I think the links look in the right positions for no additional ram board.

[nicf82]

Thanks iainfm, I will have a look into the jumper settings

[iainfm]

The tracks around the CPU look a little crusty...

[nicf82]

Yeah I did think that especially on one side, what is that I had assumed it was just an accumulation of dust! It’s not anywhere near the battery so surely not a leak?

[iainfm]

Yeah I did think that especially on one side, what is that I had assumed it was just an accumulation of dust! It’s not anywhere near the battery so surely not a leak?

Doesn't look like it's caused by a leak, which usually turns things a bluey-green.

Hopefully it's just dust!

[nicf82]

Here are a few more of the chip, and the jumpers.


I put the lid back on for the night but can get some better angles tomorrow!


Cheers
Nic

[IanJeffray]

Has anyone maybe got any pointers on what to look at first? I do have an oscilloscope to work with.

I was trying to find the board schematics also, but not able to find any?

Connect keyboard. Hold/wedge the resest button in. power up. Check that A2 is toggling fast (4Mhz or so). A3 will be toggling at half the speed and so on all the way up to A21 which will be just a few kHz. Probe these at both MEMC and at the ROMs (the other side of the address latches). If you find any that aren't toggling, or toggling at not half the rate of their neighbour, fix that first or there's no point going further.

The schematics are here : https://chrisacorns.computinghistory.or ... A5000.html (scroll to "technical drawings")

[nicf82]

Brilliant thanks Ian I will give that a go tomorrow!

[nicf82]

I was able to try this out, I calibrated by measuring FCLK at 33MHz and MCLK at 12MHz and got a good signal.

Then moving round to the address pins I was getting:

A2 - 2MHz
A3 - 1Mhz
A4 - 1MHz
A5 - 500KHz
A5 - 500KHz
A7 - 500KHz

Sometimes I was losing the signal all together on the address lines and had to reboot, other times I was getting wild readings or just good but different readings such as 1MHz on pins A5 and A6 - so pretty ambiguous!

Going to try to work out where the other ends are now and probe there too.

Edit: Same results at the MEMC, going to check the ROMs

Edit: Not seeing any continuity between address pins and the pins for the ROMs

Cheers!
Nic

[IanS]

Edit: Not seeing any continuity between address pins and the pins for the ROMs

The address bus is buffered/latched via IC36, IC45 & IC46, so there isn't direct continuity.

Did you find the TRM schematics - https://chrisacorns.computinghistory.or ... awings.zip

[IanJeffray]

Then moving round to the address pins I was getting:

A2 - 2MHz
A3 - 1Mhz
A4 - 1MHz
A5 - 500KHz
A5 - 500KHz
A7 - 500KHz

That looks more like code execution - are you sure you kept the reset button held in?

[nicf82]

Thanks Ians, that would explain it! I will have another looks over the diagrams!

IanJeffray, Yeah I did jam the button in, just the one on the back of the keyboard right? I will makes sure it is properly pressed and try it again



Edit: Yes, the way I jammed the button in with bluetac was not good enough, so it had come away I checked it again and all looks good on the CPU end now, Ill check the other locations

[nicf82]

OK I have not managed to check all the ROM address pins also, A2-18 and all appear to be getting the correct signal

I am starting to have a look in the area of the CMOS as it looks a bit crusty, I don't appear to have any continuity between R281 and IC58, which according to the schematic, I should!

[nicf82]

Progress report, I have made the following repair, to bridge R281 to pin 8 of IC58 and also C153 as it also had a break.

Still no beep when booting up but I am now getting a sequence of flashes on the FDD LED, seems it means CMOS unreadable. I assume the other codes are just status not errors.

Also no colored screens, no video at all. Not too sure where to go next but will try to find more info on CMOS unreadable code

Edit: Something else I have noticed - doing a continuity test on that little yellow capacitor C154 - the meter makes a 'blip', then nothing, If I reverse the polarity I get another 'blip', then nothing. It makes some sense to me as the capacitor presumable gets charged? But every other similar capacitor on the board gives a constant beep tone - what is normal for one for these? Is it something wrong with that 1 cap?

Cheers!
Nic

[IanJeffray]

Put a meter across pins 4 and 8 of the CMOS RAM chip. When the machine's on it should read 4.5V-5V.
If that checks out, next, with the machine OFF, check continuity of the two I2C pins on the CMOS RAM to the IOC chip - these often fail underneath the CMOS RAM chip itself, as that's where the vias are that lead to the underside of the board.

[nicf82]

Thanks Ian, just had a chance to take another look at this.

Machine on, Voltage across pins 4-8 is 4.3 volts. A bit low?

I do have continuity between pins 5 and 6 to the 2 pins on the IOC.

Also things have regressed slightly in that I am no longer getting the FDD LED flashing its diagnostic info.

One other thing that may be relevant, there are no signs of life on the keyboard LEDs either.

Cheers!
Nic

[IanJeffray]

Machine on, Voltage across pins 4-8 is 4.3 volts. A bit low?

It'll probably do.

I do have continuity between pins 5 and 6 to the 2 pins on the IOC.

On that evidence, I'd then have to suggest the CMOS RAM chip itself has died. I'd replace it.

Also things have regressed slightly in that I am no longer getting the FDD LED flashing its diagnostic info.

That could mean the POST thinks everything is ok! But..

One other thing that may be relevant, there are no signs of life on the keyboard LEDs either.

Do you mean you used to be able to hit keys and get the LEDs to toggle? (If so, RISC OS was running)
Or do you mean the keyboard LEDs used to flash briefly when you power on, and no longer do? (If so, it's simply not getting power)

[nicf82]

Ok let me see if I can find another chip.

The keyboard LEDs only ever flashed on power on, they never responded to caps/num lock and it’s still the case. Also never had had any video output. I’ve not bothered to try and check the VGA pins for any output yet but since there is not even a startup beep did not think it was worth checking!

I have just cleaned the whole board with vinegar then IPA in case it has any effect I’ll give it some time to dry.

Cheers!
Nic

[IanJeffray]

I have just cleaned the whole board with vinegar then IPA in case it has any effect I’ll give it some time to dry.

Oh no

[nicf82]

Oh is that not a good move? I read about somebody doing it and getting one working

Hopefully it hasn’t made anything worse!

[paulb]

Oh is that not a good move? I read about somebody doing it and getting one working

Hopefully it hasn’t made anything worse!

Start reading here for the most memorable discussion of this. It was an A5000 repair thread, too!

[nicf82]

Ah right, I did only give it a very light brush down with an anti static brush only just damp with 50/50 vinegar and water, then immediately sprayed down with contact cleaner. Do you think giving it a few more sprays might be a good idea to get rid of any stray acid that might be left? Or just stop messing with it!!!?

I haven't tried to power it on since last night.

Cheers!
Nic

[IanJeffray]

sprayed down with contact cleaner. Do you think giving it a few more sprays might be a good idea to get rid of any stray acid that might be left? Or just stop messing with it!!!?

Good lord. You said IPA before (not so bad), not contact cleaner (!!!). Stop it. Poor board.
Let it dry in a warm place for several days before you power up again.
(YMMV, IMVHO, etc)

[nicf82]

Oh Thought it was the same stuff TBH

Ill leave it alone for a while, while I try to find the right chip!

I have found these, so far: https://www.digikey.co.uk/en/products/d ... 518/735699, don't suppose you know if there are many variations of these?

Cheers!
Nic

[IanJeffray]

Ill leave it alone for a while, while I try to find the right chip!

I have found these, so far: https://www.digikey.co.uk/en/products/d ... 518/735699, don't suppose you know if there are many variations of these?

That's the puppy. PM me your address and I'll send you one

[nicf82]

Wow thanks Ian, much appreciated!

[philpem]

Ah right, I did only give it a very light brush down with an anti static brush only just damp with 50/50 vinegar and water, then immediately sprayed down with contact cleaner.

Wait and see what it looks like after a few days to dry - but most contact cleaners contain a lubricant to help keep switch contacts from oxidising. You might need to wash the board again with isopropyl to get the oily residue off...

It might also seal the moisture in... remember the old school science experiment where you put oil and water in a glass and stir it, and after a short while all the oil ends up floating on top of the water?