Acorn A500 repairs

[razz177]

Just stripped this down again, when cleaning motherboard a resistor (I think) came away from it's solder points maybe part of my problem then a leg on another seemed loose , (see picture) will it do any damage to start computer without these attached ?

[flaxcottage]

That is a horrible after market modification! It makes me quite queasy just to look at it.

I cannot see how it would hurt the A5000 to remove those horrid black resistors. 70R in parallel with 220R gives about 50R. Why do it?

[wiggy]

That's some pretty manky-looking soldering, too...

Just hypothesising from there being three of them (and this having an RGB-missing-a-colour problem), Is this trying to botch original TTL video levels to scart levels? And also (from the non-shinyness of the foreground one that isn't there anymore) why it's pink... ?

You might have to work out what the original cct was, then figure out what the mod was trying to do... but it the hypothesis is correct then the computer should be fine without the bits (the monitor might disagree, though).

Oh, and you might want to stick a bit of opaque tape over the EPROMs' windows while you have it apart...

[razz177]

That's some pretty manky-looking soldering, too...

Just hypothesising from there being three of them (and this having an RGB-missing-a-colour problem), Is this trying to botch original TTL video levels to scart levels? And also (from the non-shinyness of the foreground one that isn't there anymore) why it's pink... ?

You might have to work out what the original cct was, then figure out what the mod was trying to do... but it the hypothesis is correct then the computer should be fine without the bits (the monitor might disagree, though).

Oh, and you might want to stick a bit of opaque tape over the EPROMs' windows while you have it apart...

when I was desoldering the one that was still intact I thought umm three of them and three colours I wonder, I did think about the tame on the roms but I havebeen tided up with the colour problem, I will look for some tape when I have reassembled it.
Thanks

ps as an ex fuel engineer I am sure I can solder better than that but we will see, the state of the soldering makes me think not done by acorn.

[razz177]

That is a horrible after market modification! It makes me quite queasy just to look at it.

I cannot see how it would hurt the A5000 to remove those horrid black resistors. 70R in parallel with 220R gives about 50R. Why do it?

I have no Idea but i'm no electronics engineer, If the computer is safe to try without them then I have nothing to lose, I have the computer linked to an acorn ak11 at the moment.
Thanks

[wiggy]

I did think about the tame on the roms but I havebeen tided up with the colour problem, I will look for some tape when I have reassembled it.

Technically, once the lid's back on, it'll be fine without the tape - the lid keeps any UV light out. It's while you have the thing in pieces while you are working on it that the risk of being exposed to light exists!

Black insulting tape should be fine.

[razz177]

well just held my breath and restarted, still pink screen but not as bad the green acorn is actually green and not black.
now going to cover roms

[steve3000]

That is a horrible after market modification! It makes me quite queasy just to look at it.

I cannot see how it would hurt the A5000 to remove those horrid black resistors. 70R in parallel with 220R gives about 50R. Why do it?

Just be really careful, this is not an A5000. This is an A500, which is a prototype Archimedes, without an available circuit diagram, and the few remaining A500s all have lots of aftermarket patch-ups to fix production faults/issues and trial improvements.

Don't make any changes without documenting and photographing carefully exactly what you change, or you could end up with a dead A500 and little hope of restoring it.

[razz177]

That is a horrible after market modification! It makes me quite queasy just to look at it.

I cannot see how it would hurt the A5000 to remove those horrid black resistors. 70R in parallel with 220R gives about 50R. Why do it?

Just be really careful, this is not an A5000. This is an A500, which is a prototype Archimedes, without an available circuit diagram, and the few remaining A500s all have lots of aftermarket patch-ups to fix production faults/issues and trial improvements.

Don't make any changes without documenting and photographing carefully exactly what you change, or you could end up with a dead A500 and little hope of restoring it.

Thanks
Took pictures & documented where they come from, still got a picture , everything seems to be working apart from the picture being pink where it should be white, I am thinking of soldering them back in as they must have been there for some reason , maybe the picture was pink before due to the bad soldering, If they were doing nothing then you would think the person who fitted them would remove them.

I am using an AK11 monitor with this.
I still think it is the RGB lead that is the problem I cannot find a proper pin out for the scart lead, the scart lead I have was an absolute mess (see pic)
as you can see the white wire is not even connected, it goes to the center pin (6) in the DIN plug, I have checked continuity on all the wires & they are OK, Does it need power to the scart socket ?
I have only put the connections from the DIN /SCART lead back in so if I get a good picture I can resolder to the good pins that are left.

Thanks for help and comments

[steve3000]

Thanks
Took pictures & documented where they come from, still got a picture , everything seems to be working apart from the picture being pink where it should be white, I am thinking of soldering them back in as they must have been there for some reason , maybe the picture was pink before due to the bad soldering, If they were doing nothing then you would think the person who fitted them would remove them.

Possibly the person who fitted them was just trying to tweak the RGB levels for a specific monitor. However, there is a bunch of other patches that you have near the VIDC, which may suggest these were part of a wider upgrade/modification. If you want to get to the bottom of this, you could trace the circuit from the VIDC RGB pins, through to the RGB socket at the back, and compare this with the same for an A310 and A410/1 PCB, which were the successors of the A500.

If you are going to reinstate the two 70R resistors, I’d suggest shortening the legs as much as possible and using heat-shrink to cover any remaining exposed legs. Also find a third 70R resistor to fit to the last of the three PCB resistors in your photo, since it looks likely that something was soldered to the lower resistor in the past.

I am using an AK11 monitor with this.
I still think it is the RGB lead that is the problem I cannot find a proper pin out for the scart lead, the scart lead I have was an absolute mess (see pic)
as you can see the white wire is not even connected, it goes to the center pin (6) in the DIN plug, I have checked continuity on all the wires & they are OK, Does it need power to the scart socket ?
I have only put the connections from the DIN /SCART lead back in so if I get a good picture I can resolder to the good pins that are left.

Thanks for help and comments

Most modern (eg. LCD) SCART-enabled TVs require a 5v signal to switch them into select SCART-RGB mode, but the AKF11 doesn’t.

[SKS1]

Thanks
Took pictures & documented where they come from, still got a picture , everything seems to be working apart from the picture being pink where it should be white, I am thinking of soldering them back in as they must have been there for some reason , maybe the picture was pink before due to the bad soldering, If they were doing nothing then you would think the person who fitted them would remove them.

Possibly the person who fitted them was just trying to tweak the RGB levels for a specific monitor. However, there is a bunch of other patches that you have near the VIDC, which may suggest these were part of a wider upgrade/modification. If you want to get to the bottom of this, you could trace the circuit from the VIDC RGB pins, through to the RGB socket at the back, and compare this with the same for an A310 and A410/1 PCB, which were the successors of the A500.

If you are going to reinstate the two 70R resistors, I’d suggest shortening the legs as much as possible and using heat-shrink to cover any remaining exposed legs. Also find a third 70R resistor to fit to the last of the three PCB resistors in your photo, since it looks likely that something was soldered to the lower resistor in the past.

Remember the analgue RGB output of VIDC1 in the A500 is electrically different to VIDC2 found in production systems.

There exists an Acorn A5002p circuit diagram which I will wager is far closer in this area: http://chrisacorns.computinghistory.org ... iagram.pdf

[razz177]

Thanks
Took pictures & documented where they come from, still got a picture , everything seems to be working apart from the picture being pink where it should be white, I am thinking of soldering them back in as they must have been there for some reason , maybe the picture was pink before due to the bad soldering, If they were doing nothing then you would think the person who fitted them would remove them.

Possibly the person who fitted them was just trying to tweak the RGB levels for a specific monitor. However, there is a bunch of other patches that you have near the VIDC, which may suggest these were part of a wider upgrade/modification. If you want to get to the bottom of this, you could trace the circuit from the VIDC RGB pins, through to the RGB socket at the back, and compare this with the same for an A310 and A410/1 PCB, which were the successors of the A500.

If you are going to reinstate the two 70R resistors, I’d suggest shortening the legs as much as possible and using heat-shrink to cover any remaining exposed legs. Also find a third 70R resistor to fit to the last of the three PCB resistors in your photo, since it looks likely that something was soldered to the lower resistor in the past.

Remember the analgue RGB output of VIDC1 in the A500 is electrically different to VIDC2 found in production systems.

There exists an Acorn A5002p circuit diagram which I will wager is far closer in this area: http://chrisacorns.computinghistory.org ... iagram.pdf

Hi
I did look around for a diagram but gave up in the end and refitted it to the case, I have three of these resistors & was thinking of fitting them back with shorter legs & putting heat shrink on is a great idea, I was just hoping that this was a lead problem & have been searching for a diagram for the RGB cable, I ave a Philips monitor from my Amiga but it did not work on that either, so I really do not know what other monitors to try I have a few Acorn monitors.
I really want to get this finished now so I can get onto my A5000's and RscPc's.

This is the latest picture but still pinkish.

I do not no much about electronics and wish I did, I am great with fuel injection on diesel engines but that is of no use here.
So I really appreciate peoples help and advice.
Batteries are what has damaged my other computers & am looking forward to doing the PCB tracks on them.
Thanks again

[SteveBagley]

What picture do you get if you press the green button on the monitor (IIRC it works by killing the blue and red channels, so it might give a clue as the problem is with the green signal).

Also, SCART cables for the BBC usual contain resistors to reduce the TTL 5V signal to 1Vp-p analogue RGB. I suspect these aren't needed on the A500 (since it should output an analogue signal anyway?)…

Steve

[razz177]

What picture do you get if you press the green button on the monitor (IIRC it works by killing the blue and red channels, so it might give a clue as the problem is with the green signal).

Also, SCART cables for the BBC usual contain resistors to reduce the TTL 5V signal to 1Vp-p analogue RGB. I suspect these aren't needed on the A500 (since it should output an analogue signal anyway?)…

Steve

Hi Thanks for getting in touch, the green screen is green all the writing & Icons are like embossed onto the screen A bit like the BBC on flat screen with wrong lead, I cannot get a decent picture of it as it comes out green with black but the black is not there looking at it, I have re soldered the resistors to the motherboard that looked a bit dodgy when I cleaned it but it was the same without them, scart plug image attached.
Din
-----
Pin 1 Red
Pin 2 Green
Pin 3 Blue
Pin 4 Yellow Sync
Pin 5 Black 0 v
Pin 6 White (Not connected in scart plug)

Scart
--------
Pin 1 No Pin
Pin 2 No Pin
Pin 3 No Pin
Pin 4 Blue
Pin 5 No Pin
Pin 6 Green
Pin 7 No Pin
Pin 8 Red
Pin 9 Black
Pin 10 No Pin
Pin 11 Yellow Sync
Pin 12 to 20 No Pin