Resurrect BBC Micro

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Ioerror
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Resurrect BBC Micro

Post by Ioerror » Mon May 07, 2018 4:00 pm

Hi

I remember buying a BBC micro about 15 years ago at a car boot sale for a pound. I then turned it on typed a bit of basic and forgot about it. It recently surfaced in a shed and I thought I'd have a go at getting it running. Time has not been kind to the machine the PCB is filthy and 1 of the caps has leaked.

After replacing the recommended capacitors in the PSU I fired it up, I get one short beep and the caps lock LED is on. On screen there is a command prompt hidden behind the O character repeated all over the screen. I tested the PSU with a multi meter and the voltages all seem appropriate -5 is actually -4.5 but I assume this would be okay.

I tried reseating all the the socketed chips in case this helps but the screen does not change. Pressing the break key will halt the display for a second (sometimes) but releasing goes back to a screen covered in O. You can type at the prompt and just about make out what you've typed and some of the characters are not displaying the S is missing from mistake.

Can anyone suggest what might be the problem, are there any chips I can try swapping to see if it brings it back to life? If so where can I purchase those chips.

Thanks in advance.

P.S it's a revision 7 board with a wire soldered on the base which I also think is normal from reading about this.

Ioerror
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Re: Resurrect BBC Micro

Post by Ioerror » Mon May 07, 2018 4:03 pm

Images if they help.

https://imgur.com/a/90IVCUB

joachim
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Re: Resurrect BBC Micro

Post by joachim » Mon May 07, 2018 6:20 pm

The images help a lot! It looks as if the character generator is getting a bad value for bit 4 of each character (bit 4 = 1 most of the time when it should be 0). Thus space becomes '0', "BBC" becomes "RRS" and so forth. Although it looks as if the correct values are also flickering in and out.

This may be a bad RAM chip (in bit 4 of the upper 16k): we've had several threads about what to do with that, such as viewtopic.php?t=7934.

A couple of things you could try:
* Change to a mode other than MODE 7. For example try a 20k mode such as MODE 1: if four fifths of the screen looks bad then it's definitely a fault with one 16k bank. If the screen is perfect except in MODE 7 then it may be a problem with the 5050.
* Set link S25 south to disable the upper 16k, see if the machine works correctly using just the lower bank.

Ioerror
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Re: Resurrect BBC Micro

Post by Ioerror » Mon May 07, 2018 7:19 pm

Thanks for the link. I tried setting the jumper too south and the machine won't boot. Long beep sound. Where can I buy a HM4816 chip to try piggy backing to test the other ram chips? I've googled and all I can turn up is Aliexpress.

Bat
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Re: Resurrect BBC Micro

Post by Bat » Mon May 07, 2018 7:35 pm

Hi,
There was some on eBay around £10 each a couple of weeks ago...
Cheers,
Gavin :)

joachim
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Re: Resurrect BBC Micro

Post by joachim » Mon May 07, 2018 8:47 pm

Ioerror wrote:Thanks for the link. I tried setting the jumper too south and the machine won't boot. Long beep sound.
Oh right, on closer examination of the documentation for S25 (see also another good thread about bad RAM, viewtopic.php?t=2190&start=30), that's what we'd expect, because we disabled the good bank and the system is now trying to use the bad bank as system workspace. Instead, try removing S25 altogether to disable the bad bank.

Ioerror
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Re: Resurrect BBC Micro

Post by Ioerror » Tue May 08, 2018 7:40 am

Right, I’ve tried that and I get a 16k command prompt, amazing! I’ve ordered some new ram chips from retroclinic. I will update with how I get on.

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