Date   : Mon, 02 Oct 2006 14:36:15 +1300
From   : Michael Foot <mjfoot@...>
Subject: Re: One more BBC B to fix

In message <Y2AVZvBLIAIFFwuo@...> you wrote:

> In article <1ff6e36e4e%mjfoot@...>, Michael Foot
> <mjfoot@...> writes
> 
> >Do you think the fault is caused by a video circuit problem? Do you think
it's 
> >more likely to be IC related, connection related or some other component?
> 
> It's hard to tell, really.  The symptoms you describe seem to be video
> or memory related, but you say the machine will boot properly on the odd
> occasion before locking up.  Intermittent faults like this can be a
> bear.

It's confusing that's for sure. I guess no one has fixed this type of thing
before.

> Have you scoped or metered the 5v power connections?  Can you try
> another power supply?

Yes, the PSU reads 5.25V on all 3 leads. the -5V is fine too. I've not put
another PSU on this machine but I have powered up a spare Issue 7 board I
have and it booted fine. Maybe I should try a good PSU just to cover all bases.

> Leave the machine on for a while, then feel each of the RAM chips with
> the back of a finger to see if one or more is noticeably warmer than the
> others.

The RAM all feels the same temp. The 6502 and VIDPROC are warm too. Upon 
switching it on today, I got a flashing cursor in the top left corner in
what looked like a mode 4 screen. Crtl-Break did nothing so I turned it off
and back on again and it went back to the white screen with random LEDs.

> Try putting the user 6522 in the system 6522 socket, leaving the user
> one empty.
>
> Try swapping the 6502 from a known working machine.

Done all that. Everything that is socketed has been swapped. The keyboard's
been swapped too.

> On pre-issue 7 boards, it's very important to make sure all 6 5V/0V
> power connectors are attached and are clean and tight.

I'll try another PSU but I have done measurements around the board and 5V
was read where I tested it.

The fact that the machine does what it does only when jumper 26 is south
is stange. The other jumper settings don't allow Break to do a reset.

Cheers,

Mike.


-- 
Michael Foot <mjfoot(at)paradise(dot)net(dot)nz> *
http://homepages.paradise.net.nz/mjfoot/
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