Date : Tue, 13 Apr 2010 19:52:51 +0100
From : dominic@... (Dominic Beesley)
Subject: Dead beeb - easy tests
Thanks for the pointer Mark,
I got a 74LS00 and some jump leads and though it's dodgy I can get it to
boot with the inverter in place.
In Mode 4 I get stripy screen in Mode 7 jumbled characters
It looks to me as if bit 1 is stuck on, I can poke the screen with ?30000=1
or ?30000=4 to get a dot either side the stripe.
Am I right this is IC62 that is suspect or is it IC54, I'm not really
following the schematic 100%
Cheers
Dom
> -----Original Message-----
> From: bbc-micro-bounces+dominic=brahms.demon.co.uk@...
> [mailto:bbc-micro-
> bounces+dominic=brahms.demon.co.uk@...] On Behalf Of
> Mark Haysman
> Sent: 11 April 2010 21:53
> To: bbc-micro@...
> Subject: Re: [BBC-Micro] Dead beeb - easy tests
>
> Hi.
>
> I've built a small module for exactly this type of diagnosis, that
> inverts
> the signal sent to pin 3 of IC45 - that has the effect of reversing the
> 2 x
> 16k banks, so if the fault is in the lower ram bank, the machine won't
> boot.
> Reversing the banks moves the fault to the upper bank. That way, the
> machine
> will boot, albeit with a faulty looking screen, and you can do some
> simple
> memory tests to determine which bit, and therefore which chip is
> faulty.
>
> It is possible that the fault is before that link, IE in the clock
> circuit
> of IC28 and further back, but it's unlikely - it's normally a bad RAM.
>
> Mark.
>
> ----- Original Message -----
> From: "Dominic Beesley" <dominic@...>
> To: <bbc-micro@...>
> Sent: Sunday, April 11, 2010 9:38 PM
> Subject: Re: [BBC-Micro] Dead beeb - easy tests
>
>
> > Thanks Chris, Mark,
> >
> > Cigar goes to Mark - tried S25 and BRrr-beep
> >
> > Now...how to track down which chip is at fault is it possible to
> access
> > them
> > when the machine's running in 16k mode?
> >
> > Dom
> >
> >> -----Original Message-----
> >> From: bbc-micro-
> bounces+dominic=brahms.demon.co.uk@...
> >> [mailto:bbc-micro-
> >> bounces+dominic=brahms.demon.co.uk@...] On Behalf Of
> >> Mark Haysman
> >> Sent: 11 April 2010 19:33
> >> To: bbc-micro@...
> >> Subject: Re: [BBC-Micro] Dead beeb - easy tests
> >>
> >> > My London based beeb has decided to pack in this weekend. Symptoms
> >> are
> >> > continuous beep at switch on, Caps lock on from start shift lock
> >> comes on
> >> > after 1/2 second.
> >> >
> >> > Tried swapping 6522s no difference.
> >> >
> >> > No cursor on screen but I think the processor is doing something
> as
> >> the
> >> > shift lock ceases to come on if 6502 or LS245 are removed.
> >> >
> >> > Any ideas of simple tests without any test gear...if not it will
> have
> >> to
> >> > come back to civilisation where I can show it to a scope...
> >>
> >> Swapover the link at S25, that changes it to 16K, and swaps the RAM
> >> bank
> >> used for zero page, sometimes that shows up a RAM fault. Removing
> the
> >> LS245
> >> will make it a-la-dodo, as it needs that to interface to the RAM, so
> >> try
> >> swapping that out if the 1MHz bus one is socketed.
> >>
> >> Mark.
> >>
> >>
> >> _______________________________________________
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> >
> >
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>
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