Date   : Sun, 11 Apr 2010 21:53:19 +0100
From   : jumbos.bazzar@... (Mark Haysman)
Subject: 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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