Date   : Thu, 16 Aug 2007 20:07:12 +0200
From   : robert@... (Robert Schmidt-Cisternas)
Subject: Acquired Beeb with symptoms...

Thanks M./Fragula,

I removed the ULA (IC7, right?) - interestingly the second pin from the
back on the right side was bent inwards.

I powered up with the chip removed, as well as after straightening the
pin and reinserting.  The symptoms are still unchanged.

I'm suspecting IC35 (the LM324N opamps), as the Beeb came with 2
"spares" which have obviously been used.  My (very optimistic) theory is
that the bent ULA pin caused the original opamp to fry, as well as the
next 2 replacements: Now that I've fixed the pin, replacing IC35 a third
time will make all symptoms go away.  Then again, the pin might have
gotten bent in an attempt to locate the actual problem.
 
Forgive my ignorance and inability to figure it out, but where's the
UART?

Cheers,
Robert

 -----Original Message-----
From: Fragula [mailto:fragula@...] 
Sent: 16. august 2007 15:00
To: Robert Schmidt-Cisternas
Subject: Re: [BBC-Micro] Acquired Beeb with symptoms...

Might be worth powering it up after removing the cassette ULA and maybe
the UART too. Just a hunch.

Specifically, that the cassette/serial subsystem might be generating
spurious interrrupts due to a faulty op-amp, or similar.

Good luck!

M.


Robert Schmidt-Cisternas wrote:

>Thanks:
>I swapped the two 6522s, with no apparent change.  You say
"temporarily"
>- they shouldn't differ in any way, should they?
>I haven't been able to locate any shorts.
>
>The machine came with a spare Norwegian OS (Norwegian characters in
mode
>7), but using it makes no difference either.
>
>Cheers,
>Robert
>
>
>-----Original Message-----
>From: bbc-micro-bounces+robert=schmidt-cisternas.net@...
>[mailto:bbc-micro-bounces+robert=schmidt-cisternas.net@...
u
>k] On Behalf Of Sprow
>Sent: 16. august 2007 08:40
>To: bbc-micro@...
>Subject: Re: [BBC-Micro] Acquired Beeb with symptoms...
>
>In article <D27938D1A467B04B9D1B16361BA84C970182A8@...>,
>   Robert Schmidt-Cisternas <robert@...> wrote:
>  
>
>>I just acquired a Beeb (I haven't owned one since 1986 :-)), and was
>>wondering if the following symptoms ring any bells:
>>
>>- Pressing BREAK once seems to freeze the machine
>>- Pressing BREAK a second time seems equivalent to Ctrl+BREAK (or
>>power-on?)
>>- In general the machine seems sluggish, especially in graphics modes
>>    
>>
>-
>  
>
>>it is not noticeable when typing, but games run much slower than I
>>remember them.  For example, the original Repton scrolls at about 2
>>frames per second(!)
>>    
>>
>
>Sounds like it's spending a lot of time servicing interrupts which
>aren't
>really there. There's a project to do this deliberatly at
> http://www.sprow.co.uk/bbc/buildit.htm
>but you don't want this behaviour!
>
>My best bet on no other information would be something wrong with the
>system
>via or decoding of the slow bus nearby. The system via is heavily
>involved
>in interrupts from sources which might not really be there (light pen
>for
>example), it determines whether the last reset was a power on or not,
>and
>also runs some timers.
>
>Try swapping it with the user via temporarily. Also maybe take out the
>whole
>motherboard to check for shorts (washers/paper clips etc) trapped
>underneath?
>Sprow.
>
>
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