Date   : Sun, 04 Nov 2001 20:28:07 -0000
From   : "Chris Richardson" <chris@...>
Subject: Re: BBC Master problem.

Hi Alastair
Thanks for the enquiry. I am afraid that I can't help you. However, I have
sent this query to the BBC mailing list. Hopefully someone will have an idea
what is wrong.
Chris

8-Bit Software, the BBC and Master Public Domain Library
http://www.8bs.com


----- Original Message -----
From: Alastair Varnals <plinth@...>
To: <chris@...>
Sent: Sunday, November 04, 2001 8:05 PM
Subject: BBC Master problem.


> I hope you don't mind me contacting you to ask about the following:
>
> I am hoping that you or someone you know might be able to help.
>
> I have recently acquired a BBC Master. Generally, it seems to work
> perfectly. However, after it has been on for about twenty minutes the
> 'speaker begins to make a crackling noise. This gradually becomes louder
> and more rapid until it turns into a buzz. This slowly increases in
> pitch, until after about an hour it stabilises as a very annoying
> whistle.
>
> I think it is triggered by some component becoming warm, turning power
> off and on again doesn't stop it. Leaving it off for a few minutes to
> cool down starts the process again. None of the components seem to
> become excessively hot.
>
> I have been studying the circuit diagram for the audio output circuitry
> ( which I don't understand very well, it seems overly complex for what
> it does ) and doing some simple tests. I think I have ruled out simple
> interference at its input as the cause. I rather suspect that one of the
> amplifier stages is breaking into oscillation.
>
> I have also noticed that the noise becomes higher pitched when there is
> a lot of text on the display. So perhaps there is interference ( maybe
> poor power supply decoupling? ) from the video generation circuitry.
>
> I am at a bit of a loss as to what to try next. Any suggestions would be
> very gratefully appreciated.
>
> Thanks in advance,
>
>  Alastair.
>
>