Date   : Thu, 06 Oct 2005 04:15:24 +0100
From   : Mike Tomlinson <mike@...>
Subject: Re: T/shooting advice - bad UHF signal

In article <20051005113806.29037.qmail@...>, =?iso-
8859-1?Q?bbc?= <bbc@...> writes

>The picture quality is very iffy. The signal seems to loose sync regularly, and 
>when it does hold sync, the white fades from being nice and bright to very 
>dull. 
>There is no pattern to this.
>
>I also noticed that there is a static sound coming from the speaker, and it 
>definately seems to coincide with the drop in video signal quality.

The BBC speaker is normally quite noisy (hissy) anyway, the quality of
the noise changing as the machine does different things.  Sorry to ask,
but you do have the sound turned right down on the TV? (i.e. you're
certain the sound isn't coming from the TV speaker?)

>Anyone offer some thoughts? Bad UHF modulator? Noisy power supply lines (I'm 
>yet 
>to measure them to see their voltages)?

I don't think a basic voltage check would tell you much.  You need to
put a 'scope on the PSU outputs to see if they are clean.  Failing that,
can you try a known good PSU in the machine?  Those PSUs are nearly 25
years old and the caps will be well past their prime.

If the UHF output is still bad with a known good PSU, check the
modulator for dry solder joints on the two metal tabs which attach it to
the motherboard - a good earth is essential here - and if ok, change out
the modulator.  The modulator is quite a common failure.

You need to be sure the connection is good (clean and tight) at each of
the three PSU connections to the motherboard. Nip up the spade
connectors gently with a pair of pliers and re-attach them.

One other thought - check the PSU mains cable for continuity on the
earth wire.  The cable tended to break at the strain relief grommet
where it enters the PSU due to people winding the cable around the
machine to store it.

Oh, and try another UHF cable if you can first!