Date   : Thu, 06 Oct 2005 04:09:18 +0000
From   : "bbc" <bbc@...>
Subject: Re: T/shooting advice - bad UHF signal

Thanks for your thoughts Mike.

>  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?)
:-) Yep, it's muted.

>  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.
unfortunately, I'm limited in that I don't have a oscilliscope to check 
the noise on the power lines. I was considering the power supply because, 
as you say, they are well past it. Might go through it and change out all
the caps to be sure. I don't have another known good power supply to test
with (the Beebs are a very rare commodoty down here in Oz)

Best advise to decharge the filter caps first? My electronics is a bit rusty
as well, but I can wield a soldering iron and read cap marking okay.

 
>  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.
Are these easy to obtain? Just an over-the-counter-from-electronic shop 
modulator do the trick? Was thinking of this because I would get one with 
a standard antenna out socket, instead of the RCA one. I have not been able
to find an RCA Plug to Antenna Plug cable anywhere, hence I had to wire one
up myself (and it continuity tested fine - no shorts).
Should this be pretty 
>  
>  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.
Good idea.

>  
>  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.
This is also on the plans. The cable is 20+ years old, it looks okay, but
I would prefer to replace ti with a new one for safety sake.