Date   : Thu, 06 Oct 2005 16:50:10 +0100
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: T/shooting advice - bad UHF signal

bbc wrote:
>> 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)

Back when I used to just have a couple of BBC B's (don't ask!) the PSU 
died in one of them, and that machine became a testbed machine for 
various projects.

I hooked the machine up to a spare PC power supply - you'd likely want 
an old AT-class supply rather than ATX to avoid all the soft-start stuff 
(personally I much prefer a big power switch where you *know* when it's 
on or not!).

Just a thought anyway for ruling out your beeb's supply; it's still easy 
in the UK at least to find people dumping old PCs from which you can 
take the PSU.

> Best advise to decharge the filter caps first? 

I admit to not usually bothering for the size of cap present in a PSU 
like the BBC has... but that's probably not good policy!

>> 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?

Hmm, not sure about that. Personally I'd put the word out that I wanted 
a dead micro from the BBC era, as the modulator was a standard type on 
nearly all of the 8 bit machines from that time. No idea if those can be 
bought new.

> 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

Wiring up your own cable and using a 'proper' modulator sounds like the 
best plan to me to be honest - then you know you have a useful cable for 
the time when you start collecting more BBCs ;-)

As said though, check for bad solder joints first. Not only where the 
modulator secures to the PCB, but also where the output socket joins the 
modulator; I've seen them break here from years of people twisting the 
video lead to remove it from the back of the machine.

Oh, another thought - you've not got something else interfering on the 
same frequency that the BBC's outputting have you (TV station etc.)? I'm 
not sure if that would give the sorts of symptoms you describe or not, 
but it seems like a possibility.

cheers

Jules