capacitors

"James Salisbury" wrote in
message ...

Hi try RS 260-6852 in series with the inner at the source end, the cable
capacitance alone may kill the RF.

I'll give that a whirl. Thanks.

Bill

In article , Bill Wright
wrote:

"Dave Farrance" wrote in
message ...
"Bill Wright" wrote:

The 3dB rolloff point will be when Xc=output-impedance, and nobody
should notice if that's set at 20kHz, so...

If it's 600 ohms then C=13nF, and if it's 75 ohms then C=106nF.


Thank v much for that. It's just the sort of info I need. I'll get
someone young to listen to the sound while I switch the cap in and out.
I've found a few other old caps of various values, so I should be able
to optimise it with the help of a young pair of ears.

Incidentally I experimented further. Putting the caps across the phono
audio outputs of the sat box does not help if the audio is taken from
the scart output to the modulator.

It occurs to me that there is a possibility that the problem is that when
you connect the boxes the load seen by signal source is *causing* it to
oscillate. This would mean that the added capacitor isn't necessarily
filtering away an oscillation that remains. It may be preventing the
oscillation from occurring by changing the load seen by the output.

Can't speak for the items you are using. However it is a well known problem
in audio (and RF) amp design that some amps oscillate with some output
loads, and changing the load can cause this to cease. No reason I can think
of for 'video' amps to be immune to this if the designer hasn't ensured
unconditional stability.

FWIW I've encountered more than one audio amp that oscillated if you have a
load capacitance of the order of a few thousand pF, but did not if the
capacitance was either higher or lower. Reasons well understood in terms of
feedback amp/oscillator theory.

If the phono and scart audio sockets aren't closely wired together, this
could also explain why caps on the phonos don't do the same as on the
scart.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

Jim Lesurf wrote:

It occurs to me that there is a possibility that the problem is that when
you connect the boxes the load seen by signal source is *causing* it to
oscillate. This would mean that the added capacitor isn't necessarily
filtering away an oscillation that remains. It may be preventing the
oscillation from occurring by changing the load seen by the output.

Yep. Here's another. Maybe the satellite box is using some dodgy budget
audio D-to-A technique that's buzzing loudly at the 48kHz sample rate.

--
Dave Farrance

"Jim Lesurf" wrote in message
...
It occurs to me that there is a possibility that the problem is that when
you connect the boxes the load seen by signal source is *causing* it to
oscillate. This would mean that the added capacitor isn't necessarily
filtering away an oscillation that remains. It may be preventing the
oscillation from occurring by changing the load seen by the output.
I will perform an experiment. Since the crackling noise (it's like a dirty
track on an old volume control) only appears with this one combination of
sat box and modulator (both items performing perfectly in conjunction with
other equipment) I'll feed the modulator that suffers from the problem and
another modulator simultaneously, one from scart and one from phone, and see
what happens.


If the phono and scart audio sockets aren't closely wired together, this
could also explain why caps on the phonos don't do the same as on the
scart.

Caps on the phono will cure the fault on the phono outputs.
Caps on the scart will cure the fault on the scart outputs.
Caps on the phono will not cure the fault on the scart outputs.
Caps on the scart will not cure the fault on the phono outputs.
Cap on left output will not cure the fault on right output.

The intensity of the crackle varies with the operation of the receiver. It
does not vary along with picture content, but the use of menus makes it much
worse or much better, seemingly at random.

The intensity of the crackle does not vary if the box's audio output level
is altered, or if video gain is altered.

As suggested yesterday I tested the effects of the caps on the audio, using
a pair of ears only 30 years old. The owner of the ears could detect a very
slight loss of top using 47nF caps (but so could I when I actually
listened), but neither of us could detect anything when 33nF caps were used.

Further experiments will be conducted over the weekend.

Bill

"Dave Farrance" wrote in message
...
Jim Lesurf wrote:

It occurs to me that there is a possibility that the problem is that when
you connect the boxes the load seen by signal source is *causing* it to
oscillate. This would mean that the added capacitor isn't necessarily
filtering away an oscillation that remains. It may be preventing the
oscillation from occurring by changing the load seen by the output.

Yep. Here's another. Maybe the satellite box is using some dodgy budget
audio D-to-A technique that's buzzing loudly at the 48kHz sample rate.

I think the main thing is that the modulator manufacturers should have
included some sort of low pass filter in the audio input.

Supposing it is something at 48kHz or thereabouts, how odd do you think it
is that a signal (or noise!) at that sort of frequency would cause a
modulator to produce a fairly loud continuous background crackle. What sort
of amplitude would you expect the interferer to have to be to have that
effect?

Bill

"Bill Wright" wrote:

"Dave Farrance" wrote in message
Yep. Here's another. Maybe the satellite box is using some dodgy budget
audio D-to-A technique that's buzzing loudly at the 48kHz sample rate.

I think the main thing is that the modulator manufacturers should have
included some sort of low pass filter in the audio input.

Supposing it is something at 48kHz or thereabouts, how odd do you think it
is that a signal (or noise!) at that sort of frequency would cause a
modulator to produce a fairly loud continuous background crackle. What sort
of amplitude would you expect the interferer to have to be to have that
effect?

Sorry, I've no idea on that one. Can't characterise an aberration from
theory.

--
Dave Farrance

...
So to satisfy my curiousity and probably lots of other people on this
newsgroup, whats the story behind the change from school teaching to
aerial rigging? I'm sure it will be entertaining....

I did satisfy your curiosity, and I thought my reply contained enough
material to generate a hundred responses, but I haven't seen one! I was
contentious, I reminisced, and I was indiscreet. Surely someone wants to
come back with their own opinions, reminiscences, and indiscretions!

Bill


You've opened a right old can of worms! I've never been happy about my
education, but whether it was the fault of the system or of myself I don't
know.

It all started when I was at junior school. I was an underachiever and a
discipline problem. I found the lessons boring and the regime childish. I
was moved from class to class and vigorously caned (at ten years old) but
I didn't improve. In the last year I had a teacher who was strict but
actually was very good for me. She got some good work out of me, but this
was the B stream so it was assumed that I would fail the 11+. In fact we
didn't sit an exam for the 11+, we were 'assessed', and of course I
failed.

There was a bit of a fuss (I had several teaching aunts who were pretty
annoyed) but off I went to the Secondary Modern. I was devastated. All my
friends went to the brand new grammar school on our estate, whilst I was
send on a bus to the grotty mining village down the road to a decrept
Secondary Modern. Where I flourished. I was massively depressed at first
but the school had a lot of very good teachers -- non-academic
ex-servicemen most of them. I enjoyed my time there, especially the craft
lessons and the school plays. I had an inspirational English teacher
called Frank Dalton, an ex-infantryman who puffed away on a fag as he
entranced his class of rough lads with Shakespeare. The Geog teacher had
been a prisoner of war under the Japs and it showed, although he never
talked about it. The history teacher had been on Lancasters for most of
the war. The school had a young head and terrific can-do spirit. Of
course it was rough (on one occasion there was a riot just outside the
gates which resulted in several kids in hospital, the Doncaster bobbies
coming out to reinforce the Bentley ones, and mass thrashings during
assembly over the next few days). But it was a brilliant school. We were
coached for 'O' levels, which was really unusual for a Sec Mod.

But when my best pal gained an apprenticeship to a local TV dealers I was
told that I wasn't allowed to do the same because I was 'academic'. I
asked to be transferred to the Tech but again I was told that it wasn't
for me because -- well basically because I was literate. I was too young
to realise how stupid this was, so I accepted my fate, which was a
transfer to the Grammar School. This was just so weird. I was back with my
A stream pals from junior school, many of whom I'd kept up with during my
exile. Meanwhile my other mates, from the Sec Mod, were all out at work
doing interesting things like mending tellys. Then, the bombshell. I'd
gone to the GS on the understanding that I could do physics A level. But
I'd failed maths 'O' level, so physics was out. And because of the
timetable that meant that I had to do Eng Lit and History. I was crap at
both. I should have left at that point and done something else. But a
working class kid throwing away a grammar place was unthinkable.I
considered it but I couldn't face the idea of my old Sec Modern teachers
finding out. The grammar school was the most soulless demoralising place
I've ever been in. I passed my driving test at 17yrs 1 month and nicked my
dad's van, and started fixing aerials by myself in the evenings. So I was
the richest kid in the 6th form! Half way through the Lower Sixth we were
told to chose a career. Since I was obviously going to fail my A levels
the choice was simple: either teaching or banking. I chose teaching. If
I'd had any sense I would have walked out and got a proper job.

So off I went for an interview at the local TTC. I was told that I needed
only two things to get in: five '0' levels (got them) and a clean bill of
health (got it). So by the March of my lower 6th I knew that I didn't need
any A levels. I decided to concentrate on having a good time and also on
fixing TV aerials. I had a ball.

I carried on fixing aerials all through college, which wasn't hard because
the college work was easy and sparce.

In those days there was a shortage of teachers so you were guaranteed a
job at the end. All you had to do was keep your nose clean. The only
person who failed their college finals in our year was a lad who was told
he would be failed as a punishment because he organised a protest about
something or other. I went off to be a teacher. I enjoyed teaching a lot,
but there were two problems. One was the fact that (as I now know) the
headteacher was well out of line in the way he treated us young teachers.
The other was the money. I could earn as much on a Saturday fixing aerials
as I could for teaching all week. So I packed it in and started up a full
time business. And I've never looked back.

Well, you did ask.

Bill

On Sat, 23 Feb 2008 14:48:29 -0000, "Bill Wright"
wrote:

So to satisfy my curiousity and probably lots of other people on this
newsgroup, whats the story behind the change from school teaching to
aerial rigging? I'm sure it will be entertaining....

I did satisfy your curiosity, and I thought my reply contained enough
material to generate a hundred responses, but I haven't seen one! I was
contentious, I reminisced, and I was indiscreet. Surely someone wants to
come back with their own opinions, reminiscences, and indiscretions!

Nah. We all progressed uneventfully through school, learned our trade
or profession in the conventional manner and quietly carried it out.
You're the only bad boy :-)

"Laurence Payne" NOSPAMlpayne1ATdsl.pipex.com wrote in message
...
On Sat, 23 Feb 2008 14:48:29 -0000, "Bill Wright"
wrote:

So to satisfy my curiousity and probably lots of other people on this
newsgroup, whats the story behind the change from school teaching to
aerial rigging? I'm sure it will be entertaining....

I did satisfy your curiosity, and I thought my reply contained enough
material to generate a hundred responses, but I haven't seen one! I was
contentious, I reminisced, and I was indiscreet. Surely someone wants to
come back with their own opinions, reminiscences, and indiscretions!

Nah. We all progressed uneventfully through school, learned our trade
or profession in the conventional manner and quietly carried it out.
You're the only bad boy :-)

I always knew I was different to everyone else . . .

Bill

On Sat, 23 Feb 2008 19:53:22 UTC, "Bill Wright"
wrote:

I always knew I was different to everyone else . . .

Bill

Not exactly. I had a very similar secondary/grammer education
experience. Started in light engineering and ended up in IT :-)

--
Regards
Dave Saville

NB Remove nospam. for good email address