Watch Out for Oscillating PreAmps!
 

http://news.bbc.co.uk/1/hi/england/b...ts/8327549.stm

--
Ashley
For Windsor Weather see www.snglinks.com/wx

Watch Out for Oscillating PreAmps!
 

"Ashley Booth" wrote in message ...
http://news.bbc.co.uk/1/hi/england/b...ts/8327549.stm

Reminds me of all those superregenaritive sets I made when I was at school.
Got complaints from the neighbours, but thankfully never the Post Office investigators.

--
Graham.

%Profound_observation%

Watch Out for Oscillating PreAmps!
 

Ah yes, I made one of those for the airband. Mind you nobody could receive
anything for about 200 yards around it.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"Graham." wrote in message
...


"Ashley Booth" wrote in message
...
http://news.bbc.co.uk/1/hi/england/b...ts/8327549.stm

Reminds me of all those superregenaritive sets I made when I was at
school.
Got complaints from the neighbours, but thankfully never the Post Office
investigators.

--
Graham.

%Profound_observation%

Watch Out for Oscillating PreAmps!
 

In article , Graham.
wrote:


"Ashley Booth" wrote in message
...
http://news.bbc.co.uk/1/hi/england/b...ts/8327549.stm

Reminds me of all those superregenaritive sets I made when I was at
school. Got complaints from the neighbours, but thankfully never the
Post Office investigators.

Reminds me of the thick-film UHF amp boards that RS used to sell. Every one
of those I tried hooted above 1GHz. You could alter the frequency by
changing the rail voltage.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Watch Out for Oscillating PreAmps!
 

In article ,
says...

http://news.bbc.co.uk/1/hi/england/b...ts/8327549.stm

Reminds me of a problem with ITT tvs fitted with a variant of the VC200
chassis (VC207, IIRC).

Someone had the bright idea of using them on a 22 channel VHF CCTV
system with VHF/UHF upconvertors (there were two of them IIRC - one was
the TeleVerter(?))

There were lots of problems, mainly with temperature drift in the
converters but also with patterning (because there was far more RF
flying around than these devices were designed to handle).

However, the most virulent patterning problem (it affected an entire
batch on test, to the extent of completely wiping out one entire channel
on the worst affected sets) was traced to one particular receiver and
was assumed to be a fault in the converter.

When the converter was replaced, however, the problem remained!

After sniffing around with an analyser and a search coil, the problem
was found to be in the IF strip of the TV itself.

These sets had the IF strip built into a number of screened modules
mounted on the main board, so the offending module had to be removed for
further investigation. When the screening can was removed, the cause was
obvious. There was supposed to be a ferrite bead on the base lead of the
transistor. There was a bead all right - but ferrite it wasn't - it was
one of those little ceramic stand off beads for use on high temperature
resistors!

With a ferrite bead, it was perfectly stable. We later found more of
them but, of course, we only found the ones that caused a problem! They
tended to oscillate in mid-to-top Band V, with no discernible effect on
normal operation. In normal domestic use they would rarely have been a
problem unless the oscillation coincided with a channel in use locally,
so there could have been thousands of them like that!

--

Terry

Watch Out for Oscillating PreAmps!
 

"Jim Lesurf" wrote in message
...
In article , Graham.
wrote:


"Ashley Booth" wrote in message
...
http://news.bbc.co.uk/1/hi/england/b...ts/8327549.stm

Reminds me of all those superregenaritive sets I made when I was at
school. Got complaints from the neighbours, but thankfully never the
Post Office investigators.

Reminds me of the thick-film UHF amp boards that RS used to sell. Every
one
of those I tried hooted above 1GHz. You could alter the frequency by
changing the rail voltage.

Some early amps from Taylors (of German origin) oscillated just above the
UHF band, the exact frequency depending on the distance between the cover
and a heatsink. They were microphonic when tapped, if monitored on FM. The
cure was to unscrew the heatsink a tiny bit so it touched the cover.

Bill

Watch Out for Oscillating PreAmps!
 

In article , Bill Wright
wrote:

"Jim Lesurf" wrote in message
...
In article , Graham.
wrote:


Reminds me of the thick-film UHF amp boards that RS used to sell.
Every one of those I tried hooted above 1GHz. You could alter the
frequency by changing the rail voltage.

Some early amps from Taylors (of German origin) oscillated just above
the UHF band, the exact frequency depending on the distance between the
cover and a heatsink. They were microphonic when tapped, if monitored
on FM. The cure was to unscrew the heatsink a tiny bit so it touched
the cover.

....or sell them as RF microphones. Maybe they were based on an East German
design made for some other purposes... :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Watch Out for Oscillating PreAmps!
 

"Jim Lesurf" wrote in message
...
In article , Bill Wright
wrote:

"Jim Lesurf" wrote in message
...
In article , Graham.
wrote:


Reminds me of the thick-film UHF amp boards that RS used to sell.
Every one of those I tried hooted above 1GHz. You could alter the
frequency by changing the rail voltage.

Some early amps from Taylors (of German origin) oscillated just above
the UHF band, the exact frequency depending on the distance between the
cover and a heatsink. They were microphonic when tapped, if monitored
on FM. The cure was to unscrew the heatsink a tiny bit so it touched
the cover.

...or sell them as RF microphones. Maybe they were based on an East
German
design made for some other purposes... :-)

I don't know about that, but they certainly caused a bit of havoc!

Bill

Watch Out for Oscillating PreAmps!
 

"Bill Wright" wrote in message ...

"Jim Lesurf" wrote in message ...
In article , Bill Wright
wrote:

"Jim Lesurf" wrote in message
...
In article , Graham.
wrote:


Reminds me of the thick-film UHF amp boards that RS used to sell.
Every one of those I tried hooted above 1GHz. You could alter the
frequency by changing the rail voltage.

Some early amps from Taylors (of German origin) oscillated just above
the UHF band, the exact frequency depending on the distance between the
cover and a heatsink. They were microphonic when tapped, if monitored
on FM. The cure was to unscrew the heatsink a tiny bit so it touched
the cover.

...or sell them as RF microphones. Maybe they were based on an East German
design made for some other purposes... :-)

I don't know about that, but they certainly caused a bit of havoc!

Bill
Years ago when the top end of Band II wasn't full of commercial stations
and car radios were easier to tune (with a knob) you could drive down
a suburban street and listen to the local oscillators 10.7 MHz higher
than the station people had their kitchen radios tuned to. Mainly Radio 2.

Some carriers were quite clean but some were NBFM with the station
audio. Not sure which was the dominant mechanism, between acoustic
microphony, and imperfect supply rail decoupling.

--
Graham.

%Profound_observation%

Watch Out for Oscillating PreAmps!
 

"Graham." wrote in message
...
Years ago when the top end of Band II wasn't full of commercial stations
and car radios were easier to tune (with a knob) you could drive down
a suburban street and listen to the local oscillators 10.7 MHz higher
than the station people had their kitchen radios tuned to. Mainly Radio 2.

Some carriers were quite clean but some were NBFM with the station
audio. Not sure which was the dominant mechanism, between acoustic
microphony, and imperfect supply rail decoupling.

Some of the early Amstrad analogue Skyboxes radiated on UHF ch25. There an
area near here where everyone uses Belmont, with big aerials and masthead
amps. One Skybox could wipe out ITV on the whole street.

Bill