"hr(bob) hofmann at (no spam) att.net" <hrhofmann at (no spam) att.net> wrote in message
news:b218b8a5-7fb3-49ea-9794-7feac9dbf096 at (no spam) 2g2000hsn.googlegroups.com...
On Jul 10, 4:14 pm, "N_Cook" <dive... at (no spam) tcp.co.uk> wrote:
James Sweet <jamesswe... at (no spam) trashmail.net> wrote in message
news:aetdk.1248$bn3.1183 at (no spam) trnddc07...
N_Cook wrote:
Seemed a neat idea but ...
There is space inside this 1950s dashboard radio for one of those
keyfob/
matchbox size FM radios, this one based on a TDA7088T. The original AM
radio
works fine, well minimal hash from the buzzer mechanism of the vibrator
12V
to HV supply and the FM works fine with a proper linear mains derived HV
unit and the 12V to 3V ps. But try the FM off the original HV supply and
the
hash from the buzzer mechanism makes it unlistenable to. Whatever I try
in
the way of screening/ wire routing makes little difference. Any ideas
before
rejecting the project?
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
There's guys who do this regularly, I don't know the details but
hopefully some of them will reply with details. Undertaking this myself,
I would be inclined to replace the vibrator with solid state internals,
not too difficult and you get a lot of added reliability. I'm a little
confused when you say you're operating the FM receiver off the HV power
supply, are you trying to regulate down from the B+ supplied to the
tubes? If you need isolation, how about one of those little DC-DC
converters? I salvaged one from an old eithernet card for a different
automotive project where I needed isolation.
An option that would avoid modifying the original radio at all is to
use a low power AM transmitter fed by the FM receiver and just stick
that elsewhere.
I've replaced the switches with a 10 turn pot for dial tuning of the FM via
the varicap. The 3V is derived from the 12V which is positive ground which
I'm wondering if that may be part of the problem, grounding the + rail of
the TDA. The audio out buffered through a transistor, I've fed in via a
switch , at the volume control prior to the EL42.
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list
onhttp://home.graffiti.net/diverse:graffiti.net/- Hide quoted text -
- Show quoted text -
YOu need to figure out if the Hash is getting conducted or radiated
into the receiver. It should be posible to put the FM unit into an
aluminum foil shielded compartment and then filter the heck out of the
input voltage.
Bob Hofmann
Yeppers, that's exactly what I was going to suggest. I'll bet that shielding
will significantly reduce the hash. Probably need to use a shielded wire for
the antenna as well.
The vibrator is a nasty source of hash noise, and it might take a lot of
shielding to get it under control. I don't think the positive ground system is
necessarily a show stopper. You'll just have to experiment to get the right
configuration... what and how to shield and where to connect it.
