In article ,
"N_Cook" wrote:

Brenda Ann wrote in message
...

"N_Cook" wrote in message
...
Brenda Ann wrote in message
...

"N_Cook" wrote in message
...
No user switchable option. I can see its possible to rewire the
vibrator
so
you still get right polarity for the anode supplies. Heaters
presumably
happy enough powered the other way round but how to change the
biasing
of
the valves ?



Unless it's a synchronous vibrator, it should run equally well on
either
polarity. The radio doesn't get anode voltage from the car's DC, it
gets
it
from the radio's DC-DC converter (vibrator, transformer, rectifier,
filtering).




With positive ground you automaticly get a negative voltage for biasing
but
what to do in the negative ground case ?
In this case its a synchronous mechanism but its easy enough to get
inside
to rewire it for the opposite polarity, it is the biasing voltage I
cannot
fathom out.




This must be an unusual radio (by old US car radio standards). No valve
car
radio I have ever seen has used the car's battery supply for any voltage
other than the valve heaters. All DC voltages to run the valves come off
the
output of the transformer, with bias voltages developed by raising the
cathode of the valve above B- (making the grid negative with respect to
the
cathode, but not with respect to ground).



This one also, on checking.
I had not realised that the 12V to the radio is just for the heaters and the
dial lamp and the on/off switch.
Just leaves whether there is an added RFI noise problem from the "inverter".

Why would the RFI issues change? If it is a synchronous vibrator, isn't
it simply a matter of reversing two transformer connections to convert
from positive ground to negative ground, how does this change affect RFI?


Regards,

John Byrns

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/