Volker Tonn ) writes:
Jim schrieb:

I also put a pair of back-to-back glass switching diodes across the
external antenna input (across the 2.2K resistor that had already been
added by TenTec to bleed off static buildup), so in future the diode
should blow rather than the FET's. It seems that almost the whole
factory is up at Dayton.... he he..



The same many have done to the Sony ICF2010/ 2001D to prevent blowing
the FET (again).
I did 2 times 2 diodes in series antiparallel for 0.7 Volts for each
diode. So in addition there is a maximum voltage of 1.4 volts to the
antenna input.
This one diode with 0.7 volts could make you think your S-meter reading
is going nuts as it will never show more than S9+10dB or so. Even the
1.4 volts of 2 diodes in series will only give you addtional 3dBs.
Just a thought....

But there are very few cases where one would be receiving more than 0.7volts.

If you're in such a strong field that those diodes are conducting, you may
have plenty of distortion from one of the active devices. If the field
is strong enough for the diodes to conduct, the radio likely was overloading
anyway. We're talking close to an actual transmitter.

The diodes are there for protection from really strong local signals, and
I suppose even before that, in an attempt to protect from static generated
locally. Under normal use they will never conduct.

One of the keys to this problem seems to be that the gates of the FETs are
operating at a high impedance. It's not that the FETs are particularly
sensitive, but that if there's a high impedance there's less of a load
on the high voltages of static from right around you. A low impedance
point means the voltage is shorted to ground so it's not high enough to
destroy the junction.

Wait, I suppose it depends on where those diodes are. If they are just
across the antenna jack, the above applies. If they are put at a high
impedance point, like the secondary of a tuned circuit right at the antenna,
perhaps there would be enough step-up for a weaker signal to start being
significant voltage at that point.

Michael