Hi Snowy,
I don't have a schematic for that radio to look at. Nevertheless I can
make some educated guesses that may be of help to you. First of all you are
correct, a properly working SSB transmitter should put out zero power with
zero audio input. That would be if it were perfect. In the real world there
will be a few possible causes for some small RF output when there is no
input at the microphone connector. Those causes would be 1) noise in the
transmit audio stages 2) imbalance of the balanced modulator 3) sort of the
same thing as the first one is RF feedback into the transmit audio stages.
All SSB transmitters have a carrier suppression specification that is
usually expressed in decibels below peak RF output. That value is probably
greater than 30 dB for most transmitters, which means for a 100 Watt
transmitter the residual carrier should be less than 0.1 Watts. You
shouldn't be able to see it on your wattmeter, but you will be able to hear
it on a receiver nearby. The carrier balance potentiometer is adjusted for
minimum carrier in SSB transmit. Some radios switch to CW mode by applying
a DC offset to the balanced modulator to purposely unbalance it and let
carrier through.
Also when testing for carrier suppression without a microphone it is
best to have a termination resistor in place of the microphone element. The
size of the resistor depends on the impedance of microphone the radio is
designed for. If high impedance then maybe 57k Ohms, If low impedance then
maybe 600 Ohms.
With all of those voltages wrong there is a really good chance that
your balanced modulator is out of balance and letting carrier through. It
would be worthwhile to take a listen to the signal with a separate receiver
and find out how that greater than zero transmit power with no microphone
input sounds. Is it a clean CW carrier? Or does it have noise or hum on it?
And when you connect a microphone does any voice audio get onto the signal?
Clean or distorted?
Good luck,
Ken N6KB
On Wed, Jun 10, 2020 at 10:27 PM ROLAND HOWELL via TenTec <
tentec@contesting.com> wrote:
>
> Hi all
>
> Thanks to Martin and Dave for the advice so far.
>
> One thing is puzzling me with the radio and that is this, When I switch
> to SSB (upper or lower) the radio transmits what ever power level has
> been set with the drive control, I have always thought that it only
> transmitted with audio is applied via the mic, or have I got it wrong!.
>
> I am wondering if I have additional problems.
>
> The voltage levels on the 4558 appear to be correct, I have changed it
> anyway but the voltage on U1 pin 1 is only <3v not what is written, all
> other pin voltages are correct.
>
> So more head scratching today ( I must be careful of getting splinters
> ha ha).
>
> stay well and happy
>
> 73
>
> Snowy G0HZE
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