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Arv,

I get the same carrier level in the spectrum analyzer with a tone or
without a tone, just the two-tone generator output impedance as a
termination.

When you say to terminate the mike input what impedance are you
thinking of? I've attached a png of the output circuit of the two-tone.
I use the line output which basically offers a 33K impedance to the mic
input.

I don't think it is the diodes themselves. If one or both of them were
bad I wouldn't expect even 25db of suppression.

I've never been a big fan of this kind of balanced modulator. There
isn't much you can do to maximize carrier balance.? I thought perhaps
it might be unequal inter-winding capacitance in T7 so I did the
best I could to equally space all the windings around the toroid and
the wire length to the circuit board but it made no difference at all.

I haven't tried yet but I've thought about lifting C63 to isolate the
mic pre-amp to see if that makes the carrier suppression better. I've
got everything torn down now to move the circuit board closer to the
back so I can use the box as a heat sink. When I get it all back
together I will try liftin C63 to see what happens.

tim ab0wr


On Sat, 28 Apr 2018 17:10:53 -0600
"Arv Evans" <arvid.evans@...> wrote:

> Tim
>
> Interesting.? How much carrier suppression do you get without tone
> insertion?
> Just terminate the mike input and balance the modulator without any
> modulation
> being inserted.? If that dip is too shallow then it could point to the
> modulator diodes
> themselves.
>
> Arv
> _._
>
>