Hi Tom,


K7ITM wrote:

Hi Frank...

Wow, the mixer in that circuit looks really weird. No, make it: that
mixer looks WRONG. I would not expect single diodes to be directly
across transformer windings like that.

I believe they made the schematic wrong!! I checked the actual circuit
and seems just right :-)
After all it was working on the 6m band.
Though they did really many mistakes that I tried to correct.
Q6 is a transistor with Ft=150 MHz, just for a little example, but many
mistakes can be spotted directly on the schematic.


I'd expect you can get plenty of power out of a single transistor (3rd-
overtone) oscillator to drive a double-balanced mixer with Schottky
diodes, certainly. +7dBm is normal for that. That's only half a volt
into 50 ohms, or 1V rms into 200 ohms. I'd probably pull a
Minicircuits mixer out of my junque box, and not try to build one from
diodes and transformers; a Minicircuits ADE-1 (or any of several
others) in a package less than 6x8mm should do the job fine. Also
small: active mixers. I've been playing with a Linear Technology
LT5560, which works fine with 0dBm LO power.


I thougth so also, but this beast has only one mixer for RX and TX and
RF/IF ports are exchanged from RX to TX. This migth be a problem or not,
but if it worked at 50 MHz with 1N4148s it will work on 70 MHz with
1N5711 (maybe even reasonably well with 1N4148, but diodes are cheap
anyway).

Of course my own transverters are much better, I started from well known
and working schematics. I've been asked to modify this one just because
the owner isn't going to get active on the new band if he has to spend
much more than a bunch of parts. Having as many new station QRV as
possible is good, the nearest 4m station to my QTH currently is at a bit
less than 500 Km and that's after two years that we have had temporary
permits.

Thanks for suggestions, I probably will try with a simple BJT
oscillator, I was just a bit worried by the unknown collector load, I
don't have any info on T4. I would need to remove and test separately
with a signal generator.

best regards
Frank IZ8DWF