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R: Re-crystaling Microwave Modules 432/28 Transverter




----- Original Message -----
From: Bruce Robertson <broberts@mta.ca>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, March 17, 2004 4:05 PM
Subject: [amsat-bb] Re-crystaling Microwave Modules 432/28 Transverter


> Gang:
>
> I recently bought a second-hand Microwave Modules 432 -> 28 MHz
transverter
> on Ebay in order to gear up for CW/SSB LEO work. I had thought that all of
> these provide a means of working on 435, too, but it seems mine doesn't.
> I'm wondering if anyone has experience re-crystaling these models. In
> particular, since the existing lo xtal is a 101 MHz, I was thinking of
> buying the 101.750 crystal from Down East Micro.
>
> If no-one has direct experience with this model, general advice would also
> be welcomed. I suppose the question here is how broad-band the lo osc.
> tuned circuit and the rest of the filters, etc. are. I've never done any
> homebrewing in UHF/VHF territory, so I'm a bit unsure. In any case, I'm
> guessing that swapping an xtal isn't going to bust anything, so it might
be
> worth it as an experiment.
>
> 73,
>
> --
> Bruce Robertson, VE9QRP
> Dept. of Classics, Mount Allison University
> http://heml.mta.ca

Hi Bruce, VE9QRP

The 432/28 MHz transverter is a very nice piece of equipment and i was using
it on 432 MHz EME, than OSCAR-10 and 13,last on AO40 and actually i am
using it for the OSCAR-7 uplink.

Originally it is supplyed with two different oscillators using each a BF375
The first BF375 is for the crystal 101.000 MHz and the second BF375 is for
the crystal 101.500 MHz and both crystals are originally installed.

Each oscillator has a tunable coil in the BF375 collector to be adjusted for
the crystal start to oscillate.

Both oscillators are coupled with a 2p2 capacitor to a common doubler using
a BF375 to 202.000 MHz or 203 MHz and another common doubler  with a
BFY90 to 404.000  or 406.000 MHz depending on the crystal selected.

Using 101.000 MHz crystal it covers 432 to 434 MHz with 28-30 MHz IF

Using 101.500 MHz crystal it covers 434 to 436 MHz with 28-30 MHz IF

Between 432 to 436 MHz there are several combinations for TX and
RX frequency band covered depending on the crystals combination upon HI
and LO frequency switch selection.

All tuned circuits for the oscillator,first doubler,second doubler and the
rest of the tuned circuits between 432 to 436 MHz including the 10 watt
power amplifier are wide enougth to be flat provided they are a little bit
staggered tuned between 432 to 436 MHz

In order to extend the frequency agility  between 436 to 438 MHz i have
included a new crystal 436-28 = 408 MHz and 408/4=102.000 MHz

To add the 102.000 MHz crystal is very easy because one end of each
existing crystals in the circuit is connected to the ground of PCB

To add the new crystal without removing any of the existing crystals i have
opened the ground end of  crystal 101.500 MHz and i have soldered
this end to a normally closed contact of a small single pole single throw
switch that i have mounted on the short side of the box just few millimeters
from the crystals.

Than i have soldered one end of the 102.000 MHz crystal to the PCB ground.

The other end of the 102.000 MHz crystal has been soldered to the normally
open contact of the above switch.

The common of the above switch has been soldered to the ground and every
soldered connection has been made with few millimeters of  silver plated
copper wire.

Since the difference between 101.500 MHz and 102.000 MHz is very small
hopefully the BF375 starts to oscillates to both frequencies without the
need to retune the collector coil everytime i switch the crystals.

Actually with 3 crystals in place my 432/28 S transverter covers between
432-434 and 434-436  MHz with the crystal switch in position 1

With the crystal switch in position 2 it covers 432-434 and 436-438 MHz

Because of the broad characteristic of the filters in the OL as well of the
70 cm filters the output power is sufficiently flat between 432 to 438 MHz
to drive without any transverter retune a very narrow cavity amplifier with
a tube 4CX250B at 300 watt RF output but here to switching the band
from 432 to 438 MHz it is mandatory to retune the grid and plate circuits of
the cavity amplifier almost every MHz

Unfortunately the Microwave Modules is anymore in business for Ham Radio
but i have two 432/28 S transverters since 1977 and they are working very
well.

Be carefull with 28 MHz driving power wich is adjustable between 5 mW to
500 mW max for 10 watt output on 70 cm

Be carefull because the input potenziometer function is reversed in place
and 5 mW input power at 28 MHz is with the pot all in CCW position !

If you need the 432/28 S schematic driagram and the description of the
circuit that was published in English by a special edition of UKW Berichte
please let me know.

I hope this helps.

73" de

i8CVS Domenico

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