Time for some Homebrew Magic and Tribal Knowledge Tips. Several Blog posts ago I showed a notional schematic of a tube based Balanced Modulator. You can check it out. A key piece is the Balanced Modulator RF transformer.
The primary side of this transformer is in the plate circuit of the receiver mixer (6BE6) and is capacitively coupled to the crystal filter which in this case is a Heathkit Crystal Filter with a Center Frequency of 3.395 MHz.
[Note the 6BE6 is used versus a 12BE6 as this tube's filaments are in series with the 6JH8 as all other tubes are 12 VDC filaments. DC is used versus AC because we saw AC hum pickup by the 6JH8 in the 40M Wireless.]
The secondary is an interesting arrangement in that each end of the winding connects to the plates of the 6JH8 and plate voltage is shunt fed to these same plates via 47K resistors. Each Plate has a Capacitor connected from the Plate to Ground.
This is the tricky part as this arrangement has the effect of summing the balanced outputs from the 6JH8 which is then reflected into the primary side of the transformer and onto the Crystal Filter. Believe it or not but the two caps although connected to ground are in series and resonate the secondary at 3.395 MHz.
Two transformer windings and two capacitors play a huge role in the transceiver for both receive and transmit and this is one reason why I identified the Balanced Modulator as a key piece of this transceiver.
We must homebrew this BM transformer and I started with a 455 kHz IF transformer that required two modifications, the 1st of which is how to mount it to the chassis plate and the 2nd piece is to convert the windings to 3.395 MHz. Fortunately, the paper coil form has ferrite cores so the primary and secondary each can be peaked at the operating frequency.
IF Transformer and Spring Clip
IF transformers were mass produced, cheaply made and were set up for quick installation. The foundation chassis was punched so that the IF transformer fits in a slotted mount and a U shaped spring clip was placed underneath the chassis, passed through slots in the chassis and clipped into holes in the sides of the IF Can. It was a snug fit and the transformer was anchored solidly in place.
I attacked the mounting adapter plate as a 1st chore and a key tool is a set of digital calipers which are hawked by Lauen Bezos' OM. It cost a whopping $20.
Using this tool I measured the base of the IF Transformer and wrote a program for my CNC mill. Boom it was spot on like your feet in a pair of custom shoes.
Prototype IF Can Cut Out
Fit Check
Snug Fit
The IF can I chose out of a batch I have does not use the spring clip but has two lugs which likely were originally soldered to a piece of PC Board. Using my CNC Mill I will manually cut larger slots to accommodate the Spring Clips for the actual two IF cans.
The next step was to modify the transformer to work at 3.395 MHz and at this stage I removed all of the wire from the paper core and made two windings of 20 Turns each using #30 enamel wire. This was a starting point and no magic calculations involved. As a starter I tack soldered a 100PF cap across one of the windings to simulate the two capacitors in series. Using a signal generator and my 200MHz DSO I saw this resonated at about 6 MHz. I then tried a 330pF cap and tuned the cores and this is what I got. I did not forget that the impedance of the Heathkit filter is 2K which translates to about 94 microhenries. Thus 20 Turns might be a little light, which suggests perhaps more turns may be needed. Keep in mind our homebrew processes are often experimental.
3.395MHz
In the final build each capacitor would be 660pF as the series combo of capacitors are like resistors in parallel.
This is just a start and we might find that we will need further tweaking once installed in a circuit. At the very least we have a starting point.
This was a fast tour through fabricating a home brew Balanced Modulator and note no Nano VNA's were used to derive this answer.
The fabrication of this rig has relied on tools perhaps not found in every ham shack like digital calipers and CNC Mills. True my CNC Mill cost me $250K but you can buy a mill for around $300 and of course we know a $20 Bill will get you the calipers.
Them that know can make things go.
73's
Pete N6QW