[HBR] Another Receiver Project -- HBR-4, Part 12

[waltah at earthlink.net]

In order to do a decent job balancing the mixer, I bifilar-wound the 
plate transformer plate halves, then wound a secondary (to drive 
the crystal filter) over them.   I had the differential trimmer cap from 
the FT-101 VFO (couldn't use it for a push-pull VFO) so I put that in 
for adjusting the RF balance and I increased the range of the DC 
deflection plate balance by using a larger pot.  

(Dumb-de-dumb-dumb -- I forgot to put a piece of paper between 
the primary and secondary and there's 130 volts difference there ... 
Two hours work to rebuild that transformer if the insulation on my 
Radio Shark enameled wire isn't quite perfect.   But okay so far.  
The decoupling resistor for the stage is basically a 30 mA fuse, so 
no great damage will be done.) 

The test of mixer balance is what happens when you feed a strong 
9 Mcs signal in at the antenna jack.  Since that signal isn't 
converted, it isn't push pull at the mixer plates -- it's the same at 
both plates and should cancel out in the plate transformer.   I really 
didn't have that before.

Got it now though -- what a difference.   Setting the bandswitch to 
30 meters and running the preselector down to the lowest 
frequency, I can get the 90th harmonic of the calibrator at S9+20 if 
the mixer is way out of balance.   Swing the pot around and 
suddenly the signal vanishes.   The first time I hit the right setting, I 
thought I had shorted something.   

The balance circuit is:  100k resistors from each deflection plate to 
ground.   470k resistors from the deflection plates to the ends of a 
1M pot.  Rotating contact of the pot to -70VDC bias.   With the pot 
centered you get a bit less than -7V bias on each plate.   Balance 
for this tube occurs with a difference of about 3 volts between the 
two -- a surprisingly large difference.   Checking eight 6JH8's in that 
socket I found that the differences went as high as 3.5 volts with 
only three under 1 volt.   One, however, was 0.085V!

The RF balance differential cap also has an effect but winds up 
near center position -- no more correction than you'd expect from 
parts tolerances, interelectrode capacitances, and wiring 
differences.

I need a smaller value pot -- the 1Meg pot can't be precisely 
adjusted.   500k or possibly 250k should do it.  This is a 1/2" 
diameter pot with a 1/4" bushing, not the most popular item.   My 
stock skips from 50k to 1Meg ... but I have a few more places to 
look.  

I also need to select a tube that balances with a smaller difference. 
250k will only give a 25% difference -- less than 2 volts.   

This would actually be a good place for a multi-turn pot since you 
need both considerable range and accurate setting.   I might have 
one that would work.   Also I could put the pot on the ground side 
rather than the bias voltage side -- that would let me use lower 
range pots.   Yeah, there's an answer in there somewhere.

Just as we all discovered when sheet beam tubes were popular as 
balanced modulators in ham SSB transmitters, the null drifts 
considerably.   It looks to me like you can count on 30 to 40 db, 
but the 60 db I've seen quoted in a few places is only while you 
have the screwdriver in your hand.

The main importance of good mixer balance is rejecting signals (of 
all kinds) at the IF coming in via the antenna.   30+ db is a great 
addition to the rejection due to the two front end tuned circuits.   
Adding a trap is certainly possible as the final step.

Walt
KJ4KV