[HBR] Another Receiver Project -- HBR-4, Part 6

[waltah at earthlink.net]

The VFO was surprisingly easy and at constant room temperature 
drifts less than 50 cps/hour after just a couple of minutes of warm 
up.   Chalk one up to using a really high gain tube (6GK5, 
transconductance 15,000) and coupling very loosely to the tank 
circuit.  

Speaking of tubes, looking for an oscillator tube in the RCA manual 
I ran across the 6FS5, a beam hexode for VHF RF amp service in 
TV sets.   Beam hexode????   Does anyone know anything about 
this tube?    Seems like anything that odd ought to be good for 
something!

The 12AT7 Butler oscillator supplied the usual quota of new 
challenges.  Surprisingly enough it perked right away with the 35.5 
Mcs  (15 meter) crystal, but using my junkbox 18 Mcs HC-6 it 
wouldn't seem to go.   

There was grid current though, and after about two hours and 50 
minutes more than it should have taken, I realized that that had to 
be a VHF oscillation, switched my Heath IM 2410 counter to the 
high band, and there it was, running right along at 165 Mcs.  That 
had to be the resonance of the leads to the trimmer cap; a 100 
ohm resistor at the grid pin killed it and after that, things started to 
fall into place.

The trick with the plate coil is to get it the right size so it will tune 
the highest frequency (43.5 Mcs for the last 500 kcs of 10); too 
much inductance and the stray capacitance won't let you go that 
high.   But it must also go down to 16 Mcs for 160M while still 
having high enough impedence that the circuit will oscillate.  
Needless to say I had to cut it three times and rewind once to get 
it right.   

I'm using most of the larger trimmer board from an FT-101 -- 11 itty-
bitty 50 mmf trimmers on about a 2-square inch piece of fiberglas 
epoxy circuit board.   Saves tons of space but boy do those things 
tune sharply on the higher bands -- just like on the FT-101.  I 
believe the E and F used smaller trimmers for the highest bands.   I 
found it was best to set the frequency with a dip meter before 
turning on the power; that way it would usually show grid current 
and I could peak it.

That circuit can't be completed until the crystals arrive in about 
another ten days but it's fine at 18 Mcs, 35.5 Mcs, and (thanks to 
our CB friends) 41.5 Mcs.

The plate coil is a toroid; the secondary is bifilar wound with a 
grounded center tap; each half drives one grid of each of two 6J6s 
connected in balanced (pre) mixer circuit.   (The input to the other 
grid on both tubes is the 5 to 5.5 Mcs VFO.)  The plates of the 
premixer tubes drive the deflection plates of the 6JH8 signal mixer 
which feeds the 9 Mcs IF -- already wired and tested except for the 
crystal filter.

In other words, I'm getting near the part where the rubber meets the 
road.   I think the pieces will all work but the *system* can still be 
a flop in various ways.  With luck I'll get a read on that, this 
weekend.   

There is the usual amount of cleanup work -- there's too much 
audio low end, for example, I don't have the calibrator wired yet ... 

Walt 
KJ4KV