[HBR] Another Receiver Project -- HBR-4, Part 16

[waltah at earthlink.net]

Sure enough, the audio clairity problems (note plural) were in the 
6J6 VFO tube.   

Heater-cathode leakage was a big part of it.  To deal with that, I 
added a voltage doubler rectifier circuit running from the 6.3V 
filament line to give -17.5 volts and hooked the VFO cathode and 
grid resistors to that.  Allowing for the drop in the cathode resistor, 
that puts the cathode at about -12V; with a 6.3VAC filament, the 
cathode is always negative with respect to the filament and thus 
draws no current.  (There is, of course, still some capacitive 
coupling.)  Audio quality went from 'okay' to 'very good.'   

But there was still some jitter (visible as a few cps wander over a 
period of a few seconds) in the frequency and clairity still wasn't 
great.  I had used the fixed caps that came first to hand; now I 
went back and replaced them with air trimmers and ceramic caps 
known to have high voltage ratings.   The first couple of 
replacements made no difference but when I changed out the 5 
mmf N150 cap at the 6J6 socket, BINGO -- things got much better. 
 It appears that that cap -- which was on the small side and had no 
markings I could translate to a voltage rating -- was breaking down.

Clarity is now excellent.   We're back to square one on warm-up 
drift, but that's a straightforward problem -- just time consuming.   
And I'm not entirely sure about VHF parasitics:  The frequency 
counter does some odd things when waved around the oscillator 
circuit.  There was no problem before but the 6J6 will oscillate to at 
least 500 Mcs -- it was one of the very first such tubes -- and the 
circuitry is unavoidably fairly spread around.   Perhaps the bad cap 
was providing enough damping that parasitics were suppressed.

More to come ...

There was an interesting problem with the voltage doubler supply.   
When I hooked it up, suddenly there was a low level 60 cps hum 
under no signal conditions.  I had taken the AC from the filament of 
the BFO tube and sure enough there was no hum with the BFO off. 
Hummm ... the cathode of that tube is 'hot' for BFO RF.   The RF 
was getting from the cathode to the filament via leakage and being 
modulated by the switching action of the rectifier diodes.   Adding a 
right-at-the-socket filament bypass cap solved the problem.

I also tried biasing the cathodes of the band crystal oscillator.   
That had no effect.

I've been cleaning up details.   The internal loudspeaker (standard 
FT-101 under-the-chassis layout) just needs an RCA plug.   The 
BFO is hooked up to the very-well-filtered B+ for the detectors and 
low level audio stage but it pulls the voltage down enough to reduce 
the undistorted audio range; there isn't another suitable very well 
filtered line in the set so I'll have to add one.

The band crystal oscillator drift problem has to be dealt with -- that 
should be just adding loading resistors to the trimmer caps to lower 
the tank circuit Q so the crystals have more control of the 
frequency.

A parallel-tuned circuit to give a peak to the audio at around 800 
cps when in CW mode would be useful.  I wonder about that little 
Radio Shack output transformer as an inductor?

Then testing -- dynamic range and sensitivity.   There's space for 
an RF stage and much as I'd hate to do it, I'll put it in if the 
sensitivity isn't adequate.   

Getting down to the end now.

Walt
KJ4KV