[HBR] The long, SLOW HBR project

[Walt Hutchens]

> Unfortunately just about the time I got the 20M oscillator coil roughed out,
> I discovered I had some pickup of the BFO in the IF stages.  This had been
> hidden by incorrect adjustment of the AGC back apron control; it should be
> zeroed with the BFO off, then the BFO turned ON and tuned through the range.
> Any S-meter movement indicates leakage into the IFs which must be corrected.
> 
> This appears to be an easy one: It's going into the crystal filter area, as
> indicated by the fact that tuning the BFO displays the crystal filter shape
> and subsequent IFT adjustments do peak the signal.   About half of it went
> away with improved bypassing on the 1st IF cathode (suggesting that there
> may be a bit of BFO on the filaments); the other half will most likely be
> eliminated by a tinplate shield over the 1st IFT and crystal filter wiring.
> Unfortunately, this requires moving several other parts.

Well ... it wasn't an easy one.  The BFO signal definitely was entering via
the mixer/crystal filter area but getting it out of there was a bit of a
challenge. 

The 1st IF cathode bypass improvement was useful, the shield over the
crystal filter area made almost no difference, although a good idea.   I did
shield the mixer tube (new socket required) because this point is the input
to the IFs, the gain is pretty high and 1700 kcs is not a quiet frequency.

I had some power wiring run between the mixer tube socket and the IFT at the
input to the filter with the mixer plate lead looped right over it.  DUMB!
I relocated the power wiring to the other side of the mixer, between that
tube and the mixer coil socket.  Signals are larger here and on frequencies
far removed from the BFO.   I couldn't say there was any improvement but it
was the right thing to do, anyway.   Filters tend to be fussy and any stray
signal or coupling in this area is likely to cause hard to troubleshoot
problems.

Poking around with an extra bypass capacitor I noted that bypassing the
neutral lead to chassis ground at the mixer socket made some difference in
the BFO leakage.  In this receiver the B- power returns go in a star pattern
to line neutral which is bypassed to the chassis at the center of the star.
RF grounds go direct to the chassis, at each stage.    Stages are
transformer coupled to eliminate ground loops.   There really shouldn't be
enough of any signal on the neutral line to make trouble, however if there's
going to be, the BFO would be it because of the high output, and the problem
would appear at the mixer because this circuit (push-push dual triode mixer)
has a cathode that is 'hot' for RF.

The problem was, however, NOT coupling via the filaments because adding a
choke and bypass to the filament connection between the BFO tube and the
mixer made no difference.

I investigated the bypassing of the central neutral tie point.  I had used a
line-rated 0.1 mfd capacitor but adding a .01 disk ceramic across this
significantly reduced the leakage problem.   I then substituted a
combination of a different 0.1 mfd cap with a .01 disk and the leakage
disappeared.   Evidently the cap I had been using had a relatively high
impedance at the 1700 kcs IF.

The BFO signal had been reaching the mixer cathode NOT via the filaments,
but through the cathode resistor from the neutral line.   There is a
decoupling resistor/cap below the cathode resistor in that line, but it
evidently wasn't enough.   Better bypassing may be required there, as well.

The problem does seem to be completely gone with just the improved neutral
bypass.   But stray signals cause so much trouble that some overkill is
justified.  

Everything is well behaved now.  AND (I think) plenty of sensitivity -- I
was copying a VE talking to a G4 this afternoon and I could hear the G4
though he was on a dipole.   W1AW 20M code practice was hitting 80% on the
meter (copyable with no antenna!)  and the marker 4th harmonic runs 20-30%
with the RF stage disabled.    Before continuing with coil work, however,
I'm going to see if I can improve the shape of the crystal filter a bit.
One peak is higher than the other which causes uneven voice frequency
response.  

Tuning the 1st IFT gives you some shape control but the choices are a LOT of
difference between the peaks and more equal peaks with a big valley in
between.  

I need to recheck 80M to be sure I didn't break it.   There have been
several changes since I last listened there.

The BFO drifts a bit.    That should be pretty easy to fix --  the coil and
most capacitance is in an ARC-5 BFO can so it's all at the same temperature
as it warms up.

I really need to stop saying "That should be pretty easy ..."

Then measurements and clean up on the 20M coils and it's  probably time to
start looking at some temperature compensation.   The various sets of
coil-winding directions include this so perhaps it won't be too hard.  One
thing that will help:  Total power consumption is ~35W compared to 80W for
the real (W6TC) HBR.

Walt
KJ4KV