Update: SB-401 Chirp problem on CW

[Mark V. Johnson]

For those interested....
I was getting reports of "Chirp" on CW when using my Heath SB-401/SB-301
combination in transceive mode. Reports came when using it on 15 and 20
meters. Could have been a problem on other bands but those were the
bands of most concern.
When I put the transmitter in Unlock or standalone mode (het osc signal
from SB-301 not used) the transmitter was fine. What I did was put the
transmit signal into a dummy load and listened on another receiver. You
could hear the chirp or frequency shift.

Long story short, I determined that the minus 90 vdc in the receiver was
going up in voltage when the mute was applied. In other words, when you
keyed the transmitter  (in transceive mode), the mute line was activated
which caused the negative 90  vdc voltage in the receiver to climb. When
you stopped keying the transmitter, the mute was removed and the minus
90 vdc went down as load was  applied. I actually replaced all filter
caps in the bias circuit of the SB-301 thinking one may be leaky. No
change. I concluded that the circuit was working as designed. It is an
unregulated power supply. When I removed the mute cable, the voltage
stayed constant and my transmitted signal did not chirp!!.

In the end I found that careful adjustment of the Het  Osc coils in the
SB-301 eliminated the chirp.  Seems that if you over drive the Het Osc
amp (6AW8) in the SB-401, the signal will pull or chirp. By listening to
my own signal while transmitting into the dummy load, I adjusted the
SB-301 het Osc coils for proper drive (minimum required) and cleanest
sounding signal.

Also...Old version of SB-401 manual has you adjust Het Osc coils in the
SB-301 for .5 MA drive during the alignment. Later version of SB-401
manual has you adjust for max grid drive....I think the old manual was
closer to the truth..At least based on my experience.

As a final  test, I put my dual trace scope on the Het Osc output from
the SB-301 and the actual output of the SB-401 Het Osc amp.. I thought
I'd adjust the Het Osc coils in the SB-301 until I saw clipping or some
such dirty signal output from the SB-401 Het Osc (6aw8). No luck....All
 I did was bring the chirpy signal back.  This exercise also pointed out
to my just how crappy the Het Osc ouptut is from the SB-301 on the 4 Ten
Meter bands. Thought I had another problem. Even though the Het Osc
output on the 4 bands was real crappy, the actual Het Osc amp output
signal in the SB-401 was fine. I put the dual trace scope on  each end
of the small internal coax cable in the SB-301 (between output jack and
the bandswitch) and it was amazing how the signal was very different
across 6 inches of cable!!! Even replaced the cable...no
change....Again...working as designed.

As final action, I adjusted my SB-401 VOX delay so that the transmit
receive switching was acceptable for operation without excessive
switching. This  switching does cause a bit of chirp or pull as the
Sb-301 voltages are not regulated. So far, on air reports are fine....

TNX for the bandwidth. If  there are any suggestions from the Heath
team, please feedback to me.....TNX.

MV Johnson VE3DJU/VE3DDI

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