>...The fact it appears
>to be unmodulated and on an odd frequency (non-10 kHz
>multiple) may make it difficult to identify. It could
>even be a ham's transmitter accidentally in tune mode.
That reminds me of one of the most bizarre cases of interference and
Murphy's law I can remember. Back in the mid 60's I had a homebrew rig on
160-80-40. It used a single 812A in the final, driven by an 807, which was
driven by a 6V6 vfo buffer. The rig used plug-in coils.
One day I noticed a strong, stable carrier right in the middle of 160 (that
was back in the days when the total 160m band in eastern USA was 1800-1825
kHz, and the power limit was 200w day and 50w night). This carrier had no
QSB, and was the same strength day or night. It seemed to appear randomly
anywhere in the band, but did not drift. I must have spent several hours
total trying to chase it down, to no avail.
One day I just happened to turn the knob on the tank circuit adjustment of
the 6V6 buffer stage on the transmitter, and the signal went away. Upon
closer investigation, I found it didn't go away, but had shifted frequency.
Quickly I discovered that the plate tuning knob on that buffer stage was
acting like a very stable VFO and laying a strong, clean pure DC note in the
band. I pulled out the 812A and then the 807, but the signal didn't go
away. Then I pulled out the 6V6 and it was gone. The 6V6 was
self-oscillating...
But the bizarre thing was that all DC voltages to the transmitter were cut
off during receive, using a realy that opened the primary circuits of all
plate transformers. The only thing that was energised during receive was
the filaments of the tubes. The transmitter used a 5R4 rectifier for the
high voltage. When I pulled the 5R4 rectifier, the signal went away. So I
measured plate voltage on the 6V6, and noted that it had about +15 volts DC
on the plate during standby, even though the primary of the plate
transformer was disconnected from the line.
Apparently, electrons from the hot filament were migrating to the
unenergised plates of the rectifier and developing a low voltage DC
potential, even with the plate transformer cut off. The filter capacitor
would accumulate enough charge through the continuity of the plate
transformer winding to raise the plate voltage to the point of
self-oscillation, when there was no external drive being fed to the grid. I
fixed the problem by replacing the 6V6 with a 6AG7, which has better
shielding and lower interelectrode capacitance, and therefore less tendency
to self-oscillate.
Don K4KYV
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