[HBR] The long, SLOW HBR project

[Walt Hutchens]

Peter said:

> Thanks Walt. This reminds of what ham radio was back in the sixties...

This isn't the sixties?   No WONDER Sharyn says I'm out of date.

I guess there aren't gonna be any more Liz Taylor or Marilyn Monroe movies,
are there?

The warble turns out to be a real challenge.

Things it seems NOT to be:

1.  60 cps getting into an oscillator -- at least not in any obvious way.
The reason it appeared to come and go is that it only appears on a beat note
when that beat is close to certain frequencies.   In between frequencies are
perfectly pure.  Also the frequency seems to be lower than 60 cps.

I think this makes it likely to be some sort of feedback effect.

2. Mechanical, resulting from the use of a panel mounted speaker.
Disconnecting the panel speaker and using one sitting on the table makes no
change.

3. Mechanical in some other way.  I spend several minutes touring the set
with a plastic stick touching various parts and wires and nothing makes any
difference.   

4. In the audio section.   Feeding the volume control with an audio
oscillator shows no such effect at any frequency.

It does seem, however, that the audio section could be a co-conspirator.
What stages KNOW about a beat note?   And the audio section is where the
power is.   

On the other hand, the volume control doesn't seem to have much effect.  If
the audio power stage was (say) feeding back through the plate voltage you'd
expect the effect to disappear at low volume.

There is a potential feedback path from the audio detector to the AGC
detector because they both use the same negative supply.  Hummm ...

5. An obvious weakness in HV supply/filtering.    I toured this area with
another large filter cap, shunting each in turn.   No change.

6.  Involving the AGC circuit.  A substantial increase in the AGC time
constant made no difference.

Time, I think, to break out the 'scope and look at all the voltages.

> very informative and educational, and fun to read. Thanks for sharing.

Thank YOU.  More to come.

Walt
KJ4KV









very
> informative and educational, and fun to
> read. Thanks for sharing.