Scott you are correct on the waveform. I put a 1000 hz triangle wave
into the mic input with the mic removed. At low levels of audio input
and the Viking audio control level, the grids of the 807 tubes look more
like square waves and at a higher level they start to round off more
like sine waves. Really odd to me. I admitt I am not used to trouble
shooting tube circuits and AM trasmitters. Most of my radio work has
been with FM and repeaters as a hobby for about the last 40 years. My
work was in industrial instrumentation and controls.
I do have a well equipped service bench.
Here is a link that should let you see the scope pix.
https://drive.google.com/drive/folders/1kZ47jCk5ID9aSItp3nHKHJbmlJmyHO8r
?usp=sharing
The digital scope has its leads on the grids of the 807 tubes. I am
using both chanels with one inverted. That lays both chanels on top of
each other to let me see if there is much difference in the grids. They
seem to follow each other very close.
The analog scope has one channel going to a tap off of a dummy load for
the RF trace.
The function generator does show a good triangle wave when I check it.
When fed with a sine wave of 1000 hz the waveforms look ok. I
understand the difference in the sine and triangle and agree that the
triangle will show up distortion much better than the sine.
I ran some audio test with 3 of my local ham friends. They have been
licensed from about the mid 1950s and I know I can depend on them. One
was an engineer for Western Electric and Bell labs , so he should know
what he is talking about. He was using an old Collins receiver, one an
Icom 756 and one a Yeasu ft450d. They all said it sounds fine.
Thanks for the response.
de ku4pt
Post by Scott DorseyIgnore the wattmeter. Pay attention to the plate meter.
If the waveform you're putting in is symmetric (and a 1kc tone from a signal
generator is a good start), the plate meter should be rock steady. If it is
not, it is because something is becoming asymmetric (and I bet that the
waveform will NOT look so good on the scope when you check it).
First thing: check the plate and cathode resistors on the 807s. They
should be the same values top and bottom. Swap the two 807s and see if
the behaviour changes... see if the plate current goes up instead of down.
If so, replace the 807s (and the Chinese 807s seem to be okay). THEN,
check the quiescent current across the cathode resistors and make sure they
are the same top and bottom.
If all of these things are the case... then something is asymmetric in the
drive circuitry before T3. You mention that your 6AU6 had been replaced
with a 6AQ5.... I'd check that for symmetry... putting a triangle wave into
the input and then looking at it on a scope on the plate of the tube
driving T3. If it's lopsided, there's an issue with the first two audio
stages.
Totally off the wall one: The 6AQ5 needs more plate current than the 6AU6
and it's causing partial saturation of T3. I really, really doubt that's
the problem but if so, you'll see a nice triangle wave on the plate of the
6AQ5 and a terrible one on the grids of the 807s.
Even more off the wall one: the 6AQ5 stage is specifically set up as a
limiter to make the waveform asymmetric for more "punch" in a pileup.
The scope is your friend.
Post by Ralph MoweryIt does put out about 100 watts with around 250 ma of plate current in
AM and 125 watts with about 300 ma of plate current in CW, just as the
book says it should.
This one does have the 6AQ5 tube to clamp the screens of the 6146s.
Could this indicate weak 6146 tubes even though it makes the rated power
? Or should I look into something else, or normal for this transmitter
?
No, you have something weird going on with the audio deck, I suspect.
But the scope and a triangle wave generator will tell you for sure.