Drake Enthusiasts,
While working on a Drake R-4A..(11 tube) .I discovered something that
bothered me...and thought for sure it was a problem needing fixing!
Yeah...I do like things to work well.
OK...find a nice STRONG signal...maybe S-9 ++ (20 over) tune it in it
sounds great...OK...now carefully tune to the opposite sideband of that
same signal.
NB is off...not that it matters, now...listen carefully...of course you
hear the garbled opposite sideband...BUT also you hear a lot of
clicking/arcing like noise on voice peaks especially.
Loud enough to be a bother....IF you PULL the Noise blanker tube V9 and
or V10...the noise goes away! Huh! the NB is OFF, so why do we hear
that racket?????And why does pulling the tube fix it?
Well go off looking to see what's wrong so I can fix it....spent quite
a lot of time....looked at lots of things with the scope.
Discovered what was going on is:
If you scope the grid or plate of V9 the 50 kc NB amp...you see a
relatively small signal when the signal is tuned into the correct
SB...BUT if you look at it when tuned to the wrong SB...WOW huge signal
level (maybe 5-10 times as strong) ...and that signal level is getting
through the NB amp, shaper and amp...and clamping the plate of V4 the
IF amp right before the PB tuning...thus creating the annoying clicking
sounds. That's why it stops when you pull the NB tube(s).
So, got to looking at the AVC signals and how it works at the grid of
V9 the 50 kc amp going into the NB section....found out the AVC does
not throttle back as much with a wrong SB sigal as the properly tunes
one! So the signal.
Why is this a problem since we don't really listen to the wrong SB?
Well, IF you are listening to a nice signal AND right next door a
signal is pounding in and the RX hears the wrong SB for it (and it will
based on the location as to the good signal you want) you get the
clicking etc that is loud because the interfering signal is strong.
Now that I understood what was happening...I looked for a bug....not
easy to find for sure....
Ah Hah! I thought let's look at my R-4B I have here to compare the
scope patterns and voltages in question. Maybe I'll learn something
helpful?
And does my R-4C do this?
Answer is: Oh no...the R-4B does exactly the same thing! (R-4C does not)
This R-4A is not really broke! or both the R-4A and the R-4B I have
here are both broken??
Looks to me like a design issue in the AVC circuits.
1) Hey gang...try iy out, let me know what you discover. Please.
2) Anyone already fixed this with a MOD?
Thanks,
73,
Lee, KC9CDT
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