Lee -
Interesting. What is the serial number of your R-4B??
I haven't thought this all the way through, although I'm suspicious! :-)
R-4 receivers up through the R-4B, s/n 10501, had AVC applied to the grid of the Noise Amp tube.
Effective with this version, the AVC was removed, and feedback network of resistor/capacitor/diode
was added. Pretty suspicious!!
The output of the NB, V10b is a Cathode Follower, AC coupled to the input of the Passband Tuner.
When the NB is OFF, the Cathode resistor is just lifted from ground, so the pulses are coming
'through' the tube, just capacitively coupled Grid to Cathode.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
[email protected] wrote:
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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