Been diagnosing somewhat low sensitivity on my Drake 1A. I think I reported
here that I replaced all suspect paper caps with metalized Polypropylene
"yellow jacket" types. I saw no improvement with the new caps. In a
nutshell, loss of gain was affected by the severe drift of two 33K
carbon-composition resistors, one on the screen of V2, the other on the
screen of V4. Both resistors drifted up to about 105K. I replaced both
with 1/2W carbon film types and wow, what a difference! Now, I need to use
that darned 30dB attenuator even on the high bands! Proper screen voltage
is an absolute must in the early RF and IF stages in order to extract
required gain. I can now think of a few other boatanchors of mine that
likely need the same treatment.
Other resistors, (including other values of screen resistors on the other
tubes) are off a bit as expected since the set is 50 years old, but for some
reason the 33K resistors drifted way out there. So, I highly recommend that
1A owners check those values to determine if they've drifted.
Had another quirk: As I rotated the antenna trim, I noticed that Rx
sensitivity would change if I gently pushed the control. I noticed that the
ground frame of the air cap trimmer is connected to a small copper mounting
plate. That plate is connected to yet another plate, and that plate is
connected to the chassis, relying only on the hardware to make for a low
impedance
connection to circuit ground. Rather than fight the copper and hardware
corrosion, I noticed that the trim cap has an unused solder tab on its
ground frame. Easy fix: Just bring the tab down and solder it the mid
plate. I then soldered the mid plate to the copper chassis. Problem gone.
Sure makes soldering easy when the chassis parts are all copper!
I have never been a believer in allowing the chassis to function as an RF
ground return, especially on SO-239 connectors. Those connectors really
need their own reliable return path to a solid circuit ground point. Not
sure how that practice started or when...
Darn, I almost forgot how easy it is to troubleshoot VT receivers since so
many parts are unterminated at the VT. Unlike solid-state circuits, few
parts require removal of one end. That greatly accelerates the
troubleshooting process.
Also, I am going to add a 110V Zener diode at the VFO's 105VDC B+ line. The
current limiting resistor is already there going back to the +105V rail. My
utility AC lines voltage runs high at 125V so using a Variac, I dropped the
AC line voltage down to see where I achieved 105VDC. It occurs about
110VAC. I'm doing this to help stabilize the VFO by better regulating B+.
The 105V supply rail from the PS only goes to the VFO, although it's part of
a resistive divider feeding B+ to other tubes. I doubt it will help too
much as other circuit factors likely mask any improvement, but it does seem
like the right thing to do.
About a year ago, I purchased a couple inrush limiters from Electric Radio
Magazine. They do a nice job of bringing back high utility voltage down to
about 115V where this equipment really belongs. I could add the same
circuit to the receivers, but the external solution is non-invasive and
probably helps to preserve the value of the 1A.
Paul, W9AC
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