Hi Guys,
My wife would probably say that the word unbalanced should appear in the
subject line of all my correspondence. However, in this case I am
refering to a T-4XB.
All this talk from Steve and the other guys about restoring their B-Line
gear got me to thinking about the B-line I have on a shelf here in the
shack. I acquired an R-4B and a T-4XB as part of a trade for some other
equipment about a year ago. Both units have issues, but they are in
pretty good shape cosmetically. The R-4B works pretty well, except 80
meters is dead. No big deal, probably a band oscillator that gave up.
The T-4XB is completely inoperative. So you guys inspired me to take the
T-4XB off the shelf and get started on it. After firing it up, I poked
around with a scope and found the PTO and all the pre mixer signals were
present and looking fairly reasonable. But there was nothing for them to
mix with as the carrier oscillator was dead. I did some resistance
checks around V1 and found only 22 ohms of resistance between the
cathode and grid. It should be around the value of R1 (220K) A visual
inspection revealed some corrosion on and around C2 (chassis mounted
trimmer) I shot some de-oxite in there and worked it back and forth a
litttle. The DC resistance was now closer to what I expected to see. So
I turned it on and put it in tune....voila... I have some plate current.
I found C2 was not salvageable because it was acting like two fixed
capacitances when I tried to adjust the carrier oscillator. All the
signal would be going through one sideband then as I adjusted C2, the
signal would jump to the other sideband (no in between). I have a T-4XB
carcass without many parts on it, but as luck would have it C-2 was one
of the parts still there. I installed it and was able to put the carrier
oscillator on frequency.
Next I tuned it up and it behaved pretty well except low power out
(about 80 watts on 40m). I wasn't alarmed as I haven't attempted an
alignment yet and who knows what shape the finals are in. But at least
now I can call it a transmitter instead of a paper weight. Next I
decided to try an alignment. Any further issues should reveal themselves
during an alignment. It didn't take long. I first tried to adjust the
carrier balance. I had a heck of a time finding the right combination of
C6 and R4. I put a scope on the output and finally got it close but R4
is all the way clockwise and the carrier wasn't completely nulled. I was
thinking maybe I was on the outside slope of the "M" if you know what I
mean. But I cant find another null more in the middle of R4's range.
Then, for some reason, I remembered reading somewhere that Garey (I
think it was Garey) tests transmitters sometimes by playing music from a
CD through a transmitter and listening to it on another receiver in the
shack. I thought this might be a way to figure out how close I am in
this adjustment (plus it would be fun). So, after configuring a hideous
combination of plugs, cables and adapters, I hooked my computer audio
out to the microphone input on the T-4XB. At this point I should say
that if you heard Al Green or Frank Sinatra (I know, I'm weird) on 40
meters last night it wasn't your imagination. I forgot to switch the
antenna to dummy load for the first minute or so. My wife stopped by the
shack to say good nite last night and I said, "Look, I'm picking up
Frank Sinatra on my ham radio receiver". She said,"That's nice dear,
Goodnight" and off to bed she went. It is hard to impress that woman.
The bottom line is, audio sounds great on USB, but a little tinny on LSB
and the carrier balance pot is all the way clockwise. I checked the
value of the resistors in the Carrier balance pot circuit and they
seemed ok but I checked them in circuit, so who knows. Any ideas on the
likely cause of this is? Balanced modulator diodes? C6? I could haul
everything out and check it one at a time but that would take forever.
Sorry for the long winded post but I enjoy reading other folks
adventures, so why not contribute?
I remain, as always, unbalanced.
Bob K6GGO
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