"Dave Goncalves" wrote in message
...
Hello,

I have a 2A that I was unable to get the passband tuner
setup so that
the pitch of the noise was equal on both sides of the
center.

I setup a sweep generator, 0.1v pk-pk, sweeping from 40 -
60 KHz, in
on pin 5 of V4. I setup an oscilloscope on pin 1 of V5. A
marker
output on the sweep generator is used as a trigger.

Doing this, I can clearly see a narrow passband that is
shifted around
by the twist of the knob. On the 4.8 KHz position, the
passband is
greatly widened, as expected.

Now, back to the narrow mode. There is ALOT of ripple on
that
passband, and the sides are not symetrical. One side had a
slow
rolloff, while the other had a far faster one. That
explains the
greater amount of high frequency hissing in one sideband.

I opened the tuner. Four slugs are ganged together and
mounted on a
spring machanism. Each slug is mounted on a coiled wire,
which is
soldered to the part that moves them all together. Unlike
the 2B and
2C, the slugs are not adjustable. I really want the set to
work
correctly, so here is what I'm thinking of doing:

Take out the passband tuner, mount it to a wooden board.
Take the
front cover off of the tuner, and install a simple
nut/bracket to
simulate the action of the tuning knob. Unsolder and free
the slugs.
Using the same sweeping mechanism, adjust the slugs for
2.4 KHz, 6dB
voltage, and as flat on the top as possible. Solder back
into place,
reinstall into the radio.

BUT! I remember basic troubleshooting. Failures have a
cause. WHY did
the passband tuner to out of tune? Caps in the circuit
failed? A short
somewhere? I'll find out, but I am wondering if my method
here makes
sense.

Comments?

--
David Goncalves
W1EUJ

My R4-B is similar. Following the instructions gets it
usable but certainly not symmetrical. I have not yet gotten
at this in depth partly because I don't have a sweep
oscillator and I think one is probably necessary to tune
this thing properly.
If you don't know the history of your RX (meaning bought
it new) its quite possible someone else tried tuning it.
Also, perhaps there was a little mechanical drift in the
permeability tuned coils. I wish there was more service
information available for Drake stuff. I am about to
undertake fixing what I _think_ is a problem in the balanced
modulator of a T-4XB, acts like there may be a bad diode.
There is a Drake mailing list. I joined it but after getting
a confirming message there has been no traffic whatever.


--

--
Richard Knoppow
Los Angeles
WB6KBL