Hi Jim -
Some further information on the "oscillating" xmitter. With the key
down and in the CW position, with gain pot at max, the xmitter
"oscillates" with one second not xmitting, two seconds xmitting with
full power out. The anti vox pot setting will vary the frequency of
"oscillation".
This sounds like a 'gassy' 6EV7/6FQ7. It MAY be C107 in the
transmitter. The DC integrated in C99 'competes' with the DC integrated
in C93 for control of the Grid of V10b. There are several large value
carbon comp resistors involved, which may have gone up in value,
disturbing the 'balance of power'. A leaky CR13 and/or CR16 can also
upset things. Leaky caps in either of these positions could be a
problem, but I have seen VERY few cap problems.
Now, if I key (not the cw key) the transmitter with an external
switch, the output power is full and of course the xmitter does not
"oscillate" but stays in the xmit mode as expected. When I use the
external switch to unkey the xmitter, it goes into the "oscillation
mode again.
CR17 / C115 generate a 'pulse' when the key is first closed to yank the
relay closed quickly to avoid the 'dropped first dot' problem. So the
relay pulls in, but relies on the voltage integrated in C99 to 'hold' it
in.
I also have a question on the gain pot. It has always been
"non-linear" in that it does not start to produce power until about
the 1 o'clock position when in the tune positon.
The GAIN pot is actually two sections. In TUNE position the MIC gain
section varies the SIDETONE level applied to the Grid of V9b where it is
amplified, rectified and integrated in C99. (See above). A leaky diode
at CR15, which 'keys' the sidetone for CW via control of R84/R85 vs
R101/R102 can cause a reduced level vs pot rotation. Again, high value
resistors are suspect, which will alter the balance. The GAIN control
should reach full output at 11 o'clock in TUNE, where the ALC will
'flatten it out' for further rotation.
When in the CW position and the rig keyed with CW key, there is output
power (5 watts) when the gain pot is at zero. When you advance the
gain pot, the output power DROPS to zero watts at about the 9 o'clock
position, and then stays there until the 1 o'clock position. Is this
typical?
In CW position, the 'other' section of the GAIN control (R105) in
conjunction with R104/R106, apply a DC variable DC bias to the diodes in
the Balanced Modulator, unbalancing it to let the Carrier Oscillator
pass through. The more DC bias current through the diodes, the more
unbalanced the BM becomes, adjusting the level of carrier. What you are
seeing makes me suspect a dirty pot or element shorted to case.
Output power is good with 140 to 150 watts watts out on 40M with 300
mils of plate current and 100 watts on 10 meters with less than 300
mils of plate current.
Sounds good!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-B, C-Line&
TR-4/C Service Supplement CDs
<www.k4oah.com>
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