Hello,

I'm trying to fix a Drake TR6. The receive part is working quite well.
It is powered with an AC4 power supply and
I changed all capacitors and diodes on the AC4 with new ones, I checked
all voltages and they looks ok, bias
is about -57V in order to have idle current on finals to about 100 mA.
The problems seem to be in the TX/RX relay commutation. When ptt is
pressed (in SSB mode) the relay
commute but with a buzzing sound for a split second, then stabilizes.
During the buzz the s-meter goes
to the end of the scale, then it goes off as it should. Using vox on cw
does the same thing. I checked
DC voltages on the 6EV7 tube and they agree with the chart reported on
the manual.
I have no new 6EV7 so I tried to use an ECC85 instead and after one or
two rx/tx commutation the
relay gets stuck in tx mode so I have to power off the unit to release
it. I thought about a possible parassite
oscillation on the vox amplifier triode and tried to disconnect C141,
but the behaviour is the same (6EV7
buzzing before commutation and ECC85 gets stuck in tx). I changed also
the bypass 10 uF electrolitic on the
vox amplifier cathode , but obviously it didn't help. Now I don't
really know what to do next. I'm trying to find
a new 6EV7 but somehow I doubt it's really the tube that generate the
problem, the ECC85 parameters
are enough close in my opinion and it shouldn't behave so badly when in
place of the 6EV7. All resistors around the vox amplifier and relay
amplifier measure correctly. Can be a defective relay? What else should
I check?
Thanks in advance.

Francesco IS0FKQ

Clean the relay first -- it is the most likely cause of your problems.

Irv VE6BP
--


Also, clean the PTT switch contacts inside the microphone.

73,

Roger K6XQ