Drake TR4CW keying problem.

[holden]

Troubleshooting a Drake TR4CW, the break-in keying starts out working OK
but within a few minutes it fails, the relays dropping out within a very
short (less than one second) time after the key is hit. Relays are OK as
PTT works fine. Looked at the relay tube 6FQ7, and the voltage on the
grid of the relay driver is OK at first (-6V no key, .36V key down),
then it deteriorates eventually to about -7V no key, -6V key down so the
tube stops conducting. Found a bad VOX pot, but replacing did not solve
the problem. All associated resistors and resistor measurements on the
Drake chart look OK, both the 6FQ7 and 12AV6 sidetone osc tubes test OK.
Bypassed the coupling caps in the sidetone output and relay driver
circuits. Anyone have any hints out there? Also, the BAMA site only has
the schematic for the TR4C, the TR4CW circuit looks to be slightly
different in this area.

Howie WB2AWQ

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