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TROUBLE SHOOTING ANTENNA RELAY PROBLEMS
Written by: Mark Gilger, WB0IQK
This write up will cover the basics of how the PTT circuit in the TR-3 through TR-4CW and T4X series transmitters works. Non specific trouble shooting aids are supplied that pertain to any of these models. The PTT circuits in the different models are slightly different, but are functionally alike. This article teaches the basics and is not meant as a guide on specific fixes.
Before you can get any relay energized, you need a path for the current to flow through the relay coil. In this case itís designated as K1. In this first example, you have the correct voltage, +250VDC from the supply all the way to the plate of the tube. If the tube is cut off, there will be no current path to ground. There is usually a small negative voltage (-10 VDC) on the grid of the tube when in the receive mode. In this example itís pin #2. This cuts the tube off in the receive mode. If you ground pin#2, the tube will turn on and current now can flow. Once you have current flow, the relay will pull in. If you check the plate voltage, Pin #1 in this example, it will drop from itís normal +240 - 250 VDC to anywhere from 150 Ė 220 VDC, model dependant, when pin #2 is grounded. If you follow the circuit through to the mic connector, the grid will connect to the tip of the mic plug. When the PTT button is pressed, the tip is grounded and pin #2ís negative voltage now will go to zero, which now lets the tube conduct and pull in the K1 relay. The function switch will also ground this connection when itís moved to the X-CW position on the TR-3 through TR-4C, or the Tune position on the T4X series of transmitters.
T-4XB Relay Hanging
My T-4XB developed a problem where the relay would not deactivate for 1
- 2 second when going from transmit back to receive. I check all the
components, thinking some had drifted out of tolerance. I did find
several, which I replaced. The problem still persisted. I then
experimented with the value of R94. I found that if I lowered this
value, the delay went away. I replaced it with a smaller value, and it's
been working fine every since.