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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.

 The Basics:

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.




 Typical Problems:

  1. The most likely problem is the Vox control tube (V19b in TR-3 to TR-4C models and V10b in the T4X to T4XC models). The tube (6EV7 or 6FQ7) gets gassy or weak and either the relay will not energize, or its operation becomes very erratic. Replace the tube with a new one. Do not trust a tube tester to tell you if itís good or bad. The best method is utilizing a NOS (New Old Stock) tube from a reliable source.
  2. The AC3 - AC4 power supply +250 volts supply could be bad. Make sure you have 240 Ė 260 VDC at the plate of the Vox tube. If no voltage is present, the diodes have probably gone out and will need replaced.  Also check the AC ripple. If excessive, the filter capacitors in the AC3 Ė AC4 need replaced.
  3. The AC3 Ė AC4 bias supply could be bad. The voltage should be around -40 to Ė60 VDC. If itís not, make adjustment to the bias adjustment control located on the supply per the instructions in the manual. If no voltage is present, the diodes have probably gone out and will need replaced.  Also check the AC ripple. If excessive, the filter capacitors in the AC3 Ė AC4 need replaced.
  4. The Anti Vox potentiometer is not adjusted correctly. If the Anti Vox is adjusted to sensitive, it will not let the transmitter relay to turn on. The Anti Vox pot needs to be adjusted so that when receiving a station, at comfortable listing levels, you can activate the relay.  In the above circuit, you can see where the Anti Vox signal ties into the same point as the PTT from the voltage generated in the audio amplifier. If itís adjusted to sensitive, you will notice when you turn the volume all the way down, the relay works.
  5. The circuit path between the mic plug tip and the grid of the Vox tube is not good. You can take an ohm meter and verify there is very low resistance ( <1 ohm) between the mic plug and the grid input resistor, R142 in the TR4C. Your resistance should change as press the PTT button.
  6. You can take a jumper wire and with one end grounded put the other end on the grid input resistor, R142 in the TR4C. With a good 250 VDC on the plate, the relay should pull in. If the relay does pull in, but does not when using the Mic, the problem is in the PTT line of the microphone.
  7. If the relay seems to be activating ok, but intermittent results are obtained, cleaning could be in order.  See the separate write up on ďRelay CareĒ which covers cleaning on the relay contacts.
  8. If the relay hangs up, even after the tube has been removed for trouble shooting, the following possible fixes were suggested have been made others.
  • Check the wiring from the relay to the tube plate circuit, looking for any insulation problems. This could cause shorting to ground and thus causing current flow through the relay, pulling in the relay.

  • If you can tap the relay and the contacts release, suspect possible mechanical binding, or contact magnetization. Please note that mechanical problems are rare.

  • On rare occasion, relay contacts have been known to get magnetized. When this happens you will need to replace the relay or try and demagnetize it, possibly with a degaussing tool. Itís unknown if the degaussing tool will work.

  • Check for carbon tracks on the tube and relay socket. Clean with Denatured Alcohol and a tooth brush. The socket might need to be replaced.

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.