Bent - Several possibilities.
One is an 'open' component in the Cathode circuits of the PA. IF there is +650V on the Plates, 270V on the Screens and the Cathodes are grounded (via a relay contact), the PA tubes WILL draw current, regardless of the rest of the radio. This current would peg the meter, and blow the fuse on the power supply, so a negative voltage is applied to the Grids of the PA tubes to limit that current. Typically, around -60V on the grids will limit this current to about 100 mA for the three tubes.
Obviously if either the Plate or Screen voltage is missing you will see no current. OR if there is an open RF Choke or bad relay contact.
The fact that varying the BIAS voltage causes the relay to click is because the BIAS voltage is used to turn the relay OFF in the absence of VOX signal.
I would start by measuring the Plate, Screen, Grid, and Cathode voltages on the PA. Again, if these voltages are there, and the Cathode voltage is near zero, the PA tubes WILL draw current.
73, Garey - K4OAH Glen Allen, VA Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line and TR-4/C Service Supplement CDs <www.k4oah.com> Bent wrote:
Hello All I got a problem with a TR-4CW. When I switch from SSB to X-CW I only get a very little bias current on the meter. Less than 50mA. Even when I turn on the Bias potentiometer on the AC-4 nothing happend on the meter. But when I turn the Bias potentiometer back and forth around -55 VDC I hear a weak click inside the TR-4CW. It sound like a relay is activated or something like that. But I can not achieve 100 mA as required. Is someone able to inform me where to look. Thanks in advance. Br. Bent OZ5ZD
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