Author Topic: Fixing Tektronix RM503 (Read 293 times)

Henri · « **on:** April 05, 2024, 07:37:14 pm »

Hello, I have a Tektronix type RM 503, the power supply section stopped working after some use, and the Armstrong oscillator does not start oscillating on power up. I have already replaced the 6DQ6A tube, 5651 tube, C624, and R626 which was burned. On power on, the anode of the new 6DQ6A starts glowing cherry red after a minute and gets way too hot in my opinion so I switched back to the tube old tube which does not glow so hot. I have checked almost every part in the power supply section, I also have measured all of the connections on T620 all of them have a reasonable resistance. still can't find out why it does not work.

Henri · « **Reply #1 on:** April 05, 2024, 08:26:38 pm »

Any ideas on what is wrong? Here is the schematic:

Henri · « **Reply #2 on:** April 05, 2024, 08:31:10 pm »

Here is the schematic:

CaptDon · « **Reply #3 on:** April 06, 2024, 02:32:42 pm »

When the circuit is not oscillating the control circuit allows for high screen voltage causing heavy plate current draw. The new tube probably has red plate glow probably because it has excellent cathode emission. The old tube is probably about 3/4 dead with low emission, so low that the plate doesn't cherry. Even the slightest thing like a diode on the secondary with high reverse leakage will kill the oscillations. An excessive load on any of the supply voltages on the output of the transformer would also kill the oscillations as would a shorted turn within the transformer. It is really a self excited flyback circuit similar to that of an old C.R.T. television. As the output voltage rises the feedback circuit will pull the screen voltage down and become a stable regulation circuit. With main power DISCONNECTED you might try pushing a 10KHz audio sinewave into the plate lead of the 6DQ6 and run the audio voltage slowly up to maybe 50vrms and have a look at the rectified output on the secondary side of the flyback. In normal operation the P-P voltage on the 6DQ6 plate lead probably approaches 1000vpp. I have a nearly indestructable 100 watt RMS audio amplifier that I use for this type of signal injection testing. When I need larger P-P voltage I use the amplifier to drive a 120 / 24 volt transformer backwards easily getting over 200vpp from the transformer. Even though the flyback is a pulse transformer it will respond well to a sinewave. Cheers!