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Frequency, etc

Posted by Lew (A) on March 23, 2021 at 09:07:34

In Reply to: RE: Help with Beveridge direct-drive amplifier oscillation posted by fredtr on March 22, 2021 at 08:45:17:

The amplifier oscillates somewhere in the circuit that feeds the stators. The oscillation can be detected via an oscilloscope probe held within an inch or two of the 7.5M resistor (R1), seen in the schematic attached. (Can't make contact with the output circuit due to the very high voltages and lack of an HV probe.) The oscillation can also be detected, in phase with the field detected around the 7.5M resistor, with a probe touching the 1K resistor (R2) where it exits the (low voltage) input board. The wave form resembles a saw tooth. Frequency = 200kHz. Reasons to believe the oscillation arises in the output circuit: (1) I swapped input boards between the good and bad amplifier, and the problem remains, and (2) Using a dual trace oscilloscope, with one probe placed near the 7.5M (R1), and the other probe placed on the 1K resistor (R2), I started up the amplifier from dead cold. There is a delay before the onset of oscillation, and both channels detected oscillation at the exact same time. Wave forms are in phase.

The other amp of this stereo pair is configured exactly the same and does not oscillate, plays music nicely. I cannot find any difference between the two amplifiers in parts or circuit tracings. (I have actually replaced nearly every part on the oscillating amplifier output board, to no avail.)

There are two differences between both of my amplifiers and the circuit shown in the schematic. My amplifiers do not have the LED shown in the circuit that feeds g2 on the upper side of the schematic. And instead of a 4.7K resistor as shown, both of my amplifiers have 10k resistors going straight to ground. Also, R201 in the schematic is 22K, not 10K, in both the good and the oscillating amplifier. Schematic dated 1978. Inside my amplifiers, someone wrote "1979".

Edits: 03/23/21 03/23/21

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