My Tek 547 stopped working (no light on the screen). All voltages were OK except the CRT voltages. These are generated by a 6AU5 oscillator, which didn't oscillate. Measuring with a sine on the grid and a 1K resistor in the anode, I measured below 3mA/V . A replacement tube has 6.5 mA/V, and oscillates again, but at 800 kHz instead of 50 kHz (as per manual) and with much too low voltages produced. The 2nd grid was at 130V, should be 90 per manual. Dropping it with an additional resistor to 90V made the produced voltages even smaller. I think the high frequency could be the main cause of the low voltages. I believe frequency is determined mainly by the transformer and a 1 nF C between anode and 1st grid of the 6AU5. But soldering a 100 pF C over the 1n one made no difference in frequency, I would have expected a 10% drop. Using a larger parallel one like 1 nF stops the oscillation. Using another scope, I do see undistorted sines on the secondary windings. Because my 547 uses semiconductor diodes (BY711) for the high voltage (a mod), I do get secondary voltages, but much too low like 500V for the tube EHT instead of 8 kV .
What can cause this high oscillating frequency? What is wrong and how can I fix it? Could this be a faulty transformer (although I still see secondary voltages)?
Thanks, Mat Nieuwenhoven