Tektronix 547 high-voltage problem

[Mat Nieuwenhoven]

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

 

[BFoelsch]

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

It sounds like there is a short in the transformer. the loss of
reflected inductance would raise the resonant frequency as you
describe. I would disconnect all the secondaries to be sure that one
of your diodes is not shorted. While the secondaries are disconnected
check the primary inductance; it should be around 10 milliHenies.

According to my notes the plate of the 6AU5 should swing 340 volts
peak to peak.

 

[Mat Nieuwenhoven]

It sounds like there is a short in the transformer. the loss of
reflected inductance would raise the resonant frequency as you
describe. I would disconnect all the secondaries to be sure that one
of your diodes is not shorted. While the secondaries are disconnected
check the primary inductance; it should be around 10 milliHenies.

According to my notes the plate of the 6AU5 should swing 340 volts
peak to peak.

Found it at last, starting with your suggestion of disconnecting all
secondaries to start with. The 6.3 V winding of the main transformer for the
oscilloscope tube developed a short to ground, strangely enough only when
measured one way with an ohmmeter; reversing the leads showed high impedance.
I seem to missing one capacitor in the circuit, a 4 kV 10 nF flat C, which is
supposed to go to the grid of the first triode in the high-voltage regulator.
I am not able to locate it.
Anyway, I think I'll put in another small transformer with high isolation
(3kV minimum) to supply the 6.3 V - rewiring the big transformer is a big job
(if do-able at all).

Thanks for the help.

Mat Nieuwenhoven