547 scope HV transformer problem. One practical solution.


 

Hi, I am a retired electrical engineer having designed electronic equipment for measurements in oil wells for over 30 years.

About 40 years ago I picked up a 547 scope with an 1A1 plug in left as surplus by my employer. It went into my garage, and it was never powered up. After some decades, I developed a fear of powering it up, concerned over the degradation of the large electrolytic capacitors in its voltage supplies. A few weeks ago I needed an oscilloscope to troubleshoot an old record player, and I decided to bring this ancient oscilloscope back to life.

I disconnected all the big capacitors and over hours raised their voltage with an external supply while monitoring leakage current. All of them got restored! I did some general inspection and studied the schematics of all the functional blocks. I removed the timer tube and powered up just the filaments to check on the tubes. All lighted up. I reconnected the capacitors and the timer tube, and removed the oscillator tube of the CRT supply. While monitoring the regulated supplies I brought up the power with a variac. They came up fine within specs, no smoke. I was able to activate the sweep generators and see the flashing on the neon beam position indicators.

Then I placed back the 6AU5 tube of the HV oscillator, powered up, and to my amazement I saw come on that beautiful sharp bright trace on the CRT. Overcome with emotion, tears rolled down my cheeks and I started playing with the controls using the calibration signal. The oscilloscope was basically WORKING! After 40 years of being dead!

After some short time the beautiful trace started to fade away, and I realized that I had a problem with the CRT high voltage. After investigating, I learned about the now classical problem with the HV transformer. First I replaced the vacuum diodes with silicon diodes, and the power consumption of the supply improved (as measured on the pentode grid 2), but the time to failure only increased slightly. I measured it as 15 minutes after power-up cold. The failure of the transformer is an insurmountable problem. I speculated that if clamping the pentode's grid 2 at a higher voltage than 125v the oscillator could provide more power, or I could easily replace the 6AU5 with a power MOSFET after some minor modifications, and then increase drastically the available power to the Hartley oscillator. But this could be a dangerous modification if it could burn out the transformer, and then this could end the life of the instrument since there are no replacements. I could still build a new larger transformer with a ferrite pot core that would fit in the available space, with sufficient space for reliable HV insulation of the windings (and NO impregnation). But I lack the basic design of the transformer, like the turn ratios of its winding, although I could measure them on a working transformer or calculate them from the existing design.

I realize that I have fallen in love with the 547. No other test instrument, be the 7000 series, the digital scopes, etc. have this effect on me. The scope is now in my office, and each time I see it my spirit lights up. So I searched for a solution.

First, I realized that the HV supply is overpowered. If the 325v unregulated supply (about 350v in my scope) is powered down, regulation is maintained until the voltage is down to 195v. The HV could be powered from the 225v supply instead, but this one simply regulates down the 325, so this does not save power but transfers some to the 225v regulator. My experiments with an external variable supply gave me an idea: while the scope is on, why not have a switch to turn on and off the HV power and so allow the scope to be used for an indefinite time, as long as the display is turned on only when needed, and maintaining an adequate low duty cycle.

I verified that the supply tolerates a 50% duty cycle of 5 minutes on and 5 minutes off. Or, a 10 minutes on followed by a longer period off. One can do a lot of measurements in 5 minutes. I have added a toggle switch on the side cover of the scope, close to the CRT. Now after initial power up, I wait for a reasonable time for the instrument to stabilize (something that takes time with the 1A1 plug in!) Then I turn on the display and make adjustments and measurements, and 5 minutes is plenty of time. Then I turn the display off, while doing other things. I am perfectly fine using the oscilloscope like this. Cycling the HV supply does not seem to affect the stability of the scope, the readings remain the same.

One detail that worried me somewhat is the initial very bright blur of the image when turning the supply on. I am working on this issue. First I have reduced the voltage to the supply by 120v with a string of 10 zener diodes, 12v 5w, which are dirt cheap on Amazon. With 230v the supply runs fine. Next I am building with a MOSFET and some timing components a means to raise these 230v slowly, and reduce this first bright flash in the first second of power on. I had measured that the current provided to the supply is 26mA, which doubled when it reached a safe degradation. So the consumption of the HV supply will stay below 25 watt, an amount of power easy to handle.

With this last modification I will be satisfied to have brought back to life this superb instrument, which I plan to keep in my office forever, for whatever little experimenting I will fancy to do.
I might try to improve it again whenever Chuck Harris is able to produce workable HV transformers, and I would like to be one of his clients.

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