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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.


Albert Otten
 

On Tue, Feb 18, 2020 at 07:34 AM, Ernesto wrote:


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.
Hi Ernesto,

Interesting story and solutions. Though I hope my 547 keeps going on without those HV problems!
The bright blur when turning on the HV supply is not amazing.
See message https://groups.io/g/TekScopes/message/108397. The cap mentioned is C562.
This cap has no effect when the 547 is already on and you switch off/on the HV at a later moment.

Albert


John Griessen
 

On 2/16/20 4:43 PM, Ernesto wrote:
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
Sounds like a good way to enjoy your 547 while waiting for a transformer!


 

It is not only the enjoyment of the scope working, but also of the challenge of going around its HV problem.
I had incorrectly written that the plug-in on my scope is the 1A1, but it turns out the that one I have is the CA, an honorable senior dual-trace made 100% with vacuum tubes. I really like its design and the way it works competently. At the beginning waking up from the 40 years hybernation it was full of quirks, in part due to the old switches and the two gain potentiometers cracked and intermittently open. I shorted them out and I am waiting for replacements. But the more I use it the more quiet and behaved it becomes. This is the same with the whole 547. I had never cleaned it from some dirt accumulated in 40 years through the cover holes, many tubes are dirty, but I didn't dare to blow with compressed air or... god forbid ...wash it. But all by itself it is recovering from some shortcomings. The two time bases work fine, I can get delayed triggers, B intensified by A, trace separation, etc. etc. Retrace blanking and chopped blanking work fine. What a wonderful instrument!

One feature missing in the CA is a trigger signal from one of the channels so that one can use the alternate and chopped functions without having to trigger the scope externally. So I brought out the signal from the first stage of channel A, at the cathode of V3323, to the connector pin 5 through a 1K resistor, and this connects in the scope to the "Plug In" trigger input. This works fine, although as soon as I get a high voltage signal transistor or FET I will boost this added signal at the source with an emitter or source follower and send the signal with low impedance. Now the CA plug-in is perfect, it is all I need and I would not use an 1A4 if I had it. I don't know WHY Tektronix didn't add this single channel tool-module trigger, so easy to implement.


 

Hi Albert, thank you for the information.

I verified the action of the negative pulse by R562, C562 on power up of the -150V that shifts the trace to the right. I could do something similar when the 325 unreg is applied to the HV supply. This works to protect the visible screen's phosphorous, but I am not sure what other transients occur at the electrodes of the CRT. The specs list a -200V maximum between grid and cathode, and I am inclined to put two neon tubes in series between these two electrodes to protect the cathode (when my CRT is blanked out there are -85V between them. With this protection, I would try to add another blanking pulse to the floating supply where the (un)blanking signals from the sweep generators come in. In this way, the trace would turn on already stabilized and nice looking. But for the time being, by raising slowly the supply to the HV the initial brightening is reduced, and I will tackle this after further investigating the HV transformer issue


ykochcal
 

"I had never cleaned it from some dirt accumulated in 40 years through the
cover holes, many tubes are dirty, but I didn't dare to blow with
compressed air or... god forbid ...wash it."


After you get more comfortable, you might take some time to look up
washing/wash/clean in this group.

I think a little Compressed air can be good as long as it's the "oil less"
kind

John


Morris Odell
 

Hi Ernesto,

Thanks for the discussion which I found really interesting. I am also a 547 admirer and use one as my main bench scope with a 1A4 plugin. That combination really brings out the best in that scope and indeed is featured on the cover of Jim Willams' Analog Circuit Design book. I suspect the lack of a trigger connection to the plugin connector in the CA reflects its heritage when that wasn't part of the design of the older 500 series.

The switchable HV supply is a neat idea but you will eventually need to replace the HV transformer as they do deteriorate further with time. There is a drawing and specification for the transformer in the files section of this group. Chuck Harris made very fine replacements and hopefully he will start doing them again but with persistence you might also be able to do them. I have had quite a few scopes with the problem (549, 556, 547, 545B) and they are running nicely with a variety of rewound transformers. I also have a 547 with what looks like an original transformer that does not have the problem. It's worth reading through the extensive discussion in this group about the transformer before attempting it though.

Good luck and let us know how you get on,

Morris Odell
Melbourne, Australia


 

Hi Morris,

Thank you for your informative message. I went to the "files" section and found the transformer specifications. This is a big plus!

Meanwhile I am exploring other options. I started experimenting with the adjustment of high voltage.
- Lowered the CRT HV all the way from 1850v to 1655v (a 10% reduction). As expected the gain increased also about 10%. But the time-to-failure increased from 15 min to 25 minutes.
- Lowered the CRT HV further to 1500v (adding a resistor in series with the pot.) Gain increased in proportion, and the time-to-failure increased to 47 minutes.

47 minutes!! This time is quite acceptable! I don't need to keep the scope always on, since it does not need to help me warm the house in Houston, haha.

I honestly didn't see a difference in the crisp brightness of the display when lowering the HV from 1850, to 1655, to 1500 volts. And everything seems to stay adjusted in proportion, I didn't have to readjust the focus nor astigmatism. I am truly surprised at how well behaved the CRT and its supply are!

I didn't explore further to see if with even lower HV the scope could attain INFINITE time-to-failure. I am quite satisfied with three-quarters of an hour. My behind likes to get a break after sitting that long anyway...


Morris Odell
 

Hi Ernesto,

I'm not surprised that the focus and astigmatism are still OK with reduced HV but don't forget that the CRT deflection factors will be affected by the HV which means the calibration will be all wrong in both the X and Y directions. Not a problem if you are just doing qualitative observations but of you want to set up the scope to do proper measurments you will need to have the correct HV and calibrate it according to the manual..

Cheers,

Morris


 

Hi Morris,

Yes, the deflection factors, which I called the "gain of the CRT" is an inverse function of the high voltage, and I have taken this into account. It requires simple readjustments of the gain of the vertical and horizontal amplifiers. When all this is over I will care about finding good standards to calibrate the scope with. In the meantime I have made some more observations:

After being relieved with the good results of lowering the HV to 1500 V I decided to try out a "thermal enhancement".
After removing the plastic cover of the HV section, I placed over it an old PC supply whose little fan did blow mostly over the transformer. And here is what I got:

- With HV = 1500 V, the supply current raised slightly in the first minutes, then it stabilized. One and a half hours later it was still the same, and the supply was still running fine.
- With HV raised to 1655 V, the same happened and I ended the test after one hour.
- With HV returned to the nominal 1850 V, exactly the same happened. The HV supply DID NOT FAIL.

So the COOLING OF THE TRANSFORMER will solve the problem of my 547, which originally failed after running for about 10 minutes.

The easiest implementation of the cooling can be to drill holes in the cover right above the HV transformer and place on top a little fan (with a filter), blowing on the transformer with the plastic cover off.
But I don't like the aesthetics of the fan sticking out on top of the instrument, so I will do the cooling internally. A change from electrical engineering to industrial engineering, haha.

Cheers,

Ernesto


 

Hi John, thank you for pointing me to the world of oscilloscope washing, cleaning.

I read about the experiences of others and the industry washing oscilloscopes with water. The hairs on my head used to stand up at the idea, but now I recognize that it is relatively safe, although it requires WORK.
I compare it with my long experience of "don't fix what is not broken", and the good luck I have with my old scope that had a 40 years leave of absence.

I perfectly cleaned the exterior of my 547, and it is shiny. The inside is different, with plenty of dust deposited on the tubes and surfaces. I like to preserve it like that for its dramatic effect. The instrument will be perfect for my grandchildren to play with it when they are a little bigger, and it may awaken their interest in electronics. I plan to show them the inside of the scope to be impressed with its age, but after that the inside will be off limits to them.

But... if I experience any failures, I feel competent enough to trace them to the failed component, or particles of dust, and I will remove just those offending particles.

NOTE: I have a different standard for washing myself, staying clean inside and changing my underwear, ha ha ha.

Cheers,
Ernesto


Chuck Harris
 

Just to add some other flies for the ointment:

When I first started my 547 HV repair project, my scope,
which had sat idle for several years before I got it,
would only run for 15 minutes before the HV crashed.

After I experimented for several days, the run time kept
increasing, until after a week, it ran continuously.

I thought it had cured itself, and I was done!

Well, then I got busy with something else, and the scope
sat idle for a couple of months, and the process had reset
to the original 15 minutes before crash.

The issue is heat induced epoxy loss, and seems to be
related to moisture absorption by the epoxy. It seems
also to be related to a chemical change in the epoxy.

There is a temporary improvement when the internal
heating of the transformer winding dries out the winding...
but it is only temporary.

The oscillator stops because the ferrite core is operating
at a bad point in the ferrite's curves. Its frequency is
just below the frequency where core losses start to go
exponential. And, to add insult to injury, when the core
heats, it approaches the Curie temperature (about 150F),
which is where the ferrite ceases to be a magnetic material.
This causes the frequency to rise... which you guessed it,
makes the losses rise even more.

Back in the old days, when the EHT ran at 60KHz, and beeswax
was used to impregnate the winding, the HV section was right
in front of the scope's fan... out in the open. It stayed
cool and breezy.

Tektronix wrecked all that when they decided to make an
extended temperature range scope (647) and switched over to
an epoxy varnish potting material. They also decided to
unify the HV section to fit in a plastic box... to keep
it clean, and allow reuse on all of their future scopes.

The epoxy varnish, even when new, was much more lossy than
the original beeswax, so the engineers lowered the frequency,
and added a provision for heatsinking the transformer core.

They also started to think about using solid state rectifiers
to replace the old power hungry 5642 tubes.

You can achieve an improvement by cooling the transformer,
through most any means: fan, heat sink, solid state rectifiers,
lowering the operating frequency, lowering the drive voltage...

Lowering the drive voltage will make it hard to see fast, low
repetition rate waveforms... kind of like hobbling a race horse...

Lowering the EHT's operating frequency can be done by parallel
addition of a capacitor to the 1000pf resonance capacitor
C808.

Ultimately, I found that rewinding the transformer was the
only sure long term solution.

We probably wouldn't have ever noticed this problem if Tek
had chosen an MOPA (Master Oscillator Power Amplifier) design
to drive the transformer, instead of using a modified Hartley
oscillator. It only takes a slight lowering of the feedback
efficiency to reduce the oscillator's gain below unity, and
make the oscillator stop.

-Chuck Harris

Ernesto wrote:

Hi Morris,

Yes, the deflection factors, which I called the "gain of the CRT" is an inverse function of the high voltage, and I have taken this into account. It requires simple readjustments of the gain of the vertical and horizontal amplifiers. When all this is over I will care about finding good standards to calibrate the scope with. In the meantime I have made some more observations:

After being relieved with the good results of lowering the HV to 1500 V I decided to try out a "thermal enhancement".
After removing the plastic cover of the HV section, I placed over it an old PC supply whose little fan did blow mostly over the transformer. And here is what I got:

- With HV = 1500 V, the supply current raised slightly in the first minutes, then it stabilized. One and a half hours later it was still the same, and the supply was still running fine.
- With HV raised to 1655 V, the same happened and I ended the test after one hour.
- With HV returned to the nominal 1850 V, exactly the same happened. The HV supply DID NOT FAIL.

So the COOLING OF THE TRANSFORMER will solve the problem of my 547, which originally failed after running for about 10 minutes.

The easiest implementation of the cooling can be to drill holes in the cover right above the HV transformer and place on top a little fan (with a filter), blowing on the transformer with the plastic cover off.
But I don't like the aesthetics of the fan sticking out on top of the instrument, so I will do the cooling internally. A change from electrical engineering to industrial engineering, haha.

Cheers,

Ernesto




Randy Newman
 

Hi Ernesto.
As a happy 555 owner (beautiful sharp blue trace (p13 phosphor?). Re
washing, check the BAMA (Boat Anchor ....) and look in the tek section for
tektronix magazines....or maybe I am thinking of the w140.com site.
Yes...the latter...see the 111wiki, then section 38 manuals, catalogs, and
other publications...then "Tektronix magazines" Tekscope vol 8 no. 4
1976..has tek's scope washing procedure. This is a 2-parter..don't have the
other issue...might be vol 9 no. 1. Plus there is a wealth of information
on tube and xsistor scopes. I also have a 7834, 7633, and 7623. But I
really like the 555, even with its external supply(!!). Nice garage
heater....some day I will have a real shop area.
Hope this helps!

On Thu, Feb 20, 2020, 9:27 AM Ernesto <ebordon@...> wrote:

Hi John, thank you for pointing me to the world of oscilloscope washing,
cleaning.

I read about the experiences of others and the industry washing
oscilloscopes with water. The hairs on my head used to stand up at the
idea, but now I recognize that it is relatively safe, although it
requires WORK.
I compare it with my long experience of "don't fix what is not broken",
and the good luck I have with my old scope that had a 40 years leave of
absence.

I perfectly cleaned the exterior of my 547, and it is shiny. The inside
is different, with plenty of dust deposited on the tubes and surfaces. I
like to preserve it like that for its dramatic effect. The instrument will
be perfect for my grandchildren to play with it when they are a little
bigger, and it may awaken their interest in electronics. I plan to show
them the inside of the scope to be impressed with its age, but after that
the inside will be off limits to them.

But... if I experience any failures, I feel competent enough to trace
them to the failed component, or particles of dust, and I will remove
just those offending particles.

NOTE: I have a different standard for washing myself, staying clean inside
and changing my underwear, ha ha ha.

Cheers,
Ernesto




 

Hi Randy,

I had never heard of a 555 scope before. So I went online and saw a video showing it and explaining some details. There I learned about the filament voltage regulation with a saturable reactor. Smart idea! Then I downloaded the manual and peaked in it. The delay line with 12 tubes followed by multiple discrete-component stages! The two HV supplies, practically identical to my 547 scope! Multiplexing inputs with vacuum tube diodes! The magnitude of the instrument is amazing, and it must be a real pleasure to operate it. A 60 year old design that out-thrills by so far any modern $400 digital scope, no matter how modern technology has made high performance so easy. You are lucky!

Regards,
Ernesto


stevenhorii
 

I once ran into a former Tek employee at a hamfest where I was looking over
one of the 500-series scopes. He said that in the day when Tek serviced
them, they washed them with water and then warm-air dried them. I will have
to look for that publication to verify what he said. I don’t doubt it would
work with the tube-type scopes but I’d have some concerns about doing that
with the solid state ones. Some of the switches in the plug-ins don’t look
waterproof but also look like they would be difficult to dry out with air.

I’m new to this group, but not to using Tek scopes. My first was a purchase
from DoD Surplus Sales (back before they privatized it). It was a 545A and
was DOA. I found the problem quickly - a bad resistor in the power supply.
I replaced that, and the scope powered right up. I had several of the
500-series after that (a 585, 547, and a 556 - should have kept that one. I
donated the scopes to the lab I was working in when I left. By that time, I
started getting the 7000-series scopes and still have most of those (a
7704A, 7904A, 7104, R7844, 7854). I also had an oddball 7000-series that I
think was a short-lived precursor of the 7854. It had a section in between
the plug-ins and the display that, like the 7854, had a number of waveform
computing functions. I don’t recall the number and I no longer have it - I
sold it at a hamfest. Also, I don’t see it on TekWiki. I still use the
R7844 on occasion, but have mostly turned to the TDS3054C digital. I know
the analog folks have likely instantly put me on their “oust from the
group” list, but it’s a matter of practicality - portability and I’m not
worried about aliasing for the applications I have. I have found the Tek
scopes over the years to be reliable and with all the plug-ins, very
versatile.

Steve



On Sat, Feb 22, 2020 at 15:35 Randy Newman <randy.n.at.home@...>
wrote:

Hi Ernesto.
As a happy 555 owner (beautiful sharp blue trace (p13 phosphor?). Re
washing, check the BAMA (Boat Anchor ....) and look in the tek section for
tektronix magazines....or maybe I am thinking of the w140.com site.
Yes...the latter...see the 111wiki, then section 38 manuals, catalogs, and
other publications...then "Tektronix magazines" Tekscope vol 8 no. 4
1976..has tek's scope washing procedure. This is a 2-parter..don't have the
other issue...might be vol 9 no. 1. Plus there is a wealth of information
on tube and xsistor scopes. I also have a 7834, 7633, and 7623. But I
really like the 555, even with its external supply(!!). Nice garage
heater....some day I will have a real shop area.
Hope this helps!

On Thu, Feb 20, 2020, 9:27 AM Ernesto <ebordon@...> wrote:

Hi John, thank you for pointing me to the world of oscilloscope washing,
cleaning.

I read about the experiences of others and the industry washing
oscilloscopes with water. The hairs on my head used to stand up at the
idea, but now I recognize that it is relatively safe, although it
requires WORK.
I compare it with my long experience of "don't fix what is not broken",
and the good luck I have with my old scope that had a 40 years leave of
absence.

I perfectly cleaned the exterior of my 547, and it is shiny. The inside
is different, with plenty of dust deposited on the tubes and surfaces.
I
like to preserve it like that for its dramatic effect. The instrument
will
be perfect for my grandchildren to play with it when they are a little
bigger, and it may awaken their interest in electronics. I plan to show
them the inside of the scope to be impressed with its age, but after
that
the inside will be off limits to them.

But... if I experience any failures, I feel competent enough to trace
them to the failed component, or particles of dust, and I will remove
just those offending particles.

NOTE: I have a different standard for washing myself, staying clean
inside
and changing my underwear, ha ha ha.

Cheers,
Ernesto






Albert Otten
 


I also had an oddball 7000-series that I
think was a short-lived precursor of the 7854. It had a section in between
the plug-ins and the display that, like the 7854, had a number of waveform
computing functions. I don’t recall the number and I no longer have it - I
sold it at a hamfest. Also, I don’t see it on TekWiki.
Steve, that might have been a 7704A with P7001 DPO.
http://w140.com/tekwiki/wiki/P7001
Albert


fiftythreebuick
 

Hi Ernesto!

The 555 is my favorite oscilloscope ever... An absolute pleasure to operate, incredible versatility (considering the range of letter & 1 series plug-in units) and the typically excellent design and construction of the 500 series! I have had one at my bench since the mid 70s and hope to keep one operational for as long as I am... 😊

Tom


fiftythreebuick
 

If anyone is interested it more info about the 555 it should probably be in a separate thread in order to keep this one on topic...


 

Hi fiftythreebuick,

A thread about the 555 is a great idea. It could be very educative for those of us who have never seen one in person.
For example, you could describe what is your convenience of having a dual beam over a single beam with multiple channels.
And how or if you have done routine maintenance of this scope.

Regards,
Ernesto


stevenhorii
 

Albert,

Thanks! That was it! I have never seen another one of these.

Steve

On Sun, Feb 23, 2020, 09:38 Albert Otten <aodiversen@...> wrote:


I also had an oddball 7000-series that I
think was a short-lived precursor of the 7854. It had a section in
between
the plug-ins and the display that, like the 7854, had a number of
waveform
computing functions. I don’t recall the number and I no longer have it -
I
sold it at a hamfest. Also, I don’t see it on TekWiki.
Steve, that might have been a 7704A with P7001 DPO.
http://w140.com/tekwiki/wiki/P7001
Albert