Date   

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.


Re: Help diagnosing a Tek 2755 spectrum analyzer

Jaap Rusticus
 

Hello Neill,

I'm new to this group in writing. Reading allready for a long time. I'm in the Netherlands.
As I have been reading here on the Tek forum (Chuck Harris and perhaps others too), the several dipswitches can be unreliable and this can lead to strange behaviour. I changed them all in my 2756P.

Jaap


Re: 5116+5D10 intensity problem

Paul Cohen
 

Or maybe SF6 (nasty) gas which is used in HV

בתאריך יום ב׳, 17 בפבר׳ 2020, 10:22, מאת Yeun-Jung Wu ‏<
B0190@...>:

Hello Harvey:

Thank you for your hint on future diagnosis. I will start collecting more
information on epoxy coated transformer first.

5D10 seemed to work fine except read out information had been expanded
outside the screen. Now I understand why red LED "possible under-sampling"
would lit: I was using slow time base to watch two nearly identical 7kHz
since wave superimposed together to form a nice envelope. There were lot of
wave form variation during long accuqsition time but the available memory
space for storing waveform was limited. When I select faster time base such
that several cycles was within the range of one horizontal division that
warning went away.

More than 5 year's ago I bought a Tek high voltage probe from eBay. There
was an empty can supposed to contain liquid with high dielectric strength,
something similar to banned CFC freon. For 4.5kV maybe I can fill some
other liquid and give it a try.

Best Regards,

Yeun-Jung Wu




3D printable feet for TDS500/600/700

Monty Montgomery
 

Hi folks,

While putting a TDS7xx back together, I required several new feet, so
I modeled some. Files ready to print are at
https://www.prusaprinters.org/prints/22192-replacement-feetpads-for-tektronix-tds-500600700-s
for anyone who'd like them.

Cheers,
Monty


475 Module claimed

Bruce Lane
 

The vertical module I offered earlier, for the 475, has been claimed.

Thanks much.


--
---
Bruce Lane, ARS KC7GR
http://www.bluefeathertech.com
kyrrin (at) bluefeathertech dot com
"Quando Omni Flunkus Moritati" (Red Green)


Re: 475 Module: Anyone need it?

Bruce Lane
 

Thanks, Renee, but it ended up going to another fellow who has a broken
'scope he's trying to fix.

Best of the week to you.

On 16-Feb-20 10:54, Renée wrote:
Hi Bruce-
I use the 475  as my preferred scope...so If it is still available...I
will take it.
we should do this off list from here ....k6fsb dot 1 at the gmail place
Renée

On 2/16/20 10:43 AM, Bruce Lane wrote:
Fellow Tekkies,

    While hunting for a part this morning, I came across a Tektronix
module
which I think is the vertical driver board for a 460 or 470 series. The
part number is 670-3023-02.

    I don't need it, as I sold my 475 some time ago. First offer of $5
plus
postage gets it, priority to those who might need the thing to repair an
existing 'scope.

    Thanks much.


--
---
Bruce Lane, ARS KC7GR
http://www.bluefeathertech.com
kyrrin (at) bluefeathertech dot com
"Quando Omni Flunkus Moritati" (Red Green)


Re: 2215 cleaning + recap

ridderg@...
 

Thanks! I checked again and it turns out I missed a second screw on the top left that attaches to the preregulator board. So there are 7 screws in total.


Re: Repairing my Tektronix 454 oscilloscope need some advice

Albert Otten
 

Victor,

Before accusing the HV transformer ... some remarks.
How "very low" was HV at the test point? Is the oscillator amplitude at Q1430 also (in ratio) very low?
What voltage did you have at Q1423-emitter (or pin "P")? The shown voltage -4.35 V is not more than an indication; also there is ripple of 1 V or so. The oscillator starts with base current to Q1430 provide by R1425. Amplitude increases rapidly when Q1423 conducts at the negative going peaks of "P" voltage (at about -1 V). With too high load that situation might not occur.
Did you check HV pot R1401?
Did you measure the current via F1437? Normally about 0.27 A.

It might be possible to test the transformer (and load) to some extend. Remove F1437. Apply + 10 V (w.r.t. GND) or so at C1437. Apply about -2 V at "P". Apply a signal generator output (via DC blocking cap) to Q1430 collector, keeping the amplitude below about 4 V. This way no semiconductors will start to conduct. There should be resonance at about the normal oscillator frequency, about 25 kHz. Turns ratios can be checked when when you can access the secondary winding ends or rectified voltages. (Just an idea, I have never tried this in my 454.)

Albert


Re: Repairing my Tektronix 454 oscilloscope need some advice

Dave Wise
 

If I understand correctly, the 454 HV is similar to the 453. I successfully rewound a 453 transformer using a low-tech process which I documented in topic and photo album titled "453 HV Transformer Rewind".

HTH,
Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Victor via Groups.Io <vmcfer=aol.com@groups.io>
Sent: Saturday, February 15, 2020 10:40 AM
To: TekScopes@groups.io
Subject: [TekScopes] Repairing my Tektronix 454 oscilloscope need some advice

Hi members,
I have since many years a tek 454. Few years ago the trace stop to show. Since I have more spare time to dedicate to my small lab I would like to repair it.
The first step was to repair the power supply witch is working now fine. Then I discover that the CRT circuit is not working (very low voltage on the TP 1469 (-1960V)) and no voltage on the CRT heater.
I test all the parts on the Z Axis circuit board feeding the HV voltage and I couldn't find any trouble. I open the plastic box containing the HV transformer for testing and all the parts seems fine except a 1 Mohm resistor which I replace it. Is not easy to work on this circuit with few information but I think the HV transformer is dead.
A very well know Canadian supplier for Tek parts doesn't have any spare available for the HV transformer. So I'm looking for some advice or any members experience on this scope or additional test to confirm my suspicion. Also any specs about the HV transformer will be welcome.
Thanks in advance.
Victor


Re: 7904 or 7904A....which should I keep?

Dave Daniel
 

I had to make the same choice when I moved cross-country in 2017.

I chose to keep the 7904A because it has the kickstand and the 7904 does not. Having worked on both, I decided that the kickstand made working on it sufficiently better to keep it over the 7904.

DaveD

On Feb 17, 2020, at 13:24, Chris Wilkson via Groups.Io <cwilkson=yahoo.com@groups.io> wrote:

Title says it all. I have multiple (MULTIPLE!) Tek mainframes and I'm hoping to cull the herd this year. I want to keep one 500MHz scope and it's either the 7904 or 7904A.

What are the advantages and disadvantages of each? Do they both support the full range of plugins (looking at power hungry digital stuff) or is one more forgiving. What about fan noise, availability of parts, ease of repair & cal & maintenance? Control panel features? Current loop vs. voltage calibration outputs? Display quality?

All opinions are welcome!!



7904 or 7904A....which should I keep?

Chris Wilkson
 

Title says it all. I have multiple (MULTIPLE!) Tek mainframes and I'm hoping to cull the herd this year. I want to keep one 500MHz scope and it's either the 7904 or 7904A.

What are the advantages and disadvantages of each? Do they both support the full range of plugins (looking at power hungry digital stuff) or is one more forgiving. What about fan noise, availability of parts, ease of repair & cal & maintenance? Control panel features? Current loop vs. voltage calibration outputs? Display quality?

All opinions are welcome!!


Re: 5116+5D10 intensity problem

Harvey White
 

You're welcome.  I don't know if you have the particular HV transformer, but as Chuck (next) says, you're definitely looking at a low high voltage, so the question, of course, is why....

For the probe:  That's likely a P6015 or the equivalent.  It may be used with or without the dielectric liquid.  WIth, I think that the rating is 30KV, without, about 18KV.  You should be fine without it.   You may want to try using it with an adaptor with a meter, but it is designed to work into a 1 meg input impedance, and most meters are 10.  Paralleling the meter input to make it 1 meg is recommended if you try this.  I'd also try a test run (if you do that) on a known 100 volt or so supply to make sure you get the required 0.1 volt on the meter.  If not, then you're using it on a scope, and while you'll get a good idea, of voltage, it will only be within about 5% or so unless you have an A/D plugin.  The 5D10 might just do for that if you could see the readouts.

I recommend hunting the internet for the manuals for everything you have, if you haven't already.  I have an electronic library in several places.  Some of that information is available for only a limited time.

Harvey

On 2/17/2020 3:22 AM, Yeun-Jung Wu wrote:
Hello Harvey:

Thank you for your hint on future diagnosis. I will start collecting more information on epoxy coated transformer first.

5D10 seemed to work fine except read out information had been expanded outside the screen. Now I understand why red LED "possible under-sampling" would lit: I was using slow time base to watch two nearly identical 7kHz since wave superimposed together to form a nice envelope. There were lot of wave form variation during long accuqsition time but the available memory space for storing waveform was limited. When I select faster time base such that several cycles was within the range of one horizontal division that warning went away.

More than 5 year's ago I bought a Tek high voltage probe from eBay. There was an empty can supposed to contain liquid with high dielectric strength, something similar to banned CFC freon. For 4.5kV maybe I can fill some other liquid and give it a try.

Best Regards,

Yeun-Jung Wu



Re: help tek 475 power supply

Chuck Harris
 

Because you don't get a lot of room to route the heavy
traces needed in a power supply, with a two layer circuit
board, designers very often used cheats, like using the
fact that all of the negative lugs on twist lock electrolytic
capacitors are connected together, to make no cost
jumper wires used to make circuit board routing easier.

If you didn't make sure that all of the twist lugs on the
capacitors you replaced have wire jumpers to make those
connections, your supply will be very confused.

-Chuck Harris

Roberto wrote:

hello everyone after changing the electrolytic capacitors on my teak 475 I have a problem that turned on after a few minutes it turns off I checked that the transistor Q1446 2n2222 was defective due to the fact that it looks like a 24v zener diode with a 180 series resistor hom that heats up and makes you break some ideas?
Roberto




Re: 5116+5D10 intensity problem

Chuck Harris
 

When the Anode voltage on the CRT is lower than it is supposed to
be, the electrons in the beam travel proportionately slower than
they should. Because they are slower, they spend a longer amount
of time to passing between the deflection plates. This means
longer time means that the force the electric field between the
plates applies to the electrons, has a longer time to work at
deflecting the electron beam. Which further means the electron
beam will be deflected more than it would at the proper voltage.

What you should take away from this is: If the anode voltage is
low, everything on the CRT screen will be expanded (magnified) out
much larger than it should be.

That your readout is expanded outside of the screen is nearly
perfect proof that your anode voltage is low. You can guestimate
how low by how much the expansion is. But it is most definitely
lower than it should be.

-Chuck Harris

Yeun-Jung Wu wrote:

Hello Harvey:

Thank you for your hint on future diagnosis. I will start collecting more information on epoxy coated transformer first.

5D10 seemed to work fine except read out information had been expanded outside the screen. Now I understand why red LED "possible under-sampling" would lit: I was using slow time base to watch two nearly identical 7kHz since wave superimposed together to form a nice envelope. There were lot of wave form variation during long accuqsition time but the available memory space for storing waveform was limited. When I select faster time base such that several cycles was within the range of one horizontal division that warning went away.

More than 5 year's ago I bought a Tek high voltage probe from eBay. There was an empty can supposed to contain liquid with high dielectric strength, something similar to banned CFC freon. For 4.5kV maybe I can fill some other liquid and give it a try.

Best Regards,

Yeun-Jung Wu


help tek 475 power supply

Roberto
 

hello everyone after changing the electrolytic capacitors on my teak 475 I have a problem that turned on after a few minutes it turns off I checked that the transistor Q1446 2n2222 was defective due to the fact that it looks like a 24v zener diode with a 180 series resistor hom that heats up and makes you break some ideas?
Roberto


Re: 5116+5D10 intensity problem

Yeun-Jung Wu
 

Hello Harvey:

Thank you for your hint on future diagnosis. I will start collecting more information on epoxy coated transformer first.

5D10 seemed to work fine except read out information had been expanded outside the screen. Now I understand why red LED "possible under-sampling" would lit: I was using slow time base to watch two nearly identical 7kHz since wave superimposed together to form a nice envelope. There were lot of wave form variation during long accuqsition time but the available memory space for storing waveform was limited. When I select faster time base such that several cycles was within the range of one horizontal division that warning went away.

More than 5 year's ago I bought a Tek high voltage probe from eBay. There was an empty can supposed to contain liquid with high dielectric strength, something similar to banned CFC freon. For 4.5kV maybe I can fill some other liquid and give it a try.

Best Regards,

Yeun-Jung Wu


Scope auction

Robert Simpson
 

About the middle of the page:
https://www.estatesales.net/CA/Rocklin/95677/2475516

No connection, but is in a neighboring town.
That 7904A looks like it has 7S12 in it with modules
Bob


Re: The Everlasting 547 HV Transformer topic

Chuck Harris
 

Hi Leonard,

I replied to you minutes after your first request, and the same
after your second request. But your spam filter obviously doesn't
like me. I suggest you check your spam folder.

Here is what I replied:
----------------------------------------
Hi Leonard,

I sent you the below message at 12:53 on Feb 5th.
-------------------------------------------------------------------------
Hi Leonard,

I am working feverishly on finishing my new winding machine.
It is computer controlled, and will make winding HV transformers
much easier, and more reliable.

I have been promising that I would be ready really soon now, for
about the last 5 years... I don't like making promises that I can't
keep.... so I won't do that anymore.

However, I am now retired, so in theory, I have more time on my
hands.

I will post an announcement on the tekscopes, and tekscopes2 groups
when I am ready to start winding again.

-Chuck Harris
----------------------------------------

leonard scheepsma wrote:

Guys,
I've once again restored a 547 almost at the edge of the dirt container, got it working (except then both time-bases A & B do not trigger but that's for later as I run an 1S2 right now which doesn't need it anyway) but the scope shows all symptoms of a defective HV transformer (20 mins show time and then dies on me..)

So, I read Chuck Harris is the man who could possibly make me a replacement. For a 549 I have as well, I got an Xfrm winded locally, but that was no success (spraying), for that scope a small blower entered the HV case which solved the problem as the temperature doesn't rise so much. For this 547 I'm very interested to have a real good replacement.

I've sent a direct email to Chuck a few times but got no response yet. He is around right?

Thanks Leonard




Re: 475 Module: Anyone need it?

Renée
 

Hi Bruce-
I use the 475  as my preferred scope...so If it is still available...I will take it.
we should do this off list from here ....k6fsb dot 1 at the gmail place
Renée

On 2/16/20 10:43 AM, Bruce Lane wrote:
Fellow Tekkies,

While hunting for a part this morning, I came across a Tektronix module
which I think is the vertical driver board for a 460 or 470 series. The
part number is 670-3023-02.

I don't need it, as I sold my 475 some time ago. First offer of $5 plus
postage gets it, priority to those who might need the thing to repair an
existing 'scope.

Thanks much.


475 Module: Anyone need it?

Bruce Lane
 

Fellow Tekkies,

While hunting for a part this morning, I came across a Tektronix module
which I think is the vertical driver board for a 460 or 470 series. The
part number is 670-3023-02.

I don't need it, as I sold my 475 some time ago. First offer of $5 plus
postage gets it, priority to those who might need the thing to repair an
existing 'scope.

Thanks much.

--
---
Bruce Lane, ARS KC7GR
http://www.bluefeathertech.com
kyrrin (at) bluefeathertech dot com
"Quando Omni Flunkus Moritati" (Red Green)