high pitch noise from a tek 422 oscilloscope

[crazyhog]

Hi,

I got an old tek 422 oscilloscope from ebay.  It's from the 60s or early 70s.  I am guessing it's been in storage for decades.  When its power on, there is a high pitch noise.  Its like the squeaky sound a mouse mades.  Is it coming from the transformer?  bad filter caps?  What would cause this?

Please note this scope is completely sealed; there is no fan.  No dust bunny in this machine!  Aside from the common problem with dirty potentiometers and switches, the scope mostly works!  I really like the aesthetic of this scope.

[Armadillo]

Likely bad filter capacitors at the power supply.

[voltz]

Would check the EHT transformer. Its basically part of a DC-DC converter driven by a high frequency clock. If the transformer starts to break down or is just loose internally, you will hear a high pitched squeal. Hopefully its not breaking down, your trace looks bright enough and i assume there are no bad smells from the scope like burning plastic. May just need a slight tighten of a screw or a dab of silicon.
Would also check the schematic for any other DC-DC converters in there. Same thing applies to those, if any. Not got the diagram in front of me so just general advice here.

[Toasty]

See: TekWiki 422 for pictures and schematics.

As voltz said, the squealing is in the high voltage section (EHT aka Flyback transformer) that supplies the anode of the CRT.  It's the only area that runs at the frequency you describe.  Follow the the large grey wire and cap in the pictures that's hooked to the side of the CRT.  Per the schematic, it's running at 4.9kV.

Open the unit up and see if you can localize the noise.  You can check for an ozone smell then and perhaps run the scope in a no light environment looking for any corona glow on that HV lead.  You could also have a condensation issue from its time in storage.

T

[crazyhog]

Thanks everyone for your help.  Today the scope blew it's fuse.  I ended up troubleshooting that and haven't got around to follow up on the EHT. 

Two electrolytic caps blew their top and failed short.  To my surprise, I found the failed caps are hot glued to the circuit board!  I don't believe Tektronix would use hot glue in the manufacturing of this beautiful machine!  As such, I am probably following the previous owner's foot steps.  Anyway, after replacing the capacitors, the scope is working again. 

Unfortunately, the high pitch noise is back too.  I will dig into the EHT.

[Armadillo]

Then you should look for further electrolytic caps, not just 2.
Don't wait for it to show smoke signs, ESR all those, it's old enough to give it a new life with all new caps   

[crazyhog]

Yup, I've checked all the caps.

[Armadillo]

Yup, I've checked all the caps.

 

[crazyhog]

I've confirmed the noise problem is not the EHT driving the CRT (no noise under the high voltage shield of pic 1).  The noise is from the power supply module (pic 2).  Good thing since I don't know know how to fix a failing EHT transformer .

There is an AC only Tek 422 model and a AC/DC Tek 422 with internal battery pack model.  I have the later and I suspect the power supply maybe using SMPS circuit to deal with DC power source.  Will read the manual and dig around more.   I hope the noise in the power supply is not inherent in the SMPS tech of the 1960s.  I like a silent scope.

While digging around, I cleaned up a really dirty pot use to control intensity (see pic 3).  This pot is not like the others.  The factory used a green stuff in making this pot.  The green stuff has coated the wiper and the track and has dried and hardened over the years.  The green stuff is non conductive!  No idea why this green stuff is needed for the intensity control.  Anyway IPA worked well cleaning and restoring the pot. 

[Toasty]

The "green stuff" is a lubricant and insulator that keeps any wear and possible arcing that might occur on the pot's wiper to a bare minimum.  It also provides some mechanical resistance so the pot feels "smooth" when adjusting it. 

I would use a high quality silicone grease available today.  You want something that's more of a paste.  The intensity control is in a 1400V circuit, with the focus control next in line.  See the schematic "CRT Circuit" in the link I posted.

Since you've narrowed the noise to the power supply, it's probably time for recapping the electrolytics! 

You can't depend on value checks alone, so get your ESR meter out an have at it. 

Is the battery pack connected?  If it is and it's bad/dead/shorted, that could be causing the power supply to squeal too.

... Good thing since I don't know know how to fix a failing EHT transformer

There's usually no "fix" to those transformers.  However, fixing the rectifier/multiplier can be quite easy.  Just HV caps & diodes.

T
 

[crazyhog]

Thanks Toasty for your help.

The noise is coming from a toroidal transformer (big metal can in pic 1).  The Tek AC/DC model has a power supply with a boost converter to handle a wide DC power source (11.5v to 35v).  The boost converter has a 7khz oscillator that drives the toroidal transformer.  Even when the scope is power by AC, the power supply would rectify the AC to DC and then feed the DC into the boost converter. 

My only idea for quieting the toroidal transformer is to fill the the metal can with silicon grease.  Hopefully the grease will lower the pitch of the noise.  It's a bummer.

[Armadillo]

My only idea for quieting the toroidal transformer is to fill the the metal can with silicon grease.  Hopefully the grease will lower the pitch of the noise.

Better to use Shellac instead of grease as a norm practice.

[crazyhog]

Thanks Armadillo!

[Toasty]

Loosen the screws top & bottom that mount the coil and try moving it about a bit while running. See if that quiets it down.  Push on the wires with a plastic tool.  You might just find a sweet spot.  You can secure it with some silicone sealant and let if fully cure.

While shellac is the correct choice, the best method is to submerse the coil in a sealed container and apply a vacuum.  That gets the shellac into the nooks & crannies in the windings.

I'd still check those caps.

T

[crazyhog]

Thanks Toasty. 

I share your concerns about the caps.  The first thing I did after finding the 2 exploded caps was I checked all other caps for both capacitance and ESR.  I did take one shortcut by doing mostly in circuit measurements.  I look for capacitance that is inline with the spec which give me confidence the rest of the circuit didn't influence the in circuit measurements. 

[xwarp]

Just curious, is the NiCad pack still in there?

I think I have an AC supply somewhere around, if you are interested, I'll look for it.

[crazyhog]

Hi Xwarp,

The nicad were still in the scope when I got it.  There were 20 D cells and were quite heavy.  They were all dead and some had corrosion.  The first thing I did was removing the battery.  After I get this scope working to my satisfaction, I plan to convert it for li-ion battery.  I really like the idea of having this as my portable scope that can do floating measurements!  The tactile feel and aesthetic of this old timey machine is a joy forever! 

Thank you for your offer for the AC power supply.  I don't need it at this time.

[bd139]

Just popping in to say thanks for the decent photos. Lovely bit of engineering that scope. I hope you win the battle

[crazyhog]

Pic 1: Tektronix wants you to keep your dirty paws off the donut!

Notice the can holding the transformer is soldered  all around the entire edge!  It is also a giant heat sink.  All of my soldering equipments were defeated.  I am sure my propane torch can force it open.  Unfortunately, it will be a pyrrhic victory since the can's content will be cooked too!

I thought about giving up.  But I am doing this for fun and to settle seems to miss the point!  Therefore, brute force it is!  A large metal file and half an hour of elbow grease later, I am in like flynn (pic 2)! 

The transformer is coated in a soft rubberly material.  Let's see if this can be improved...

[crazyhog]

I plan to submerge the transformer in a potting compound (i.e flood the metal can).  Other folks has already suggested electrical resin. 

Thinking it over, I realized there are many other options that I can experiment with for the specific purposes of sound dampening.  I am currently considering hot glue and silicon chalk(i.e the stuff used to seal bathroom tiles).  I like them because they are soft which should work well for sound dampening.  Removing the material is also very easy, which means I can do over if I am not satisfied.

My concern with resin is that I had to get it into all the crevices of the transformer in order to stop the coil from vibrating.  I don't have a practical way to do this.  Secondly, there is no do over if the results are not satisfactory(i.e cannot remove hardened resn).  Lastly, resin when cured will harden.  Therefore it may not be as good at sound dampening as a soft material.

Am I right to think soft potting compound is better for sound dampening? 

Any other potting material I should consider?

[tautech]

Two pot encapsulation epoxy. Electrician suppliers will have something that doesn't cure fully hard and if warmed will flow nicely into crevices. Ensure the curing time is not too quick as to let any bubbles escape. Some gentle rocking before cure will help them find their way out of the windings.

The one I have is probably some rebrand, Plasti-Cast from Fraser Brown & Strathmore Ltd., Wellington, NZ

[Armadillo]

Would it be too troublesome to hook up to the ac source and confirm where and how exactly the noise emanate from, since now it is more readily visible from the open transformer, and then conclude your approach.?   

[Toasty]

I find it hard to believe any noise would be coming from that coil. 

It is well covered with, as you said, a soft rubbery coating.  If there is noise coming from it, it must have been like that from the factory.  The only conceivable place would be from bad factory encapsulation where that compound didn't get into all the places it should have.  Adding anything over top of what's there, quite honestly, won't make it any quieter.  If pushing on the attached wiring didn't make -any- difference, I seriously doubt anything you could add will.  You'd have to completely strip that compound off (without damaging the enamel insulation) and vacuum it down submerged in a new batch. 

I'm just wondering if there is something wonky in the circuit that's causing the noise.  Or, you may simply have a noisy toroid that will never be quiet.

You may be stuck living with it, or adding some insulating and/or sound dampening material exterior to the can.

Can you do what Armadillo asked?

T

[crazyhog]

The sound is definitely not the 60 hz AC.  I have a isolation transformer on my bench and I know the 60hz hum well.  Here is a 7khz tone on youtube and the noise from the power supply sounded like that.

https://youtu.be/-vAqiFrG9cg

To test the transformer in isolation, I would need a power source that oscillates at 7khz and sources about 1 amps.  Note that the oscillation is not sine, it's PWM.  I would need to rig something up from scratch. Hmm, I could use my function generator to switch a large power transitor to power the transformer.  I'll need to look into this idea more.  Although filling the whole can with silicon chalk is easier.

Stepping back, what other noise makers can there be in this power supply?  I've review the parts list and checked the likely candidates.  Here is the manual I found http://exodus.poly.edu/~kurt/mmm/tek-422.pdf.  Note that we need the AC/DC power supply and not the AC only supply.

Regarding the white stuff on the can lid and wall, it looks thick but its foam.  No sound dampening.  The rubbery coating is only on the winding and its about the thickness of a finger nail.  It just cannot provide much sound dampening, if any.  I won't bother removing this surface coating; there will be crevices within the windings that I will never get at.  At 7khz, the frequency is high enough that I am hopeful that dampening the noise is do able.

 

[Toasty]

According to the PSU schematic I pointed to in Post 4, the frequency sounds about right.  The reference trace they show is ~125µ sec which comes out to 8kHz.

http://w140.com/tekwiki/images/3/3d/Tek-422_lvps2.png

I believe Armadillo was asking if you could reconnect it back in the circuit with it open, not build something.  If you do that, can you more effectively isolate the noise?  Just touching components would change the frequency or intensity.

If you're absolutely convinced it's the transformer, perhaps some of that "tool grip" rubberized coating?  Once you reassemble it, you could dip the whole coil, casing and all...?

T