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 Post subject: Tektronix 647A scope noise
PostPosted: Nov Thu 23, 2017 3:57 pm 
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Joined: Jan Thu 15, 2015 4:56 pm
Posts: 114
Location: Winona, Mn.
A buddy of mine was cleaning house and gave me a bunch of electronic stuff. This 647A was the grand prize. It cleaned up pretty nice and seems to work as it should. However I hear a tiny high pitched tone coming from the rear of the scope. It sounds like one of the high pitched tones when getting your hearing tested. Not loud but you can hear it. From what I could asertain it is coming from the vertical amplifier area. (See pictures) 2nd through 4th are LH rear views. The 3rd is looking down from the top just above the vertical amplifier board. I don't want to fry anything if this tone is hazardous to the scope. Thoughts?


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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Nov Thu 23, 2017 4:52 pm 
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Posts: 660
Odd that it would come from the vertical section, especially if you are hearing the sound with no signal.

My first guesses would be high voltage section, which is the usual culprit in video devices, or it may be something in the blanking circuit. Does the tone change when you change the TIME/CM control?

It's hard to say if it could damage the scope. It may be a component on the edge of going out, but then again it just may be something a little dirty or leaky. Either way I don't see it getting 'better' without an intervention. Maybe you could get in there and spray around with some cold spray to try and better locate the cause? Or, is it all solid state? perhaps you could consult the schematic and pick out a few key tubes to pull, one at a time, and see if that affects the noise.


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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Nov Fri 24, 2017 1:34 am 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
You might want to try a length of plastic hose or tubing held to your ear to localize the sound. It may be coming from below the circuit card.

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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Nov Fri 24, 2017 1:05 pm 
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What a great Gift!!! Thats a 100 meg scope.

Tek used power oscillators to generate their CRT High voltages for decades, and if memory serves, their frequency was on the order of a few KHz, so that's perhaps what you're hearing. Typically their power supplies were located at the rear of the scopes.

Get a COPY OF THE SERVICE MANUAL here, and go to page 122 to check out the schematic of the supply.

If you go to the table of contents, you'll most likely find the theory of operation for the supply/oscillator.

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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Nov Sun 26, 2017 4:50 pm 
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Joined: Jan Thu 15, 2015 4:56 pm
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Location: Winona, Mn.
Well, I was finally able to sit down and play with this thing again. The tiny high frequency sound is there only when the calibrator selector is on a voltage selection. Sound does not change in amplitude within the voltage range. When in off position there is no sound. (?) The calibrating voltages are pretty much right on. The 100 VDC reference is 600 mv short of 100.

When powering up the scope it takes about 30 seconds for the sweep trace to appear at the bottom of the CRT. It then continues to rise and stabilize, to the previous setting, after a couple of minutes of warming up. (?)

There are 3 5642 half diode tubes and 1 7586 Nuvistor. Rest is solid state components. Picture shows the 3 5642's behind a plexiglass cover.

There are 2 miniature flanged base lamps for illuminating the graticule. Both are open. Mouser has them.

My next move is to check the power supply voltages and also see what ripple is out of spec. Service manual specifies a factory calibrating plugin when checking voltages. I don't have that. (?)


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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Nov Sun 26, 2017 6:30 pm 
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Posts: 660
Perhaps there is a leaky cap in the calibrator multivibrator circuit. Sounds like it's somewhere around there.


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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Nov Sun 26, 2017 6:52 pm 
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Disc ceramic capacitors are capable of being an audio transducer, converting audio range electrical signal to physical audio vibrations. I do not know if that effect has any increase with age. You may find the actual sound you hear coming from a disc ceramic.

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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Nov Sun 26, 2017 7:47 pm 
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My father used to use a stethoscope to isolate such difficult to pinpoint sounds, as have I n a couple occasions. Granted around high voltage, this technique would need to be exercised with an abundance of caution, though most stethoscopes use rubber or neoprene tubing, so your head wouldn't necessarily become a current path. But still...

As has been essentially said already, if there's a "singing" component or mechanical part in the device, merely applying a bit of pressure with an insulated rod of some sort selectively throughout the suspect region, ought to alter the volume or tone of the sound when you push on the offending part.

Just a though.

- Scott


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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Dec Thu 07, 2017 4:57 pm 
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Joined: Aug Mon 04, 2014 9:42 pm
Posts: 11
I had a 647a operational a few yeas ago, and it, too produced a faint high pitched noise. Something went wrong in the sweep, I never got around to trouble shooting it. I think, however that noise is "normal"


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 Post subject: Re: Tektronix 647A scope noise
PostPosted: Dec Fri 08, 2017 1:41 pm 
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Posts: 414
If the noise is only present when the calibrator is on, I wouldn't worry about it too much unless you need a "project" :-D. Quite possibly, that's normal. It could be that the square wave generated by the calibrator is somehow causing some odd effect with the power supply oscillators... or it could be one of the other suggestions noted. Unless you want to get in there with a "sound tracer" (plastic hose, plastic rod for contact) ... it'll be really tough to localize. Doesn't sound harmful unless it's happening all the time.

Question .. do you hear it with the covers ON ???


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