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Author Topic: flat topping on scope trace of filament voltage = hum?  (Read 6742 times)
NZ5N
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« on: March 06, 2012, 09:07:55 AM »

Hi,

I have been working on an old Heathkit Twoer, it is working OK now on both TX and RX, except I still have some hum in the speaker.  The hum has been traced to the 12AX7 audio amp.  Have gotten several recommendations and tried a lot of things (replaced all electrolytics, checked for loose grounds, etc) and finally yesterday I borrowed an oscilloscope.  All DC voltages looked OK, no ripple. 

I was told that hum can often be caused by the filament voltage.  When I put the scope on the 12AX7 filament (or any other tube), I did not see a perfect sine wave.  Instead, there was flattening on the top and bottom of every cycle.  Does this suggest that the filaments are the source of the hum?  If yes, is there any cure?  Some have suggested that changing the filament voltage to DC might silence the hum.

Thanks for any thoughts,
Bill NZ5N 
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KD8GEH
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« Reply #1 on: March 06, 2012, 10:15:27 AM »

Sounds like cathode leakage to me. If you havent already take a peek here: http://www.ohio.edu/people/postr/bapix/HW29A.htm

Soulds like fun, remember this is a regen reciever.

73   Dave KD8GEH
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KE3WD
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« Reply #2 on: March 06, 2012, 10:35:37 AM »

Pull the tubes one at a time, each time checking that filament line on the scope.  Replace tube and do the same to the next tube, etc.  See if you can isolate the section in this fashion. 

73
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WX7G
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« Reply #3 on: March 06, 2012, 11:32:47 AM »

The flat topping is not the source of the hum.

BTW, the 12AX7 has a heater and not a filament.
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AC5UP
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« Reply #4 on: March 06, 2012, 11:34:51 AM »

Any tube with an indirectly heated cathode... And a 12AX7 is definitely one... Has a thin layer of a ceramic powder like insulation between the heater and cathode sleeve. Should that insulation break down the tube will add 60 Hz hum to the signal.

I second the motion to watch the filament winding on a 'scope as you pull each tube individually. Off the top of my head I can't think of any reason for the filament line to be distorted, except for maybe crosstalk or a short in the transformer and that isn't likely. OTOH, you have a fine example of Benton Harbor engineering in that lunchbox and that makes almost anything possible..............  Tongue
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K7MDO
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« Reply #5 on: March 06, 2012, 11:45:21 AM »

Even though you have cathode tubes you might still see if the filament wire leads are "twisted" pairs.  They probably are but for some reason that was not standard or may have been skipped.

Also, in a previous thread of mine I recently asked about the termination resistors to ground at the last tube on the filament chain.  From comments by the "elmers" I learned about their ability to help keep the "hum" down as the filament winding in my device was not center tapped so had no ground connection.

For reference see the wiring diagram for an Eico 730 filament chain.



73 and good luck, Tom
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NZ5N
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« Reply #6 on: March 06, 2012, 11:50:54 AM »

Thanks for the replies.

We did try replacing the 12AX7 with another one, but that had no effect at all on the hum.  Maybe both suffered from insulation breakdown.  I know some audio buffs pay like $100 for what is claimed to be a special hum-free version of the 12AX7.

The filament (heater) wire leads are not twisted pairs.  A single wire runs from tube to tube.  The other heater pin on each tube is grounded.

It does seem that lunchboxes are prone to hum.  The one I had 40 years ago had hum.  And often when you see one on eBay, the seller says something like, "I don't know if this works, but I turned it on and heard hum from the speaker."

I will try tonight removing tubes as suggested.

73, Bill
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WB4SPT
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« Reply #7 on: March 06, 2012, 01:09:27 PM »

Core saturation of the 12vac transformer?   Anyway, try 12vdc.  I'm sure you have a source of that nearby; don't forget to disconnect the xformer, of course. 
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N0WOP
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« Reply #8 on: March 06, 2012, 06:51:53 PM »

You might look at the 120 volt ac line out of one of your outlets with your scope.  Be careful not to create a ground loop and blow the scope ground lead apart.  Measure the amount of flat topping on the 120 line and compare that in percentage with that on the output of the filament transformer. 

I see a fair amount of flat topping on the utility line here.  Apparently this is not uncommon.  It is caused by all of the electronic devices that only draw current at the peak of the ac waveform, charging those input filter electrolytic caps.  This happens all throughout the grid, causing the flat topped utility power everywhere.

Giuseppe
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NZ5N
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« Reply #9 on: March 06, 2012, 07:14:22 PM »

Thanks for the replies.  I did further testing this evening, lots of new info but still no fix for the hum.

* Removed tubes one by one as suggested, no change in the scope display of the heater voltage, some flat topping.  There's a photo of the heater line scope display at http://qsl.net/nz5n/Twoer%20heater.jpg.

* Replaced the heater voltage with regulated DC.  Somewhat to my surprise, no change, the hum persists even with DC.

* The hum persists even if the grid of the 12AX7 is grounded.  The hum goes away if the grid of the 6AQ5 audio output tube is grounded, so it sure seems like the problem relates to the 12AX7.  With the scope connected to the plate of the 12AX7 or the grid of the 6AQ5, we get an odd-looking display pictured at http://qsl.net/nz5n/12AX7%20plate.jpg. Does that tell us anything?

I am attempting to find another 12AX7 or two, just in case both of the two I have already tried are bad.  Both check OK in the tube tester.  Another than that, I am about out of ideas.

Any further thoughts?

73, Bill

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WB4SPT
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« Reply #10 on: March 07, 2012, 04:48:45 AM »

Hum that persists with DC on the filaments and "no ripple" on the DC supplies pushes this to a new level.. Is the power transformer shielded in these radios?  Is it located close to the 12AX7?  Are there primary or secondary transformer wires in the area of the 12AX7?  Your testing does indicate that ac is coming in between the driver and audio out stage.  Also, what frequency is the hum in the scope waveforms?  I can't quite read the horiz. sweep rate in the pix.  My comments on the transformer/wiring would relate to 60Hz, not 120Hz.  Does the radio use full wave rectifiers?
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N2EY
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« Reply #11 on: March 07, 2012, 07:09:06 AM »

Soulds like fun, remember this is a regen reciever.

No, it's a super-regenerative receiver. Quite a different thing.

It's probably a grounding/shielding issue, since the heater circuit and B+ are not causing it.

73 de Jim, N2EY
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NZ5N
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« Reply #12 on: March 07, 2012, 07:23:16 AM »

Not sure about the hum frequency, it is quite low, you can hear it at:
http://youtu.be/G_lhIs7hups
The same hum was there long before I did the selectivity mod, so don't think we can blame the mod.  The hum persists even when the volume control is turned to minimum.  

Pardon my ignorance, but this is the first time in over 20 years I have used a scope, how do we read the horizontal sweep rate?  That are no numbers on the display or the controls.

The Twoer as a whole is small, but I would not say there are transformer wires near the 12AX7. The power transformer is on top of the chassis and the connections to the 12AX7 are under.  The rectifier is a full wave voltage doubler.  There's a schematic in back of the manual at:
http://qsl.net/nz5n/Heathkit%20Twoer%20HW-30.pdf

There is a photo of the underside of a Twoer (not mine) at:
http://farm7.static.flickr.com/6159/6204139249_d65f386dbd.jpg
The 12AX7 is in the upper left hand corner.

Thanks for the help, keep the ideas coming,
Bill
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WB4SPT
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« Reply #13 on: March 07, 2012, 08:17:31 AM »

Not sure about the hum frequency, it is quite low, you can hear it at:
http://youtu.be/G_lhIs7hups
The same hum was there long before I did the selectivity mod, so don't think we can blame the mod.  The hum persists even when the volume control is turned to minimum.  

Pardon my ignorance, but this is the first time in over 20 years I have used a scope, how do we read the horizontal sweep rate?  That are no numbers on the display or the controls.

The Twoer as a whole is small, but I would not say there are transformer wires near the 12AX7. The power transformer is on top of the chassis and the connections to the 12AX7 are under.  The rectifier is a full wave voltage doubler.  There's a schematic in back of the manual at:
http://qsl.net/nz5n/Heathkit%20Twoer%20HW-30.pdf

There is a photo of the underside of a Twoer (not mine) at:
http://farm7.static.flickr.com/6159/6204139249_d65f386dbd.jpg
The 12AX7 is in the upper left hand corner.

Thanks for the help, keep the ideas coming,
Bill


I'm not sure that Leader scope has calibrated horiz sweep.  No problem, just compare the horiz distance between peaks of the audio ripple (hum) at the speaker to a known 60Hz source, like the filament 12vac.  If the horizontal distance between peaks is the same for each, then that is 60Hz.  If it is half the distance, then it is 120Hz.  This is critical to know, since that will steer this effort to either a line source (60Hz) or a power supply DC ripple source (120Hz).
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NZ5N
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« Reply #14 on: March 07, 2012, 08:52:27 AM »

OK, checked that, it is 60hz hum.
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