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Author Topic: 60 cycle hum  (Read 1067 times)

KA9OID

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60 cycle hum
« on: July 27, 2021, 01:46:38 AM »

Why do I get a 60 cycle hum when I plug my headphones into my SX-117 Hallicrafters?
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K1KP

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Re: 60 cycle hum
« Reply #1 on: July 27, 2021, 05:53:56 AM »

There could be multiple causes for hum with headphones.

Looking at the schematic, I see that this is one of those old boatanchors with a 2-wire power cord, and the chassis is connected to the line through capacitors. This is a moderately dangerous design, because if one of the caps short you could have AC voltage on the chassis. It would be wise to replace the cord with a new 3-wire cord and directly ground the chassis to the power cord ground. You might already have this mod. It won't necessarily cure the hum problem, but it might.

Next, is the hum coming from the early audio stages? If the level of hum varies with the AF Gain control the hum is coming from the product detector or earlier. If it is unaffected by the AF gain control it's getting into the later stages.

Hum can be caused by routing of the filament wires too close to sensitive nodes (early grids).

It is possible (though not very likely) that there's a bad tube in the output stages. You could try swapping tubes.

GL,

-Tony, K1KP
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WA9AFM

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Re: 60 cycle hum
« Reply #2 on: July 27, 2021, 06:35:45 AM »

Ditto on what K1KP says.  Go through the unit while listening on the headphones and tap each of the tubes and listen for anomalies.  This is a faster way to do an initial check of the 'hollow-ware' devices.  If you still have the two-prong power plug, try reversing the plug in the wall socket.  The 117 was, and still is, a very good receiver.
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KM1H

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Re: 60 cycle hum
« Reply #3 on: July 27, 2021, 09:06:21 AM »

Quote
Looking at the schematic, I see that this is one of those old boatanchors with a 2-wire power cord, and the chassis is connected to the line through capacitors. This is a moderately dangerous design, because if one of the caps short you could have AC voltage on the chassis. It would be wise to replace the cord with a new 3-wire cord and directly ground the chassis to the power cord ground. You might already have this mod. It won't necessarily cure the hum problem, but it might.

60 cycle hum is usually due to leakage in the AC line bypass caps which should be replaced with modern ones. I rarely switch to a 3 wire line cord and with good line bypass caps there is no danger.

120 cycle hum is often due to the 3 power supply filter caps in the can especially if original.  It seems that Halli had some hum problems since they added a pair of .1mf 50V ceramic disc caps from the filament hot side to ground, those might be acting up as Ive had problems with those low voltage ones several times in other sets of the same 1960's time frame.

The headphone jack might not be making a good ground contact; have you tried a speaker since the same transformer is used; also note that 3.2 Ohm headphones are required.

Enough rambling, hope it helps.

Carl
Ham since 1955
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G3RZP

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Re: 60 cycle hum
« Reply #4 on: August 04, 2021, 08:46:54 AM »

If the cold side of the headphone socket relies on a connection through the chassis back to the output transformer, that can be the problem, because  the chassis isn't zero ohms. Low, but not zero. Running a lead back from the cold side of the headphone socket, with the socket insulated from ground, is a good answer. I know of some transceivers where the  cold side of the loudspeaker was run back in the same laced cable as the 'cold heater return and actually used the same wire in the cable. Produced the same effect...
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W9AC

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Re: 60 cycle hum
« Reply #5 on: August 07, 2021, 04:45:06 AM »

It's been too long since I owned an SX-117, but one source of 60 Hz hum is inductive field-flux coupling between the power transformer and audio output transformer, including any inter-stage audio transformers.  Back in the day, hum/buzz problems were largely masked by the limited frequency response of Hi-Z headphones. 

Two examples:  Collins 75S-3A and the solid-state Hammarlund HQ-215.

The Hammarlund hum problem was so bad when using modern headphones, that I gutted the entire AF section and constructed a class AB push-pull amp in its place.  In the original design, the power transformer is placed adjacent to two audio transformers.  Even after moving the transformers on the chassis and re-routing cabling, enough hum remained that that it was intolerable to me.

Similar problem with a Collins 75S-3A.  Although my Collins 75S-3C has zero hum issues, the 3A's output audio transformer is also placed near the power transformer.   You can remove all audio going into the output transformer and 60 Hz hum is still there and it's not in any way related to headphone jack bonding.  Instead of a re-design, I simply took the AF control wiper arm contact and routed it with RG-316 coax to the receiver's rear panel "spare" RCA phono jack and bypassed it with a 0.001 uF cap.  When using the 3A, a separate audio amp is used with much better performance than the original design -- and no hum/buzz. 

When the chassis is used as a tube's filament return, the entire chassis has 60 Hz current.  What we may think is a safe grounding point often isn't.  In these cases, hum can sometimes be reduced by re-tightening a tube socket's ground ring to the chassis.  I think better designs use balanced twisted filament leads to keep 60 Hz currents off the chassis -- and a means to control 60/120 Hz current from a power transformer's center tap by segregating grounds, then bringing them together at one point, away from high-gain audio stages.

For me, tracing hum and buzz is one of the most difficult parts of a restoration because there are so many ways for it to affect an audio circuit.   

Paul, W9AC
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W3SLK

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Re: 60 cycle hum
« Reply #6 on: August 07, 2021, 05:44:44 AM »

Paul said:
Quote
For me, tracing hum and buzz is one of the most difficult parts of a restoration because there are so many ways for it to affect an audio circuit.
What a profound statement. Several years ago I purchased a Valiant I from a well known AMer and BE. I had all kinds of reports about the audio was good but there was a persistent hum. I could see it on the scope. When I dug into the chassis, I found it full of 'modifications' where leads were never trimmed, parts 'RTVed' to the chassis, etc  to the point I didn't know what the former owner was trying to do. Later I think they were trying to 'Hifi' the audio with 6146B's. I bought it at a good price but now know why: He couldn't get rid of the 'hum' either! I've since shotgunned the speech amp but became so frustrated that I decided it would best to serve as a 'hangar bay queen', (a reference to us carrier sailors!), and be a parts rig should I get another Valiant.
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W9WQA

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Re: 60 cycle hum
« Reply #7 on: August 07, 2021, 06:58:16 AM »

Quote
Looking at the schematic, I see that this is one of those old boatanchors with a 2-wire power cord, and the chassis is connected to the line through capacitors. This is a moderately dangerous design, because if one of the caps short you could have AC voltage on the chassis. It would be wise to replace the cord with a new 3-wire cord and directly ground the chassis to the power cord ground. You might already have this mod. It won't necessarily cure the hum problem, but it might.

60 cycle hum is usually due to leakage in the AC line bypass caps which should be replaced with modern ones. I rarely switch to a 3 wire line cord and with good line bypass caps there is no danger.

120 cycle hum is often due to the 3 power supply filter caps in the can especially if original.  It seems that Halli had some hum problems since they added a pair of .1mf 50V ceramic disc caps from the filament hot side to ground, those might be acting up as Ive had problems with those low voltage ones several times in other sets of the same 1960's time frame.

The headphone jack might not be making a good ground contact; have you tried a speaker since the same transformer is used; also note that 3.2 Ohm headphones are required.

Enough rambling, hope it helps.

Carl
Ham since 1955

"since they added a pair of .1mf 50V ceramic disc caps from the filament hot side to ground, those might be acting up as Ive had problems with those low voltage ones"

im curious, just what were those caps supposed to do??
scope that point,  add a dozen caps. did the waveform change?
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KM1H

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Re: 60 cycle hum
« Reply #8 on: August 07, 2021, 11:15:42 AM »

Quote
"since they added a pair of .1mf 50V ceramic disc caps from the filament hot side to ground, those might be acting up as Ive had problems with those low voltage ones"

im curious, just what were those caps supposed to do??
scope that point,  add a dozen caps. did the waveform change?

Supposedly part of TVI prevention. In a receiver they helped bypass power line noise.

Ive had those disc ceramics go bad on several commercial ham TX from 100W and up.
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AC2EU

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Re: 60 cycle hum
« Reply #9 on: August 07, 2021, 12:04:18 PM »

It's been too long since I owned an SX-117, but one source of 60 Hz hum is inductive field-flux coupling between the power transformer and audio output transformer, including any inter-stage audio transformers.  Back in the day, hum/buzz problems were largely masked by the limited frequency response of Hi-Z headphones. 

Two examples:  Collins 75S-3A and the solid-state Hammarlund HQ-215.

The Hammarlund hum problem was so bad when using modern headphones, that I gutted the entire AF section and constructed a class AB push-pull amp in its place.  In the original design, the power transformer is placed adjacent to two audio transformers.  Even after moving the transformers on the chassis and re-routing cabling, enough hum remained that that it was intolerable to me.

Similar problem with a Collins 75S-3A.  Although my Collins 75S-3C has zero hum issues, the 3A's output audio transformer is also placed near the power transformer.   You can remove all audio going into the output transformer and 60 Hz hum is still there and it's not in any way related to headphone jack bonding.  Instead of a re-design, I simply took the AF control wiper arm contact and routed it with RG-316 coax to the receiver's rear panel "spare" RCA phono jack and bypassed it with a 0.001 uF cap.  When using the 3A, a separate audio amp is used with much better performance than the original design -- and no hum/buzz. 

When the chassis is used as a tube's filament return, the entire chassis has 60 Hz current.  What we may think is a safe grounding point often isn't.  In these cases, hum can sometimes be reduced by re-tightening a tube socket's ground ring to the chassis.  I think better designs use balanced twisted filament leads to keep 60 Hz currents off the chassis -- and a means to control 60/120 Hz current from a power transformer's center tap by segregating grounds, then bringing them together at one point, away from high-gain audio stages.

For me, tracing hum and buzz is one of the most difficult parts of a restoration because there are so many ways for it to affect an audio circuit.   

Paul, W9AC

I think you are right about the intrinsic design issue.
Even with all new caps, I believe the S-117 had a residual hum in the headphones- just enough to be annoying.
It wasn't very noticeable in the speaker,though.

KM1H

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Re: 60 cycle hum
« Reply #10 on: August 07, 2021, 07:11:23 PM »

Quote
For me, tracing hum and buzz is one of the most difficult parts of a restoration because there are so many ways for it to affect an audio circuit.   

Paul, W9AC

Some of the most aggravating ones to find and time needed to fix are the radios that used the the ground lugs on a socket that was riveted to the chassis without a star washer. No fun on something as complex as a NC-400 as one example I worked on many times as a National tech. There were two production runs and the first 3 digits of the serial were the Engineering Release. I seem to recollect somewhere around 400 total with a large number going to the FCC and other government agencies. Sure easier to work on than a SP-600. It was pretty much an updated NC-183D in functionality which was an updated NC-183.

Never owned one but a few came thru here for overhauls and all had star washers and rivets which indicated the second run. First run ones done in the Service Dept had 6-32 hardware and rework line women drilled out the rivets first.

What a great QRZ web page Paul, Im still not thru reading it. Arguably the best on the Internet.

Carl
Ham since 1955
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W9AC

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Re: 60 cycle hum
« Reply #11 on: August 08, 2021, 05:39:41 AM »

Carl, you might be interested in the HRO and NC-101X restorations near the bottom of that very long QRZ page!  Both receivers are rock-solid and really enjoyable to use.  There's just no comparison between the 1930s era Nationals and the Hallicrafters Skyrider series.  Put a SX-25 next to either National receiver and the difference is night/day.

By the way, there's nearly no audio hum from either receiver although I think I told you before that the NC-101X sometimes exhibits "hum modulation" on 20 and 10 meters.  Others may not know what this is -- it makes CW carriers sound "fuzzy."  That's the best way I can explain it.  It's strange.  I can tap the HFO tube (and even try a half dozen) and it comes and goes.  I'm in the minority, but I'm a believer that the 2.5V filament tubes as used in the early HROs was the better choice when using an HFO that runs at the true operating frequency. 

From the 1946 ARRL Handbook under the heading of "The High Frequency Oscillator":

"...However, in these two circuits (three Hartley HFO circuits shown in the Handbook) the cathode is above ground potential for RF, which often is a cause of hum modulation of the oscillator output at 14 MHz and higher frequencies when 6.3-volt heater tubes are used. Hum modulation usually is not bothersome with 2.5-volt tubes, nor, of course, with tubes which are heated by direct current. The circuit of Fig. 723-C overcomes hum, since the cathode is grounded. The two-coil arrangement is advantageous in construction, since the feedback adjustment (altering the number of turns on L2 or the coupling between L1 and L2) is simple mechanically."

Paul, W9AC   
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KM1H

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Re: 60 cycle hum
« Reply #12 on: August 08, 2021, 11:17:07 AM »

Carl, you might be interested in the HRO and NC-101X restorations near the bottom of that very long QRZ page!  Both receivers are rock-solid and really enjoyable to use.  There's just no comparison between the 1930s era Nationals and the Hallicrafters Skyrider series.  Put a SX-25 next to either National receiver and the difference is night/day.

Very nice work as usual Paul, what I found decades after leaving National is that the HRO and all the moving carriage Nationals need a complete overhaul of the plug in coils as well as removing all the NC-100, etc, variety coil boxes.

Using my Run 2 1935 HRO, after the overhaul including all hand made resistors plus all paper caps & 'lytics, at only 180VDC results in rock solid stability and excellent sensitivity thru 20M but useable on 10. Minimal images with those high Q double RF stage coils. It uses the 6.3V tubes as that was the PS I found at Dayton in the 80's
Similar detailed overhauls with the early and late NC-101X, NC-200 and 240D here.

My SX-17 blows away the current SX-25 which is unfortunately a great looking radio so I was forced to get another one! In 1956 I graduated from a HB AA5 tubed regen to a bandspreaded BC-453/455 and wanted to upgrade to something that covered 15M which was early Cycle 19 hot for my friends. A ham friend of my dad suggested his SX-25 and to make it short I "made" him give it back after a month and with my own money bought a clean used HQ-129X at Harrison Radio in NYC Radio Row. Not the hottest on the highest bands but nothing a 717A in the RF amp and a 6SG7 first IF couldnt cure.

My Mentor was Brother Pat, W2GK (SK) who already did that to the W2ZLK Bishop Loughlin, Brooklyn HS club station HQ-129X. Look up his life long RCA vacuum tube research, collection, etc. He took a group of us on tour to the RCA Harrison NJ plant.....WOW!
https://www.qcwa.org/w2gk-09194-sk.htm


Quote
By the way, there's nearly no audio hum from either receiver although I think I told you before that the NC-101X sometimes exhibits "hum modulation" on 20 and 10 meters.  Others may not know what this is -- it makes CW carriers sound "fuzzy."  That's the best way I can explain it.  It's strange.  I can tap the HFO tube (and even try a half dozen) and it comes and goes.  I'm in the minority, but I'm a believer that the 2.5V filament tubes as used in the early HROs was the better choice when using an HFO that runs at the true operating frequency. 

I never experienced that but Id first suggest a poor connection where the coil box plugs into the socket under the tuning cap. TarnX, loads of Q Tips and disc brake spray cleaner works wonders on the scores Ive overhauled for others and myself. A minimal drop or so of DeOxit D-100 on variable cap wipers is another necessary step IMO.  With Hallicrafters and some Nationals I solder the wipers to the cap frame.

Quote
From the 1946 ARRL Handbook under the heading of "The High Frequency Oscillator":
"...However, in these two circuits (three Hartley HFO circuits shown in the Handbook) the cathode is above ground potential for RF, which often is a cause of hum modulation of the oscillator output at 14 MHz and higher frequencies when 6.3-volt heater tubes are used. Hum modulation usually is not bothersome with 2.5-volt tubes, nor, of course, with tubes which are heated by direct current. The circuit of Fig. 723-C overcomes hum, since the cathode is grounded. The two-coil arrangement is advantageous in construction, since the feedback adjustment (altering the number of turns on L2 or the coupling between L1 and L2) is simple mechanically."

Paul, W9AC   

I never noticed a difference between 2.5 and 6.3V but I know Millen loved the oldies until WW2 when he was forced to use octals.

Carl
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WB2WIK

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Re: 60 cycle hum
« Reply #13 on: August 09, 2021, 05:26:22 PM »

I always believe that when a rig exhibits a hum, that's because it doesn't know the words.
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KL7CW

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Re: 60 cycle hum
« Reply #14 on: November 17, 2021, 09:45:33 AM »

I have solved problems by using EXTERNAL TOOTH lock washers with probably 6-32 hardware on various devices which depend upon a good chassis ground.  Even a one ohm resistance may be a problem.  Tube socket rings, and terminal strip ground tabs are prime suspects.  As long as the washer has prongs that point outward you should be OK.
             Rick  KL7CW
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