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   Home   Help Search  
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Author Topic: SB303 headphone hum  (Read 896 times)
KD0ZGW
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Posts: 1056




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« on: June 20, 2019, 02:51:36 PM »

Just got this working.  seems to work fine except for very noticable hum when using headphones.  Can't hear the hum using the speaker.

Looking for someone to direct me to the probable components to be replaced.

thx
KD0ZGW
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N4UE
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Posts: 927




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« Reply #1 on: June 20, 2019, 04:29:11 PM »

Hi. Do your headphones match the 303's headphone jack impedance?

Do you have another set to  check with?

What happens if you plug an 8 ohm speaker into the headphone jack?

I have a 303 and have no such issues, but I do have a LOT of headphones....

ron
N4UW
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If you're not the lead sled dog, the view never changes......
G3RZP
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Posts: 1321




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« Reply #2 on: June 21, 2019, 02:16:35 AM »

60 Hz hum or 120 Hz?
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W1VT
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Posts: 3401




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« Reply #3 on: June 21, 2019, 07:25:11 AM »

Is the volume setting higher with the speakers than the headphones for "normal volume."  If so, you may want to resistively attenuate the headphone jack to make them similar.
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AD4U
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Posts: 2543




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« Reply #4 on: June 22, 2019, 01:59:59 PM »

If the SB 303 needs high Z phones and if you are using low Z phones, you will get hum.

Dick AD4U
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N6KD
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Posts: 0




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« Reply #5 on: June 27, 2019, 01:49:06 PM »

Several possibilities:
1.  Your headphones are for Hi Fi and they reproduce low frequency sounds much better than your speaker
2.  You have a bad tube in the audio section.
3.  You need to replace the filter caps in the HV section because they are not doing their job anymore.
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K5WLR
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Posts: 313




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« Reply #6 on: June 27, 2019, 03:05:37 PM »

The Heath SB-303 is 100% solid state; no tubes present.  Grin
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K6BSU
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Posts: 66




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« Reply #7 on: June 27, 2019, 04:13:03 PM »

The SB303 uses a multi-transistor push-pull audio amp capacitively coupled to the speaker terminals.  It will therefore drive just about any impedance equally well.

Audio hum can be coming from many places in the radio.  However, if no hum in a loudspeaker, there likely is no problem.

One suspect:  unshielded headphone cable picking up 120 Hz from nearby fluorescent lighting?
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KD0ZGW
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Posts: 1056




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« Reply #8 on: June 27, 2019, 04:56:15 PM »

Thx for suggestions.  No nearby flourescents.  Club member that restores old stuff is going to look at it on an upcoming project day.
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K6BSU
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Posts: 66




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« Reply #9 on: June 29, 2019, 09:46:11 AM »

Lighting may not be problem.  I once chased a buzzing noise in a home brew receiver.  Finally traced it to my soldering station!  Seems like the SCR in the temp regulator was making RF "hash". 

My computer monitor (an old-style CRT) makes noise that I can hear in my HF receiver at least 20 ft. away!
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