Hum question

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  • Jr. Member
  • Posts: 24
Re: Hum question
« Reply #20 on: 24 Mar 2013, 05:24 am »

The electrolytic in the B+ you speak of, is that C103 ?
I have replaced C106, C304 & C305. But not C103 and C300R, C300L.

I have not tried replacing the power chord but will give that a shot too.


  • Full Member
  • Posts: 616
Re: Hum question
« Reply #21 on: 24 Mar 2013, 05:01 pm »
I want this:
Two small wires to essentially bypass each resistor?
Also, I went back and tried to power off the Clarinet again, with my ear right up against the speaker.
The hum is gone immediately, both when I turn the switch off and when I pull the power cable out of the wall.
Not gradually like I mentioned before. I performed the test twice just to make sure I wasn't imagining it.
Hmmm... It is a different case. The shorting out resistors trick should be useful too, but I now think the same as JH: it seems the filter capacitors in the B+ circuit (C103, less possible: C300) are the main suspects. I can't imagine what should happen with the power cord that will led to the symptoms.


Re: Hum question
« Reply #22 on: 25 Mar 2013, 04:13 am »
A bad C103 would cause these symptoms.



  • Jr. Member
  • Posts: 24
Re: Hum question
« Reply #23 on: 31 Mar 2013, 09:40 pm »
I replaced caps C103, C300L, C300R to no avail. Also tried the resistor bypass that member poty mentioned, no effect.
However, I think I may have stumbled upon the problem. One of the resistors that I had updated (Clarinet resistor update) a few months ago, R310R, had a poor solder job on one of the joints.
I discovered this because the sound from the left channel woofer was crackly. Like the sound one gets from poor contact. Probably didn't notice until now because I been listening for the hum with my ear right up against the woofer.

Oddly I always suspected the left channel sounded lower and not as full as the right. 
Any how, at first I reversed the interconnect and the sound moved to the right channel. I figured it was a bad interconnect cable, but after swapping interconnects with another pair, it was still there. Then I started to look for suspicious solder jobs on the board and resoldered that one and now I can not perceive any hum from my normal listening distance. And the volume on the left channel is now louder and more distinct and clearer than before (specifically the lower region). I only hear some hum now when I crank the volume past 12 which is way beyond my listening level. Than you all for your help.