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Hum and No Sound in Left Channel

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reaper996

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Joined 2009
#1
I recently completed a Bob Latino ST-120 amp build and I cannot run down why I have a hum and no sound in the left channel. The right side does play and is stone quiet. The hum does not vary with volume. I have re-soldered all ground connections as well as re-soldering all connections from the step attenuator to the inputs and driver board. I have re-soldered connections to the recitifier and power tubes. I have shorted the inputs to rule out external factors and the hum persists, so it is somewhere in the amp itself. I checked the top side of the amp to make sure none of the underside wiring was potentially touching the amp chassis. I have swapped both the driver tubes and power tubes, as well and trying a different rectifier tube to see if the bum follows, it did not. My tube compliment is 3 matched RCA clear top 12au7's for the driver section, a new Mullard GZ34 and a matched quad of Genalex KT-88's. I tested all tubes prior to installation.

I have experienced the 60 Hz hum before and it generally winds up being lead dress or a suspect solder joint, but poking around with my trusty wooden chopstick I cannot get the hum to increase or decrease. I did notice that when I moved a screw driver or anything metal across the left side there is a noticeable change in the hum volume. When I do the same to the right side it is quiet.

The tubes will bias and I get the correct voltages for both the left and right side power tubes.

I know this is a long post, but I wanted to include all that I have attempted to do to run down this issue before posting this on the forum. I have searched the Net in hopes to find someone else that has experienced a similar problem but no avail. I also spoke to Bob directly, but was limited in help regarding the issue. No fault to him.

I am looking for some direction from a forum member(s) that can suggest what I should try next to solve this issue of hum and no sound as I have run out of ideas. I have a DMM available for troubleshooting.

Thanks for taking the time to read this.

Patrick

Gut Shot.jpg
Input Section.jpg
Power Section.jpg
Left Side.jpg
Right Side.jpg
 
#4
reaper996 said:
I have removed the step attenuator from the circuit, going directly to my Preamp and no improvement.
Click to expand...

Then assume that both the hum and the lack of audio are caused by the same problem, and signal trace
starting at the input for the cause of no signal. Once you fix that, if the hum is still there it may be due
to layout, such as a filter choke coupling to the audio circuit.
 
#5
I re-soldered the left channel pentode/triode switch after hearing it pop when switching between modes with no volume. Hum is barely audible with volume turned off in both speakers. I understand that this very low level hum is possible with the tube amps. I am going to check the resistance on the OPT output side to determine if this is causing the lack of audio in the left channel.
 
#14
I have missed the forest for the trees, I had one resistor solder joint that I overlooked this entire week. I guess the benefit from this mental exercise is that I have learned a bit more on how to logically troubleshoot a given issue. Lesson learned.
 
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