This is my first post as I haven't been able to find any clues on this excellent forum but somebody may have had this problem before, which is a hum in right channel only with or without the pre-power jumpers fitted so it is the power amp only at fault, not the power supply as only one channel is affected. I have changed the electrolytic caps in both power amps and it would be nice to fix it!
SX-850 very low level hum, right channel only
- Thread starter Vintage53
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MIJ_Pioneer
Super Member
Did you replace any other components other than caps? Transistors, diodes etc.
Welcome to the forum, you came to the right place.
Welcome to the forum, you came to the right place.
do a thorough check of the right channel grounds from the power amplifier board to ground.
Be aware that we are looking for BOTH open grounds (high resistance) AND ground shorts to inappropriate grounding places.
So look for broken wires, and pinched wires.
Sometimes the manual does not illustrate the care and feeding of grounds.
For example the ground returns for left and right speaker terminals are kept separate all the way back to the star ground.
Their minuscule resistance with the currents involved could develop millivolt level voltages that could interfere if inadvertently.
I could include a whole course on this, most of which you don't need now.
pin 24 (zobel output filter ground)
pin 21 (input circuit ground silence reference)
"POWER IN" amplifier input jack ground may have a small ceramic disc cap between the ground connection and the chassis metal.
but the pre-out / amp in jacks are special as far as grounding is concerned.
first: see if a shorting plug at the right channel power amplifier input silences the channel.
Be aware that we are looking for BOTH open grounds (high resistance) AND ground shorts to inappropriate grounding places.
So look for broken wires, and pinched wires.
Sometimes the manual does not illustrate the care and feeding of grounds.
For example the ground returns for left and right speaker terminals are kept separate all the way back to the star ground.
Their minuscule resistance with the currents involved could develop millivolt level voltages that could interfere if inadvertently.
I could include a whole course on this, most of which you don't need now.
pin 24 (zobel output filter ground)
pin 21 (input circuit ground silence reference)
"POWER IN" amplifier input jack ground may have a small ceramic disc cap between the ground connection and the chassis metal.
but the pre-out / amp in jacks are special as far as grounding is concerned.
first: see if a shorting plug at the right channel power amplifier input silences the channel.
Thank you for the speedy replies, no I have not changed any other components yet as after following the procedure for setting up the amp the measured values are within the correct range but both VR1 and VR2 are near the end of CW travel (and a PITA to adjust) and the reading is always changing by 2 or 3 mV even with no load and shorted inputs. (is this indicating a problem!) With headphones connected there is no noise corresponding to the small changes in DC level but the hum is still present on the R channel. Regarding the ground returns I agree they can be very tricky and I have checked the wiring and it seems to all be factory fresh, so no displaced or damaged wiring.
If not, connect a jumper between pins 18 and 21 (does the same thing basically)