SX-1080 Loud Hum on Phono Only

kapohoguy

Active Member
My SX-1080 receiver has developed a very loud hum and a lot of distortion when playing records. It was professionally serviced a couple of years ago and a bunch of capacitors were replaced. It has worked fine until now. This only happens with Phono mode--it sounds fine in AM, FM, Aux, & with both Tape Monitors. The hum is the same with either Phono 1 or Phono 2 selected and regardless of whether a turntable is connected. This leads me to suspect a problem in the preamp. Does it need to go back to the shop? Any ideas would be appreciated.
 
Does this occur if the receiver is in PHONO mode and the turntable cables are disconnected?
 
The low frequency hum becomes noticable at about 1/4 volume, whether or not a turntable is connected. With a turntable, records sound tinny and distorted even at low volume.
 
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That eliminates the cables or table picking up EM. Since the EQ amp in the 1080 uses ICs, I was hoping it was that simple.

Cleaning the PHONO1 and PHONO2 switches would be a good start. Similarly, you may also want to check the wiring to the microphone jack, and the contacts on the leaf switches in the jack itself. A little deoxidizing cleaner and exercise by repeatedly inserting and removing a ¼" phono plug might clean things up a bit.

It may also be worthwhile to check grounding in the GWS-125 board and the RCA Phono input jacks (GWS-127 board).

Failing all of that, the caps in the EQ circuit may be suspect.
 
That eliminates the cables or table picking up EM. Since the EQ amp in the 1080 uses ICs, I was hoping it was that simple.

Cleaning the PHONO1 and PHONO2 switches would be a good start. Similarly, you may also want to check the wiring to the microphone jack, and the contacts on the leaf switches in the jack itself. A little deoxidizing cleaner and exercise by repeatedly inserting and removing a ¼" phono plug might clean things up a bit.

It may also be worthwhile to check grounding in the GWS-125 board and the RCA Phono input jacks (GWS-127 board).

Failing all of that, the caps in the EQ circuit may be suspect.

Thanks Watthour. I'll try those fixes before hauling it to my AV repair guy. Just disconnected the 1080 and lugged it down to my workshop. Temporarily replaced with an SX-D7000 I've been trying to sell for weeks on CL. Now I'm glad it is so underappreciated.
 
I haven't taken the SX-1080 to my repair guy yet, so it's been sitting in my office where I've been able to observe the problem in more detail. It appears that the PHONO mode low frequency hum only occurs when the receiver is first turned on, becoming noticable at about 1/3 volume and higher. However, after about 15 minutes the hum dissapears, leaving background static that becomes noticable at around 1/2 volume--again, only in PHONO mode.

Any ideas?
 
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