Pioneer SX-780 static kind noise when activating loudness switch

[calavera68]

Hello,

I found an SX-780 for sale for $50 dollars including a pair of modern Sony speakers. I decided to replace the capacitors and relay even though I thought it was working OK when I tested it. I completed the recap and I also cleaned with Deoxit all the pots and switches. I powered it on and everything seems to work OK except for a static noise introduced to the left channel when I activate the loudness switch, I cleaned the switch a few more times and this does not make a change. Unfortunately I didn't test the loudness switch before the work I performed and I don't know if the issue was there before I replaced the capacitors.
Can someone point me in the right direction to figure out where the problem may be? The static noise sounds similar to the noise you hear when you don't have your radio tuner set to a broadcast station and sounds like white noise.

Regards,
Pedro.

 

[markthefixer]

loud static overwhelms or substitutes for say... FM stereo audio? or background static where the program material is predominant?

left channel. does rotating the balance control change things? Does mono / stereo change things?

use an ohmmeter to check your loudness switch.

 

[calavera68]

I did more testing and the static noise can be heard only when the Aux input is selected, I can’t replicate this issue in FM nor phono. I noticed that the static noise can be heard if the treble switch is moved all the way clockwise, activating the loudness switch just makes it louder. I can still hear the music, but the hiss noise overwhelms it at higher volumes. The static noise can be heard in stereo on both channels and only on the left when in mono. I realized that I had the switch set to mono yesterday when I first turned the unit on. When the volume control is below the fifth notch, then the static noise seems not to be there, or maybe I can’t hear it, but it may be still there.

 

[markthefixer]

stereo / mono switch connects / blends / mixes both L & R channels together when the lever is DOWN, in MONO.
the stereo / mono switch is positioned pretty farl downstream from the function selector switch.

left AUX line. try shorting plugs in the AUX inputs. What happens?

basically this is casting aspersions on the left aux rca jack, the shielded cable to the GWK-118 audio assembly pin 52 (R = pin 48), the tracks to the function switch and the function switch. From there FM and everything else travel over the same path..
as a note, pins 48 & 52 are in close proximity to the function switch.

It's been my experience that the Function Selector switch gets REALLY corroded, and takes a bunch of cleaning & deoxiting. Most of the time I end up disassembling the switch and polishing the blades with a pencil eraser.

 

[larryderouin]

"polishing the blades with a pencil eraser. " I knew they were good for something other than tickling frontal lobes......

 

[calavera68]

Thank you Mark, I shorted the Aux inputs as you suggested and it sounds the same as when I switch to phono and there is no turntable plugged in.
I will work trying to clean the input switch with Deoxit, and if needed I will attempt disassemble it to clean like you said.

 

[calavera68]

Mark,
I took the input switch apart and cleaned the blades with an eraser like you suggested. I installed it back up and the noise continues to be present. I connected the audio input directly to posts 52 and 48 to rule out a problem with the wires coming from the aux input plugs and tried it again and nothing changed. I forgot to take a picture of the blades after being cleaned but all the black stuff showing in the uploaded image was gone. Any tips on where should I check next?

 

[raycarr00]

out of curiosity, did the static appear after the recap job? i'd go back and see if something's shorting(possibly the cap leads). had a similar issue with a preamp and one cap had its leads touching- the "bow legged" lead version. maybe a solder bridge.

 

[calavera68]

Unfortunately I only tested the FM tuner before recapping. I discovered something else right now. If I connect the audio input to either tape1 or tape2 play inputs while the input selector is at the FM position the static is there also, which makes me think it may not be the input switch. I will go ahead and check my work on the cap leads like you suggested @raycarr00