I recently bought a used ST-120 for quite a steal and ever since I got it home, I'm chasing hum. At the first owners home, I checked everything out. Some dust and a few questionable solder joints, but all worked and sounded great. no hum whatsoever. The owner was several hours from home so I stopped at my parents house along the way. At my parents house, I plugged the amp in to a set of test speakers, plugged my phone in using an AUX cable, and still...no hum and sounds great. I get home that night, hook everything up in my main system, and the frustration begins. I immediately notice a strong hum/buzz coming from the speakers.
After a few days of troubleshooting, checking ground connections, and trying different setups, I managed to quiet things somewhat. I learned that I cant put any interconnects anywhere near the main power transformer (ie, passing them under the front of the chassis to the back), the amp was quieter plugged directly into the wall, and the main Mullard driver tube was a little noisy and sounded quieter when I plugged in a $10 JJ. I was able to at least listen to my turntable this way, but it was still very annoying. In my configuration tests, I confirmed that the amp is quietest (nearly dead quiet) if I just plug an AUX cable into it and use my phone. All of the noise from above dissapears if I'm useing the AUX cable. Another discovery was that my preamp/headphone amp seems to be part of the problem. When plugged in but turned off, I get some quiet hum. When turned on, the hum gets louder and lower in frequency.
My solution to the noisy preamp was to just cut it out to of the chain. I ordered the step attenuator which is on its way, but I discovered what appears to be a major problem with this solution last night. When I plug my phono preamp from the turntable directly into the amp inputs, I get a MASSIVE ground hum. As soon as I touch the amp chassis or the grounding post on the preamp (or both), the hum dissapears. I tried plugging into different outlets, grounding to the ST-120 chassis, moving things arround, and nothing helps. What has me completely baffled is that I don't have this issue when my phono preamp is plugged into my integrated amp, even at full volume.
So after troubleshooting all week, and finally feeling like I had things figured out, I'm back at square one. The common denominator seems to be the ST-120 itself. I'm feeling defeated and need some help.