2. amps-preamps
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What might cause LOUD hum in a Conrad Johnson pv6

The PV6 today blared out a loud hum after it had been running fine for a couple of hours. I shut off all power to the amp, pre-amp, tuner etc. I noticed when I moved the preamp that one of the output cables was disconnected.
I don't know if it somehow disconnected and it caused a hum or became disconnected when I moved the pre-amp. In anycase, I later plugged the cables back in securely and used only a tuner STILL A HUM then a turntable and STILL A HUM.

I am looking for someone who might have an idea what is causing the hum. I opened the preamp No smell all tubes appear to be working. So I am puzzled.
randall
randallvar
johnsonwu
469 posts
 
One RCA socket lost ground from the cable yanking. Love the tube dampers. Add heat and cause users to buy lots of replacement tubes:)
rogerbent
2 posts
 
Even after replacing filter caps, I had hum even with the volume turned down. I swapped tubes. No change. I checked voltages on the zener diode string and each was reading 36 volts instead of 33 volts. The regulator transistors Q2 & Q3 were bad. I used a MJE15032G for Q2 and MPSA42G for Q3. I replaced Q1 also with MPSA93G though it checked ok. Quiet as a mouse now! Ground rings on many jacks are bad. Next job is replacing them with jacks with nuts on inside of jack panel.
hifigeek1
770 posts
 
Given it's age it does sound like a power supply issue.
jtimothya
612 posts
 
One RCA socket lost ground from the cable yanking.

This was my first thought as well. I lived with CJ preamps in the PV-5 through PV-8 series and the internal RCA wire connections tend to weaken over time with just normal use, partlyh due to the less than robust pop rivets holding the phenolic that is the RCA plug support attached to the back panel. Plus, modern RCA cables tend to have heavier plugs and cables which may put strain on the ports. Beautiful vintage CJ sound from these units.
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