2SA750 -> ZTX795A = hum? (Rotel RX-803 phono)

[grandpaslab]

My Rotel RX-803 had a noisy phono section, so I though I'd try swapping out some transistors, starting with the PNP input pairs. I replaced the original 2SA750's with Zetex ZTX795A's. Now I've got serious 60hz hum. I've double checked the pinout: 750's are ECB, 795's are EBC, but I've got the subs in correctly, at least if I can trust the silkscreen. What bonehead maneuver have I committed this time??

 

[redk9258]

Is the cover on? Maybe you need the shielding from the metal cover.

 

[grandpaslab]

I wouldn't have believed it, but you're right! Cover reduces hum to just about nothin'. Bonehead city, population: me.

 

[grandpaslab]

Looking closer, I now see that the wires from the RCA jacks to the phono section are not shielded--they're just 2 conductor side-by-side...maybe that's where my noise was coming from in the first place. I might just try putting some coax in there.

 

[redk9258]

I bought a receiver from the auction site and was cleaning the insides. I noticed the PHONO section had a lot of hum. I was ready to recap, etc. I put the cover back on and the hum was gone. Glad I didn't go crazy trying to recap, because the hum would have still been there! At least until I gave up a put the cover back on.