HUM IN VINTAGE HIFI'S
The sources of hum are bad cables, cables not secured tightly enough, oxidized/corroded receptacles on receiver/amp or oxidized/corroded cable end connections.
The cables, or patch cords, RCA's, fail because the connection right at the plug assembly will open or make intermittent connection. The RCA connection will loosen over time due to usage. The manufacturing process in the 70's were not as precise and exact as today is. As a result, if you precisely measures patch cords from 10 or more brands made in the 70's, you will find slight variations in size. That is also true for the receptacles into which these plugs are inserted.
Anything, electronic or otherwise, will deteriorate if stored in cellars and attics as temperature and humidity reach their extremes. Especially here in New England as we go through the seasons. Many customers come in and say "It was working perfectly and It stored it in my attic/cellar and now it won't work." Those extremes of temperature and humidity hasten the oxidation process mentioned earlier. Additionally, lead used in solder is more porous than metal and I believe is more susceptible to oxidation.
That being said, hum and intermittent channel/s are generally caused by a combination of all of the above. These are the steps that I take to address hum and channel issues:
1. Use metal polish (Brasso,etc.), and polish the amp/receiver receptacles till they shine like a mirror.
2. Take a stiff bristle brush with a little metal polish and give the cable ends a good brushing until they too shine.
3. Push the outer sleeve of RCA's inward so that when plugged into their receptacle, you feel a snug/tighter connection than before.
4. Squirt a small amount of wd-40 onto the cable ends and work the cable a little by rotating it in the receptacle socket. WD-40 also helps to prevent future oxidation.
The above was a general method to get the most out of your cabling. If you still have these hum and intermittent channel issues, there may be a problem with your receiver, amp, or turntable that a qualified technician should look at it.
PHONO HUM TROUBLESHOOTING
All of these above symptoms apply to troubleshooting phono hum problems as well but are complicated by the fact that the phono signal is much weaker than the aux/tape/cd signal.
The phono signal strength is from .002V to .008V, whereas the aux/tape/cd signal is from .140V and up. The signal is much smaller making oxidation connection issues more noticeable.
From the phono cartridge, through the turntable internal wiring to the phono RCA cables, there is no amplifying. Just a series of connections each of which may cause a problem. These connections include the wire clips on the cartridge, perhaps a muting switch(used to short the signals while the turntable is going its automatic mechanical functions), and a soldered connection to the RCA cables.
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