Kenwood Basic C2 Earth Hum

Chazola

New Member
Hi, I have a C2 that works well except it has a 50hz earth hum through the output- headphone and main out- it's set level, i.e. doesn't increase/decrease with the volume control. I've disconnected all connections except mains power obviously and it's still there.
Checked for dry joints etc all over the boards and re-soldered some of the RCA connections and I've re-capped it, still hums. The balance control was knackered of course when I got it so I did the simple jumper by-pass.
It stops humming if I turn off the output using the front switch.
I've seen some C2s have a capacitor on the rear panel output RCAs, mine doesn't have this...
Any ideas appreciated! :)
 
I have a C2 that works well except it has a 50hz
Did you confirm with an oscilloscope or other that it's 50Hz? Could it be 100Hz?
50Hz will be due to mains "ripple" finding its way into the audio path, normally something like a bad ground connection (wire or solder) outside
chance would be failed rectifier.

100Hz will be ripple after rectification, again it could be bad GND connections (RCA GND's etc...) Could also be tired caps (eg, filters), transistor....

Hum on both channels?
 
Thanks, not confirmed with an oscilloscope but with 50hz generated in a sound editing program to compare, plus I'm in the UK with 50hz mains so I guess you could say an educated guess! Hum is steady on both channels, reasonably low-level and only noticeable when near the speakers.

I'm wondering if it could be to do with taking out the balance control as I just linked the pins and didn't add the resistors shown in other threads, though others have said they did the same with no issues...
 
50Hz hum means the "leak" is occurring before rectification. If rectifiers
were tired I would expect the hum to be more noticable.

However, looking at the sm shows a simple rectifier/single diode and
power supply. Single diode means half wave rectification ie, 50Hz "pulses".

Since both channels affected points to power supply, could be either pos
or neg voltage rail. Candidates are,

Q29, Q30
C79,80(47uf/16V)
C83,84(470uf/25V)
C85?(??/25V)
Q25,26
C77,78(1000uf/35)
C71,72(100uf/25V)

Really need an oscilloscope or maybe an audio probe to track this down.
If the C2 is unrestored then you could take a punt, most likely candidates being
C77,78 (1000uf/25V), C71,72(100uf/25V) and Q25,26, these are the main components
for the opamp power supply
 
Just an update on this- turned out to be the headphone amp- bypassed it completely as I never use headphones, and all nice and quiet :)
 
Just an update on this- turned out to be the headphone amp- bypassed it completely as I never use headphones, and all nice and quiet :)
I have (probably) the same hum with C2 - it is constant, volume does not affect its loudness, pressing output switch button removes the hum. How did you do bypassing the headphone amp?
 
I have (probably) the same hum with C2 - it is constant, volume does not affect its loudness, pressing output switch button removes the hum. How did you do bypassing the headphone amp?
If it is caused by a groundloop; Try over the top coppershielded ground(ing) / wiring from amp ground to tuner ground to preamp ground post and to cassette deck/TT etc if you got all this hooked up in your vintage setup. And lead that joint ground wire to a CV radiator, metal copper water or gas pipes or into the fysical earth ground if you live groundfoor / detached / townhouse / mobile home etc. Kenwood especially and older gear in general can not be grounded sufficiently to the ground terminal of your power conditioner / surge protector / AC wallsocket alone. Thorough connecting all components in a setup hard wired / grounding solved it for all my Kenwood and humming Pioneer gear.
But again: Connecting ground wirering from humming component ground post, to the ment for this groundscrew of my UltraPower powerconditioner and surgeprotector HDC-150RM in my case, made the humming even worse!
Ground all your gear, not only TT's and or Tuners. If not it will go kaputt on you. With or without humming at you first!
 
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Thanks TX-9100 for the detailed answer. I have connected a ground wire from C2 to electric extension cord's ground pin. It was like a hum reset. Finally. However noticed that a hum also builds up depending on the placement of the preamp comparing to the source... And there is another thing that I am going to replace - it is the interconnects between C2 and M2. Fiddling around but I can feel the solution is near :)

P.S. Also I wonder if the silent noise is coming from the M2 transformer? Should it be there or should it be dead silent?
 
Not dead silent . But you shouldn't be able to hear the transformer hum over the speakers. And on touch it shouldn't vibrate neither. If so you do have a worn out loose stretched copper wiring around the core of the " trafo".That can cause the vibration / hum. But these transformers were not the quietest around, I happen to know!
Aside from that I'm no real tech. But I hope for you the ground was the real problem!
 
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