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Kenwood KR-4070 Receiver - LOUD "Motorboating" sound in right channel

redpackman

redpackman

Active Member
The volume control has no effect on this. It is a loud static sound coming from the right channel only. I've cleaned the controls with De-Oxit and this isn't acting like that in my experience.

Any suggestions would be appreciated.
 
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Sounds like a transistor - however these have printed-on-board resistors that have given me problems in two of this model.
 
Check the AF board (left front, below the meters) for any of the devices on the "top eleven" list of suspect transistors. There may be some 2SA950s in it.
 
Watthour said:
Check the AF board (left front, below the meters) for any of the devices on the "top eleven" list of suspect transistors. There may be some 2SA950s in it.
Click to expand...

One question, I don't see the 2SA950 on it? Is it more reliable than the "worst eleven" but still prone to fail more often than most transistors?
 
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In as much as the loud, continuous static sound is coming only from the right channel and it totally unaffected by the either the volume or balance controls, doesn't that indicate that it's in the amplifier section?

I have checked and there are no SA950's in the amp section. But in Q1-Q4 (which call for 2SA640E or F, or 2SA841) instead I find transistors marked CEN PN4 249 in all four spots. What are these and are they a match? Also in Q5 which calls for 2SC1980T, which are found in Q6-Q8, I find an NTE287, which I presume is a match for the SC1980T.
 
Obviously, someone has been in the unit since it left Japan. The NTE devices are alway suspect, and changing them out for a know reliable device might be prudent.

The data provided for the PN4249 does NOT indicate Ft (transition frequency) but does show a test pulse width of 300 µS. If that is any indication of the Ft of the device, the equivalent frequency would be in the range of 3.5 KHz. If that were the case, oscillation at even normal audio frequencies is quite probable. It would be prudent to change out all of those for more reliable and appropriate low-noise devices, such as KSA992Fs.
 
pmosher said:
60 hz noise? Getting some AC through the transformer?
Click to expand...
I sure hope so....Power transformers don't work with a straight DC voltage. And they don't filter out 60 hz..Rectifiers convert AC to DC .Then smoothing capacitors filter out any remanence of 50/60 hz signal riding on the DC voltage.
 
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