I'm starting to run out of ways to introduce all of these receivers that I've been finding lately. I've been extremely lucky to come across so many this winter—it's usually the in spring and summertime that this happens, mosty likely because people are cleaning out their garages.
This Kenwood KR-6400 is rated at 45 watts/channel and has pretty much all of the standard features you can ask for on a vintage receiver. From what I could find, it seems to be related to the KR-5400 and the KR-7400. All three share a similar faceplate design with the tuning and volume knobs protruding through the tuner dial glass. It looks like you can pass a quadraphonic signal into the back, but I'm unfamiliar with how it works. Normally this feature is a huge selling point, so I found it strange that it wasn't mentioned anywhere on the faceplate. The owners manual seems to confirm that you can in fact listen to 4-channel sound with this receiver, but it probably converts it down to stereo or something.
As usual, most of the lights were burned out when I got it home, so once again, I ordered replacements from djwogo.com. The original style of bulbs were unfortunately no longer in production, so I had to make due with a slightly different crimp style.
The replacements fit pretty good in the original rubber holders, but I had to stick the bottom end in a little further to angle them up so that the bulbs weren't resting against the internal plastic enclosure.
With the new bulbs installed, it was looking fairly pretty, but there was still another small problem. Whenever I powered the receiver on, no sound could be heard at all until I cranked up the volume. Once I did that, the sound would kick in and I could reduce the volume back to a normal level. I've had this problem before with my Sansui AU-5900, and the culprit was the protection relay. The tiny little contacts get dirty with oxidation over the years and cause trouble. With the Sansui, I was able to simply remove the plastic cover from the relay and gently rub the contacts with a piece of paper soaked in contact cleaner. It did the job, but I found out that they still sell the exact same relay used in this receiver today, and it was only $13 on Digikey. So I figured I might as well do the job right and replace the part entirely. For anyone wondering, the manufacturer is Omron and the part number is MY4-02-DC24.
To remove the relay, you just need to remove a couple screws on the amp board and lift it out, then tilt it back to access the solder joints.
The new relay was almost identical to the old.
I've gotten a lot of soldering practice lately so the installation was fairly easy. It helps if you bend a few of the pins back to hold the relay in place while you solder it in.
With the relay replaced, it was working perfectly again! One useful tip to test whether a relay is giving you trouble or not, is to gently tap on the plastic case while the receiver is running. If you hear static through the speakers, you should probably replace it. Or at the very least, try to clean the contacts.
And of course, as usual, I made one final adjustment to the bias current, setting both channels to 50mV by adjusting VR1 and VR2 on the amp board. Perfect!