Beogram 3400 Muting Switch Issue

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Aaron Posted: Sun, Jan 17 2021 8:23 PM

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Hi everyone. The other day my MMC20EN was sounding a tad "worn" on my 3400, so I pulled it off the tonearm and did an examination with a loupe. Needless to say, it was filthy. I did a careful cleaning and removed every trace of built-up dirt and dust. I installed the cart back in the 3400 and it went back to playing perfectly. The next day, I fired up the 3400 and suddenly the left channel was doing nothing but humming; as if one of the leads from the cart to ground had let go. I checked all my cables (which I replaced two years ago) and everything was fine. The pre-amp was fine as well. However, I noticed that when I hit the STOP button, the left channel would clear up for a second before the muting switch kicked in fully.

I brought the 3400 down to my bench to clean the muting switch as it was showing some dark corrosion. However, that did nothing to solve the problem. With the 3400 tipped on its side so I could watch the mechanism, I found that when the "ARM" (part 1579) slides back to engage the muting switch and lowers the tonearm, it seems to bring the muting switch back too far so that it seems like the switch is causing a dead short to ground. It mutes the channel, but introduces the hum. If I manually rotate the turntable so the cam-lifting wheel forces the "arm" to back off the muting switch a fraction of an inch, the hum goes away, and the left channel unmutes and sounds normal.

I've been playing with this for hours now and I can't find any way to adjust the muting switch so it finds that "sweet spot." I also went as far as bypassing the muting switch completely, but that did nothing but kill the left channel entirely. I checked all the pins on the MMC20EN and none are shorted to the case. I checked the pins from the tonearm to the terminals on the muting switch and there's no short there either; only when the switch is engaged.

Any thoughts? Thanks very much.

Aaron