Monday, February 20, 2023

Repairing a "mute" Technics SL-1600MK2 turntable

 The Technics SL-1600MK2 is basically an older brother of the famous SL-1200MK2. It has an identical heavy platter and a similar overall body weight. On top of the robustness and features of its younger and famous sibling, it has the benefit (to some, a downside) of an auto-return, soft touch buttons for cueing.

A nice shot of a SL-1600MK2

Unlike its popular cousin, the SL-1600 series were only manufactured from 1977 to 1981, meaning all models you may be lucky to find on the market are over 40 years old. Adding to the fact that this model has a more complex circuit board, tone arm and cuing mechanisms, proper maintenance is a must to keep this turntable spinning!

The auto-return and cuing mechanisms are made possible by means of tiny plastic gears that unfortunately are made of plastic which tends to became brittle over time. You may find several posts of owners of these auto-return vintage turntables where these gears have cracked and finding replacements for them are not easy and the the labor required to replace and align them is far from trivial. If you have one of these vintage turntables, it is wise to add a bit of epoxy glue on its centre to avoid any cracks to spread out throughout the gear. I was lucky with my turntable as the previous owner took care of the plastic gears by adding a bit of epoxy glue on them: 

An example of epoxy applied to contain the spread of crack on the plastic gear

Aside from potential cracks on the plastic gears, you will need remove the old lube and apply a new one to ensure there is reduced resistance to move the gears agains each other. I like to use white lithium grease mixed with a bit of synthetic oil to make it less thick (you can see it applied on the photos below).

These turntable models also feature an auto-mute switch which is connected to the cueing. This fancy (but not super useful) feature cuts out the potential noise while you are handling the tonearm, allowing the sound picked by the cartridge only when the arm is placed over the record. Because the muting process is controlled by a mechanical contact switch, this is yet another component that needs to be maintained or replaced in order to function properly. 

About a year ago, my turntable suddenly became muted due to oxidation of this contact switch. I used De-oxit and I made it work again. However, it only lasted about 6 months until it got stuck on "mute" again. Therefore, I decided to remove it from the circuit altogether as I don't really find this muting important for my overall listening experience (I kinda of expect some noise from such an analogue equipment experience).

Disconnecting the mute switch:

Disassembling the unit is relatively easy, just follow the detailed service manual that can be found here: mkII service manual

First step is to remove the heavy platter, then gently put the turntable upside down and remove the 4 feet

Then you need to remove the screws holding the back of the platter

Now it was time to disconnect the logic board connectors

Don't forget to remove the screws holding the phono audio cable

Next, you need to remove the tonearm from the chassis

With the tonearm removed, flip it upside down to expose the gears

Remove the screw holding the shield metal plate and finally remove the metal plate

Underneath the metal plate you can see the 2 audio channel cables that are connected to the muting switch

I unsoldered the small shield wire from each of the channels, taking the muting switch off the circuit (I found these would do the trick after tracing these with a multimeter) 

After disconnecting these, I used a piece of kapton tape to keep them isolated from the circuit board

Done! My SL-1600MK2 will never get muted by that switch again!

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