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New 10/2017
This project started after my first SP-10 rebuild project, here. I was concerned that the parts in that turntable, especially the PCBs, were 40ish years old, so I kept an eye out for replacement spare parts. I found a set of SP-10 parts but without the chassis or power supply. It did have most of the parts, including the PCBs, the motor & platter, the start/stop and speed assembly, and brake assembly. I immediately re-capped all of the boards and built a new power supply to confirm the components worked correctly. Eventually, I decided that I could build a working turntable around the parts rather than storing them in a box, and this webpage documents that process.
This photos, from my first SP-10 project, shows all of the parts in an SP-10 except the power supply. The ones that started this project are circled in red. PCBs on the lower left (with the speed control PCB), brake assembly at the top left, platter top right, and motor below that.
This chassis is the key component that was missing from the parts package I purchased. Basically, everything mounts to this chassis.
I've seen similar projects on the web (building the SP-10 into a new chassis/plinth), mostly with lots of exotic materials and machining - and huge price tags. I wanted to keep this a bit simpler, and was planning to omit both the strobe (which always shows the speed as right on target thanks to the PLL circuit) and the brake mechanism. After most of the work was done, I decided I could add the brake hardware as well, as you'll see below.
Above: New power supply, identical to the one I built for my first SP-10 project. More info including schematic at the link above. Note that it only supplies 5V and 32.5V, as the 140V supply for the neon strobe light won't be needed.
Since I has no metal chassis for this table, I started with a 1/8" aluminum plate to mount the PCBs on. I added the green sorbothane to the plate to dampen any ringing.
I found a combination of various length standoffs to replicate the Technics chassis and align all of the PCBs. I also added a aluminum bar between the PCB mounted heatsink (for the power transistors) and the main aluminum plate (bottom center front above), using heatsink compound to ensure good thermal conductivity. The above photo is actually the bottom of the assembly (as in the original, the PCBs will face down when assembled),
Another view with the PCB assembly oriented as it will be in the turntable. At this point, I could connect the motor and speed control assembly and confirm that everything worked correctly. The standoff's (legs) here are just to temporarily hold it up. When installed, the assembly will be mounted on standoffs going up into the bottom side of top panel.
I spent some time selecting wood for the plinth/base. I found some nicely figured kiln dried American Walnut from a sawmill in Michigan, shown above.
The framework/inner shell of the turntable was made of 3/4" Baltic birch. Above is the basic frame.
Above shows the top panel being fitted. The top panel, where the motor mounts, is made of baltic birch and MDF laminated with soft glue - a technique I learned from George Short of North Creek Music for speaker design to minimize resonances.
Above: 1" walnut was cut for the raised area around the platter, then a 12+" circle was routed for platter and a thru-hole cut for the motor. The edges were finished with a 3/8" roundover bit on the router. That configuration starts to look like the missing chassis shown at the top of this webpage.
Laying out the walnut sides before assembly, using the most interesting piece for the front. These 1/2" pieces are simply glued to the 3/4" baltic birch sides.
Above: Sides in place, top and bottom edges rounded with the router. The front top panel that will hold the speed controls and start-stop switch (on the right) is also in place. I've also installed oak battens to mount the armboard, and added threaded inserts for the armboard and the motor. The woodworking is basically done at this point.
I decided to move the speed controls and start-stop switch to the right side of the table, allowing the platter to be moved as far to the front and left as possible, to allow fitting either a 10" or 12" arm to the table.
I finished the base with a coat of Waterlox to bring out the grain of the wood, followed by several coats of spray lacquer. Above shows the finishing process mostly complete.
I decided to use a 10" Jelco SA-750E tonearm on this table, and this shows cutting the arm mounting hole on the armboard.
Above: Test fitting the controls (on the right) and the motor in the base. You'll notice that I used the stock speed switches but I replaced the square Technics start-stop switch with a round momentary contact pushbutton switch, illuminated blue when the unit is powered on. That switch is an E-Switch brand part # PV3H2B0NN-341. I also changed the colors of the LEDs in the speed selector switches, discussed below.
Meanwhile, I disassembled the motor, cleaned and re-lubrcated it, and replace the thrust pad. In this case, the new thrust pad is made of Torlon 4203, and is yellow as you can see above.
I disassembled the speed selector assembly to replace all of the microswitches (as the originals are 40 years old) as I had done in my first SP-10 rebuild. I also decided to add some variety to the speed LEDs which are all red in stock form. Since 99% of my listening is 33.3 RPM, a replaced that LED with a blue one to match the start-stop switch (on the right in this photo). The 45 RPM remained red, and the 78 RPM was changed to green (on the left). More photos of the completed assembly in action below.
Above: The bottom view of the base, showing the speed selector assembly (with a new longer wiring harness I made) and the start-stop switch on the right. The front of the unit is at the top in this photo.
The other end of the base showing the PCB assembly in place and wired up. The front of the unit is to the left in this photo.
Once everything was installed and tested, I adjusted the PLL period trimpots (VR101 and VR102), per the service manual. Above shows the 33.3 RPM setting of 6.3 ms. This process is described in a bit more detail in the webpage for my first SP-10, link at the top of the page.
At this point, I decided I could (and should) add the brake mechanism. However, this should have been done very early in the build process, as it required routing a series of cutouts in the top plate shown above. I disassembled everything, and then carefully cut the recesses for the solenoid and the adjustment mechanism, being careful not to damage the walnut and finish.
Above: The brake mechanism in place. Its basically a felt loop (attached to a metal band) around the inner platter, and a solenoid pulls it tight to stop rotation. The Technics engineers did a really thorough job: the solenoid is powered up at a reduced voltage whenever the turntable is not running to allow cueing, and full power (32VDC) is applied when you turn off the table to stop the platter rotation quickly.
In the process of testing the braking mechanism, I inadvertantly connected the solenoid in reverse polarity, which killed the transistor, TR213, that controls the solenoid on the logic board. I had the same problem with my first SP-10 (although I wasn't the guilty party on that one), so I simply replaced the transistor with a Fairchild KSC2690AYS from Mouser.com, circled in the photo above. Note that the legs of that transistor are too big to fit the holes of the PCB, so they need to be carefully filed down to fit into the PCB.
Above: original Technics platter screw on left and my DIY replacement on the right. I was missing the platter screws, and had no luck tracking them down, so I made replacements with stainless steel M3 x 12mm screws, flatwashers and a couple layers of clear heatshrink to replicate the original.
Interesting note: The original SP-10 (Mk1) had no screws attaching the platter - it just sat atop the sub-platter. The Mk2 has so much torque, if you neglect to screw the platter to the sub-platter, the sub-platter starts spinning immediately at the correct speed, leaving the platter standing still and slowly catching up. The turntable equivalent of "peeling out".
Since I was using a Jelco tonearm with this table, and they are sold without cables, I made a phono cable, above. I used a Cardas TIDP DIN connector, Cardas SLVR RCAs, and Neglex 2534 shielded cable. Although I used different RCAs and cable, I used the recipe here to build a fully shielded cable.
Above: The bottom of the chassis (3/8" Baltic birch) in place, after adding the three Stillpoint footers.
A trio of photos showing the speed controls with the new LEDs. First, 33.3 RPM with the blue LED to match the new start-stop switch...
... then 45 RPM with the stock red LED...
... and last, 78 RPM with the green LED.
Finished unit playing for the first time: One From The Heart by Tom Waits and Crystal Gayle. The Jelco SA-750E arm is in place and adjusted with a vintage Grado MCX cartridge for now.
The completed table with arm. No mat that came with my kit of parts, so I purchased a new Technics mat, part number RGS0010A (also the same as my first SP-10). I had a New Old Stock (NOS) Technics dust cover from another turntable and added 4 spring loaded dust cover hinges from ebay to mount the cover. I experimented with the hinges before adding the dust cover, and found I needed 4 to reliably keep the cover in the upright position. One side note: since I had the dust cover before this project started, I actually sized the base size to fit perfectly under this cover.
View from the front. You can see the Stillpoint footers here.
Rear view of the setup. On the left is the tonearm cable, and to the right you can see the connector panel (which is usually beneath the stock SP-10 chassis) containing the power cable, ground lug, and jack for the remote start-stop accessory (which is just a momentary contact switch in parallel with the main start-stop switch). As a finishing touch, I added an original SP-10 maker's tag identifying the table and power requirements - another ebay find.
Another photo of the finished table, from above with the dust cover closed.
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