TECHNICS SP-10 MK2 Refurb Project

Above: Top view of the SP-10 at the start of this project.

With the bottom cover off, showing the missing the Control PCB, which should be on the right side.

The first step was to get the unit operating at all.  I was fortunate to find the missing control PCB from Angus MacDonald in the U.K.  Angus was also very helpful with some repair and debugging advice, so thanks very much!

1. Power Supply

My unit didn't come with a power supply, so time to build one.  The stock power supply provides 5VDC and 32.5VDC for the logic and motor, along with 140VDC for the strobe lamp.  Since I intended to replace the existing strobe with an LED (mostly for reliability), I only needed the 5 and 32.5V supplies.  I designed a supply based on LM317 regulators, with some of the enhancements from the datasheet schematics, mostly based on Gary Gallo's design published in The Audio Amateur magazine, issue 4/90, p.47.  The LM317HVT regulators were suggested for this application by Dave Cawley here, see post 82. Note that the Technics power supply holds off the 32.5V supply until after the 5V supply is up and running.  I believe they only need come on simultaneously for the unit to work.  The capacitor in the leg of the LM317 determines charge-up time, so in my supply, I used a larger 47uf cap in the 32.5V supply than in the 5V supply, where I used a 10uf cap.  This works just fine.

Above: My power supply schematic.  Click to enlarge.

Above: Photo of completed power supply.  Larger transformer and top PCB are the 32.5VDC supply, smaller transformer and lower PC are the 5VDC supply.  Transformers are from Antekinc.com.  The 32.5V supply is a dual 15V transformer with the secondaries in series.  The PCBs are from ebay, most other parts are from Mouser.com.

Above: rear view of the power supply.

2. Work Stand

The SP-10 Mk2A manual shows a work stand to raise the table off the workbench allowing you to lower the single PCB for test and maintenance.  No such stand is shown in the Mk2 manual, as the four PCBs cannot be easily dropped down for service.  I built a simple stand to hold the table level, as normal, and also flip it up 90 degrees to give full access to the underside and PCBs.  NOTE: I suggest you always remove the platter when standing the turntable upright to avoid stress on the main bearing, and the danger of it falling off if not bolted on.  Also, since the table was not made to be run without the platter, test connections should be made with the table upright, then it should be lowered, the platter put back on, and then the unit can be run for testing.

Stand in normal position.  Notice the tall legs in the rear to balance the unit when vertical.

The stand vertical.  Note the platter has been removed.

The business end, with test connections being made.

3. Capacitor refresh

Before powering up the unit, as a precautionary step, I replaced all of the electrolytic caps on all of the PCBs with Panasonic FCs (rated at 105 degrees) at the stock values.

Control PCB with new caps and pots (Thanks to Angus).  The pots are used to adjust the PLL circuit (see below).

Drive PCB with new caps.  Dave Cawley suggests that C2,C4,C6 at the top are the most critical to replace, his comments here.

Above: After heating the switch cover, I was able to gently pry the cover off.  This shows the double sided adhesive tape that was holding it in place.

Above: closeup of the cover, after cleaning.  Note that its just a very thin aluminum stamping.

Above: switch with cover sanded & refinished.  Note the brass screw in the center of the plastic base of the switch, used to adjust the switch action once re-assembled in the base. Once you re-glue the cover back on, there is no access to this adjustment.  Note the very dark yellow color of the 33, 45 and 78 RPM labels - to be fixed shortly.

Above: Internals of the switching assembly with the new start-stop micro-switch in place.  These are the 3 LEDs for speed indication.

Above: The three new speed switches installed (bottom view), not yet wired up.

Above: The completed rebuilt switch assembly ready for installation.  The start-stop button cover was glued on after assembly into the base and adjustment of the switch action, as described above.

5. Next problem: No brakes!

The brake works by activating a solenoid that tightens a loop around the inner platter.  The solenoid never worked on mine.  Thanks to Angus again for advice, I replaced TR213 (along with the diode on the solenoid, just as a precaution), and the brake worked again.  I couldn't find the stock transistor (2SC1384A-Q), but I found a reference to a substitute: Fairchild KSC2690AYS available at Mouser.com.  Note the legs on the replacement transistor are too large to fit thru the PCB holes, and rather than risk damaging the PCB, I filed the legs down a bit to fit the board.  There is some additional maintenance that should be performed on the solenoid.  When I fully disassembled the SP-10 to have the base powder-coated, I discovered there is a rubber washer on the solenoid that had fallen apart.  That washer measures 14mm outside, 8mm inside by 2mm thick.  I found a close replacement at the local hardware store, and just had to trim the outside diameter to fit.  There is also a rubber stop on the end of the plunger that was completely missing from my unit, so I made a simple replacement as shown in the photos below.  A bit of white grease to lubricate the plunger was added during reassembly.

Disassembled solenoid plunger.  Original rubber washer at upper right (in pieces).  New washer below it, cut down from larger rubber washer.  Once I had installed the solenoid and plunger, it looks like I didn't have to trim the washer - a larger outside diameter would have fit fine.

Above: Assembled plunger with new rubber washer and stopper (on the right).  Since the stopper was missing in my unit, I guessed on length, and ended up trimming a bit shorter than shown here for proper operation.

Some good additional SP-10 brake info is here.

6. Base Refurb

The base of my unit had numerous scrapes and marks and showed a bit of corrosion, so after considering various options (including trying to clear coat it with polyurethane) I decided a new paint job was in order.  Instead of normal paint, I decided to have the unit powder coated in the hope it would be more durable.  After removing all the bolted-on parts, I used a heat gun to soften the adhesive under the Technics logo.  Unfortunately, the logo was made of a thin copper, and although it didn't look great when I started, it definitely looked worse for the wear after I removed it.  I eventually decided to have a new custom nametag made at Frontpanelexpress.com.  

I used a company called Imperial Metal Finishing in Stratford, CT (website here, talk to Vinny, the co-owner) to do the powder-coating. They had the unit bead blasted and powder-coated in a color that I chose, somewhat darker silver/grey than original.  Since I had trouble cleanly removing the Technics logo, I didn't even try to remove the maker's plate on the rear of the chassis - I asked Imperial to mask it over during the bead-blasting and powder-coating process.  They did so and it came out just fine.

Above: My DIY SP-10 disassembled.  This photo was actually taken just before I started reassembly, so the base has been powder coated and looks much better than before. Disassembly wasn't very difficult: first the plug-in PCBs, then the motherboard with the attached switch assembly, ground lug, and power connection plate/cable.  Then remove all of the standoffs, and the strobe lens.  Last is the brake assembly and motor.  I suggest taking some notes as you go and keeping the screws in separate labelled bags for each subassembly (as shown above) to facilitate getting it all back together correctly.

Above: Chassis stripped is all parts, including the Technics logo.

Above: Chassis after powder coating, before I reassembled everything.

7. Motor Refurb

While the base was out for powder-coating, I overhauled the motor.  When I removed the top portion of the motor, the shaft showed minimal wear, but the plastic thrust pad had a significant dimple where it rotates on the ball bearing (which sits stationary via friction fit in the bottom of the well).  I ordered a Torlon replacement here for $35.  Next I removed the bottom nut from the bearing housing.  Note that it is held in place with some form of threadlocking material, so you need to heat up the end cap with a heat gun before trying to remove it.  Although I've seen references to people removing it by hand once heated up, I had to use the additional leverage of a pair of water pump pliers to free mine.  Important: to ensure that the nut is tightened to the same place/tightness as at the factory, you should mark the nut and the housing BEFORE disassembly so you can align the marks during reassembly (see photo below).  Note that the ball bearing is a tight fit, and you need to gently drive it out with a wooden stick from above.  When I disassembled my unit, it had a fair amount of dark grease (formerly oil) that I cleaned with isopropyl alcohol.  The ball bearing in my unit showed wear marks on the upper side facing the plastic thrust pad, so after cleaning, I inserted the ball other side up, so a clean smooth side faces the thrust pad.  Note that the ball bearing does not rotate - its a friction fit into the bottom of the spindle shaft well. When reassembling, I used Loctite 242 (removable) threadlocker to secure the cap.  I added fresh spindle oil during re-assembly from KABUSA.com, part number SFWO 010, just $6, which is apparently Anderol 465, and is the oil Technics originally specified for this use. 

Bottom of motor, showing alignment marks I made before removing the end cap.  You can see the manufacture date of June 17, 1976 on the motor.

After removing end cap, ball bearing in the bottom of the shaft well.

Ball bearing shown next to thrust pad.  Note wear marks on each.

Another view of the old trust pad, clearly showing the wear dimple.

For comparison, the new Torlon thrust pad installed.

The Loctite 242 threadlocker used when replacing the bearing cap.

The bottom with the Loctite applied just before installing the end cap.

The motor: cleaned, re-lubricated, and with a new thrust pad (although you couldn't tell any of that from the outside).

8. Reassembly & testing

After I picked up the base, I reassembled the SP-10 in the reverse order I disassembled it.

Above: Photos showing the heatsink on the drive PCB, with heatsink compound applied, before re-assembly.  Note the area where this mounts to the base has had the powder coating removed with a wire wheel.  Although there was no heatsink compound when originally manufactured, I felt that better heatsinking would keep the drive transistors cooler, and less prone to problems.

9. New LED Strobe

I decided early in this project to eliminate the stock neon strobe lamp and the associated 140VDC supply.

There are a couple resources on the web with suggested circuits for an LED strobe for the SP-10:

DIY thread showing LED strobe

LED Strobe design link 2 (see post 4665)

DIYAudio thread on SP-10 LED strobe

I started with the first, and most simple version.  After a bit of experimentation with various orange LEDS, I selected a Kingsbright diffused orange LED, P/N WP7113SED (Mouser.com part number 604-WP7113SED, here).  I ended up with 3 LEDs and associated dropping resistors on a small perfboard about 1" by 2".  Note that you'll need to drill two 1/8"  holes to mount the board, and the LEDs should be mounted near the opposite edge of the board, as shown below.  Also, I used two short (8mm) M3 standoffs to mount the board (Mouser P/N 706-740-342470), here.

Above: LED PCB with dropping resistors and 3 orange LEDs.  Note that the LEDs are tilted a bit away from the mounting holes to better align with the platter window.

Above: The LED board installed, with two supply wires coming from the motherboard, detail below. Off to the left, you can see the two pins originally connected to the stock strobe lamp (originally at 140VDC) , now unused, covered in white heatshrink.

Above: Motherboard connections of the LED strobe: 5V and N2 pins.

Above: New LED strobe in action.

10. PLL adjustment

The only adjustment (outside of the 5VDC and 32.5VDC power supply voltages) in the SP-10 is the PLL circuit, documented in the service manual on page 4.  The two pots (VR101 & 102) on the control PCB make the adjustments at 33 and 78 RPM, with scope leads attached to the O, S, and T test points on the control board.

Above: Connections to the O, S and T test points.

Above: The finished product, bottom view.