Friday, April 30, 2021

Turntable Final Assembly

Now that all the main components of the turntable are done, let's put it together!  I had a few hangups in assembly because, well, my blueprints were a bunch of scrap pieces of paper in a folder collected over the course of 3-4 years, and stuff didn't line up.  Anyways, in no particular order, here it is.

First, I needed the driving belt.  I bought two of these because I was originally planning to make two turntables (though after realizing how much hard work it was, I'm settling for one).  This was a standard 3/8" V-belt and, as noted on the label, they are 38" long.  These belts were actually too wide for the pulley that came stocked on the motors, but I couldn't find anything smaller.

Next, the plug outlet box.  As much as I love my Technics 1200s, the one feature they don't have is removable RCA and power cables.  This was one feature I missed about my first cheap set of Stanton turntables, so I decided to replicate it.  This box was to be made from some aluminum sheet metal.  The piece was actually left over from my old bike basket mount, which explains the unnecessary preexisting holes.  I drilled new holes for the connectors, mounting holes, and rivets prior to bending.

Here I am filing the opening for the outlet box.  The rectangular hole was created by four 1/2" holes for the corners, and I then cut it open with my newly repaired jigsaw.

Here I've installed the outlet box with RCA and power connectors from the electronics store.  The power connector I chose was for a 6V wall adapter, which didn't end up working because it couldn't supply enough amps (in the testing videos I used a variable voltage power supply).  Since the RCA connectors weren't color coded, I just letter stamped the identification for left and right into the aluminum.

An inside view of the outlet box prior to soldering everything.

Alright, now that we've beat that subject to death, lets look at the belt again.

I was originally going to mount the motor with the gearbox on top, so the motor would be inside the base. However, due to the thickness of the base casting, the original pulley would not stick through far enough to clear it.  I resorted to mounting the motor upside down extended with two bolts to make room for the belt.  I hated this configuration.  It did work, but the belt was practically interfering with the mounting bolts.  On top of that, tightening the bolts and nuts to adjust the height was very tedious, and having to thread the belt through at the right step made it even worse.

As if that wasn't bad enough, the belt was rubbing on the high points of the casting.

Fortunately, I was able to mill down the aluminum to make more clearance.  The high lip around the opening in the casting was a consequence of that sheet metal shield I had to make in the casting phase, so it served no functional purpose at this point.  I just hadn't bothered to grind it down earlier.

While I had it on the mill, I went ahead and milled down the top of the tonearm linkage boss as well.  I realized this would've otherwise made the tonearm way too high to level with the platter.

The answer to my pulley routing woes was to just make a new bigger pulley.  This also solved the other problem I had of the platter not spinning fast enough even at full speed <insert youhavemiscalculated.jpg here>.  My new pulley included an offset to mount onto the motor shaft with a set screw, thereby making it tall enough to stick through the base casting.  I also cut the V groove wide enough the match the belt better than the original plastic pulley.

This diagram should make it make more sense.

Here it is with the new pulley installed.  Much better!  This way the motor doesn't look ugly sticking out the top of the turntable either.  By the way, if you're wondering what that is wired in series with the motor, that's an on/off switch on the left, and a potentiometer in the middle.  Due to my lack of electronics knowledge, I didn't even have a correctly rated pot to vary the speed, I just picked one at random I had lying around to have one as a placeholder.

Now let's look at the tonearm.  The first order of business was cutting it to length.  The idea here is to make sure the needle is tangent to the record grooves at all times.  This is less possible with a straight tonearm, since the geometry causes the tangency to vary across the record.  I wasn't too concerned with this, so I just adjusted it to where it was tangent at the most places.  Here showing the original length before I cut it, definitely not tangent.

After cutting the tonearm to length, notice how the needle is in line with the grooves now.

Now for the tonearm wires.  If any of you masochists out there feel like recreating this headshell connector, here is the schematic for the pinouts.  I gathered this by measuring the resistance between the headshell wires and the RCA outlets on my Technics. The audio routing on turntables is literally that simple, the cartridge wires connect directly to the RCA outputs.  I'm not talking about those lame new turntables that have built-in analog to digital converters here.  Diagram view is looking toward the inside of the connector.

Here I've soldered the tonearm wires.  These four wires attach onto the rear of the connector pins with the same removable pin connectors that came extra with my needle cartridges.

After threading the wires through the tonearm linkage, I soldered them directly onto the respective RCA outlets.

Here's the underside of the tonearm.  The tonearm itself is a very simple piece, just 1/2" tubing with two holes on the front for mounting the headshell connector with #4-40 screws, and a hole (later opened up to a slot) in back for the wires to exit.  On regular turntables, these wires are extremely small, almost to where you can't even see them.  I couldn't find any wire that size at the electronics store, so I bought the smallest I could find.  Consequently, the stiffness of the wires interfered with the tonearm counterbalancing, since they'd put upward pressure on the tonearm.  This is the reason I opened that hole into a slot, to give the cables more slack room.

I also milled some extra clearance on the tonearm swivel for the same reason.

Speaking of the tonearm linkage,  I found out the real reason why Technics has that complicated gimbal assembly.  It's to reduce friction.  You see, the tonearm has to rotate from the mere force of the needle advancing to the next groove on the record.  Too much friction, and the tonearm won't move at all and the needle will just keep skipping back into the same groove.  Well, long story short, that's exactly what happened with my turntable.  My design intent to make the tonearm linkage as heavy and rugged as possible backfired since all that extra weight made the friction too high (let's just call it "unintended anti-skating").  To remedy this problem, I machined a small brass washer to go between the tonearm swivel and bushing.  I made this from a scrap valve component I found in the street.

Alright, now let's look at the platter.  I wanted to clean up that ugly finish on the side.  Now that I'd filled the inside with Bondo, it no longer had that awful chatter problem.  The finish came out sufficiently better.

Before I completed the V-belt groove in the platter, I accidentally cracked the part where the set screw insert was doweled in.  This in combination with me not having the patience to make a proper lathe setup at that point led to the V groove coming out lopsided.  The result of this is the record speed varying as it plays.  It's almost the same effect as if you drilled an eccentric hole in a record and played it like that, but more subtle.

What can I say... the platter was almost written off as a total loss enough times over the span of this project, I was impressed I even got it to work at all.  I have plans to remake it with the proper thickness, proper shaft mounting mechanism, proper V-belt groove concentricity, or maybe even convert it to a worm gear drive since the current motor gearbox is FREAKING LOUD!!!

Well, after all that, the turntable works... to a degree, and I'm satisfied with that for now.  Even if the electrical is only half baked, the motor is super loud, the platter doesn't even spin at a constant speed, and the tonearm wires mess with the counterbalancing, it was a fabulous learning experience!  I think the tagline of this project should be: "Technics 1200's, because you probably aren't going to build your own turntables".


1. Base Pattern

2. Base Casting

3. Base Machining

4. Platter

5. Tonearm Linkage

6. Headshell Connector

Return to turntable project directory

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