TNT Mk-5 bearing upgrade
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The improvement made when I re-lubricated my Mk-1 main bearing, made me very curious
as to
how much better the newer Mk-5 bearing might sound.
After checking that it would be compatible with my table, and swaying to and fro about waiting
for the TNT-6 platter/bearing upgrade to emerge, I ordered up the Mk-5 bearing.
Note: This is not the bearing used in the later Mk-5 and Mk-6 TNTs. Those tables
now have the lighter, clear acrylic platter and use an inverted bearing with a
Teflon thrust plate (like the HRX). I don't believe the inverted bearing is
compatible with the older (heavier) TNT platters.
Jumping to the conclusion first, the Mk-5 bearing is a major improvement.
There was a very significant drop in noise floor,
with more clarity, focus, depth, transient snap and bass tightness apparent.
This is not a subtle effect, the increased clarity is observable outside the
listening room! Frankly, I'm amazed that a bearing swap could produce such a
dramatic improvement in sound.
If you're using the Mk-1 bearing, this is a no-brainer - very highly recommended!
Here are the two bearing sets - Mk-5 on the right with the huge 3" securing nut:
The bolts for attaching the Mk-1 bearing housing (which is aluminium) look very
inadequate compared to the massive 3" nut that threads directly onto the Mk-5's
(steel) housing.
The Mk-5's nut has holes on opposing sides so pins can be inserted to tighten it
down very firmly to the turntable plinth (I used
a couple of nail punches for this). While the Mk-5 exhibits closer tolerances
than the Mk-1, I suspect it's superior method of attachment to the plinth has much to
do with the improved sound.
The Mk-5 nut also provides an earth lug.
The Mk-1 housing had tape wrapped around the top to centre it in the hole
provided in the turntable's plinth. I did the same when installing the Mk-5 to
keep cartridge alignment essentially undisturbed. When assembling the bearing I
placed about 6 drops of 'Mobil 1' engine oil in the well and a couple on the
shaft. (several weeks later I found the VPI provided oil in the packing box!!
- at the next oil change I'll use this).
My platter had to be re-levelled, using a hex key to adjust the three grub screws at it's centre. I
used a second hex key, positioned on the plinth with a blob of 'Blu-tack' (so
that the short arm
just overhung the platter's top edge), as a handy runout
tool for levelling the platter. Platter level can be adjusted to within a
fraction of a mm using this method.
A Possible problem with the Mk-1 acrylic pulley base:
Notes:
1) I don't believe this is a problem if you have the later Mk-2 steel
pulley base, which is apparently more compact.
2) Since I've now upgraded to the one-piece motor/flywheel, this information is no longer
relevant to my system.
After I had fitted the new bearing and replaced the turntable plinth and
platter, I noticed there was no clearance to the side of the pulley base. Even after adjusting motor assembly position for the biggest gap,
I only had a clearance of about 1mm.
A few solutions to increase clearance are:
1. Raise the height the turntable feet (In my case, by placing more
sand in the box).
2. Put a groove in the acrylic pulley base.
3. Reduce the motor assembly height.
I'd made my turntable platform flush with the top of the sand box and didn't want to
raise it. To reduce the motor height would require changing the feet.
So
I chose option 2.
Putting a groove in the middle of the acrylic base is very easy -
acrylic is extremely workable. I started with a 'bastard' (coarse) file to create the rough
shape, then smoothed with a 'rat tail' file and finished with some wet & dry
paper. The final 1200 grit paper produced a silky smooth finish, so it looks like
the groove is meant to be there - I had to stop because finish on the groove was
outshining the rest of the base. At the bottom of the groove you can see what
appears to be scratch marks, but the acrylic has actually gone translucent(!)
and what you can see is the inside edge of the base!
The new groove in the pulley base:
Mk-5 bearing sitting in the groove - clearance is now about 10mm:
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