Thursday, 24 October 2019

Hitachi HT-L33 turntable repairs.

I was given this by a kindly benefactor...

It's a Hitachi L33 turntable from about 1982.

"It plays too fast, I've tried to adjust the speed, 33 is just about OK, but 45 is wrong"

Great, this will do as a gift to my mate Alan, who's been on the lookout for a sensible small turntable for a while.

I check both speeds, which seem to be far too fast, and varying badly.

Here's the speed circuit from the manual...

The motor PWB (printed wiring board, don't ya know!) is housed inside the motor. It's a brushed DC motor, controlled by a small regulator on the board. Normally the speed is adjustable via a small hole, operating a pot inside the motor. In this case, that connection is made available outside the motor, via the orange wire.

The speed is first set to 33 RPM, whereby the FET Q01 is switched on by the micro processor, and is effectively short circuiting R42 (the 45RPM control) and R41. Once R40 (33 RPM control) is set, you can then adjust 45. Doing it the other way round won't work, as 33 RPM will also adjust 45! The whole motor and speed arrangement is fed by a regulated 9.9V supply from Q11. Q10 is there to rapidly bring the motor up to speed, under control from the micro.

OK, the pots are already set to minimum speed. Damn... why so fast ?

Incidentally, be careful to secure the platter in place with some tape (or remove it) before turning this upside down. If you don't the platter will hit the stylus and probably ruin your day.

Ah! The belt has ridden up the pulley! Effectively changing the gearing..

The belt is moved into the correct position and re-tried. It's now running too slow, and followed a few seconds later by too fast...

The belt had ridden back up the pulley.

Removing the motor shows that the motor mount hadn't been installed correctly.

Now it's running steady in the correct place, but too slowly.

Using the cueing button, just moving the arm to the left starts the motor at 33, whilst we still have the lid up...

Adjust for 33.33333 RPM!

Seems reasonable !

and then press the speed select switch, and adjust for 45 RPM.

Close enough.

A quick look at the stylus under a microscope shows it's in good health, so off to Alan it goes!

An easy fix, and a stylish linear tracking deck. All Alan needs now is a phono pre-amp!

Another saved from landfill!

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