 |
|||||||
|
|||||||
Technics SL-1200mk4 Turntable
1. Platter rotates backwards then forwards.
The MK4 was a special model made between 1997-2010 and
available only in Japan.
The MK4 had a special blend of features, notably 3
speeds, pure copper tonearm wiring.
RCA jacks and dual spring footers. The
MK4 had a very different motor PC board to accomodate
the components needed
to establish 78rpm speed.
Many of these units suffer from a
strange start up issue. At power up, upon hitting the start button,
the
platter would rotate backwards and then go forwards.
Usually, this will
only happen once per session.
The cause of this was traced
to a change in the grounding scheme that
allowed stray current from the
strobe lamp to enter the sensitive phase lock control circuitry during power
up.
This would cause IC201 to latch for 10-30 seconds upon power on.
During this time the direction control voltage (EC), Normally 6.6V was actually
just 4.0V
When the EC voltage is below the ECR reference (5V) the platter
is directed in reverse.
A study of the older mk2 boards
revealed what was wrong.
Isolating the strobe lamp currents with a dedicated
ground wire solves the problem.
The correction is shown in the photos
below.
I hope you find it useful.
.
If these pages have helped you service a 1200 and perhaps saved you
time, effort and aggravation.
And you would like to make a donation, you
can do that right here:
