HOWTO - FIX Sankyo Super-8mm projector problem = blown power transistor in motor drive circuit.

THE PROBLEM

In 12 years ownership of my Sankyo 800 Stereo Super-8mm projector, it has only developed 1 major fault. 

Symptoms:     Unsteady projection speed, usually too slow, although
                        after a long time running it can get better.

Cause:            Faulty power transistor, "T...D633"

FIX IN BRIEF

Replace with MOSPEC MJE3055T or equivalent.

FIX IN DETAIL

When the motor drive started having problems, I knew enough about electronics to have some idea what was wrong as I knew that power transistors get hot and can fail because of this stress.  The Sankyo motor control circuit is a classic "feedback" circuit of this kind.  A circuit measures the motor speed and uses this information to increase or decrease the power supply to the motor as required to correct to the standard speed.  The power supply is controlled via the power transistor.

The original power transistor only has the markings "T .. D633" on it.

I could find no such number in any references.

I asked at an electronics parts shop, and the assistant there suggested "BD743C".  This got the projector running a little better but gave a poor result with the "24" speed no longer working.

I therefore turned my attention to solving the problem, buying examples of the most common kinds of power transistors and trying them out in various configurations until I found which worked the best.

The result:  what worked was the most common and therefore most obvious type.

            NPN Power Transistor, package TO220, with pin layout:

                        1 = B

                        2 = C

                        3 = E

Case = C

The one I used was the MOSPEC MJE3055T which cost about $3 from Dick Smith Electronics.

At the time of writing, April 2005, this fix has continued to be successful for 3 months of fairly heavy work copying films on to video.

Various manufacturers add other letters and numbers to the "3055" to get their name.

What it looks like:

Remove the back of the Projector.

There are 2 circuit boards near the front.  We are interested in the upper one of these.

The Sankyo is well designed for easy removal and replacement of this circuit board. 

There are about 5 plugs to remove and these are all differently shaped so they can

only go back where they came from.

Then undo the screws and remove carefully. You need to twist and turn the board to

manoevre it past some frame obstructions.

Motor speed is controlled by a small daughter-board soldered in a vertical position.

Beside this daughter-board is a heat sink with the transistor on it.