Sony AV-3650 service manual

[RJMiranda]

After jlb2 let me have the European model (AV-3670CE) service manual, which I studied very carefully, I got the full AV-3650 service manual, including all the supplements. When you look at all the changes Sony made to this product in the course of several years (sometimes the supplements were separated by just a few months) you may think this was a very popular VTR at the time. Otherwise, they would not have bothered at all with so many updates.

In this model, the capstan is powered by a DC motor that has a frequency generator (FG) inside. The output of this generator is used by the servo circuits to control the speed and the phase of the tape in relation to the incoming video signal, so when you press Record to start editing, the transition is smooth. (Not having rotary erase heads, "smooth" is a little too benevolent term.)

Well, I tried measuring the output frequency of the FG, and got only silence. There must be an open in the coils. That's why the capstan overspeeds, because the servo doesn't get any reference signal.

I trust I may get another motor from a similar machine that belongs to a friend of mine. He has it only as a showpiece, and is not interested at all in restoring it. I hope his machine is a late model, like mine, because among the changes Sony did to the 3650s, there was a new capstan motor in the post-1974 machines.

[RJMiranda]

3650 capstan motor.jpg

This is the AV-3650 capstan motor. The connections to the motor itself are in the V-shaped plate at the right. The two purple wires are from the PG, and they measure infinity.

The brown thing in the upper part of the motor (left in the picture) is not made of metal, but instead is a thin rubber-like cilyndrical cover that goes over half the thing.

If my friend's machine has a different kind of motor, I may have to repair this one. Nothing really difficult, I think, except that the tiny Allen wrench to open it may be hard to find. (I live in an embargoed country, you know, and can't have all the tools I would like.) But if 20 years ago any technician here could replace individual VHS or Beta heads in their home workshops, and adjust the penetration too, this can't be too difficult. (No, we can't splice damaged filaments in vacuum tubes, yet )

Thank you very much for your kind attention. I will keep you posted, and maybe in January this 3650 will be in working order again.