Betamax PALsite: SL-F25 : Technical

Technical

Head layout

Pendulum

hall effect sensors with glue
Hall effect sensor attached with glue

Fault and repair guide

Stops in play or record

This is becoming a more common problem these days. The first thing to check is that there is adequate torque on the take up spool. If there isn't much then the thing to do is to try to increase the foreword torque of the reel motor to compensate for wear. There is an adjustment for this on the servo PCB. This is the lower board in the machine. The adjustment needed is a yellow pot labelled 'FWD'. Turn this to maximise take up torque.

If this fails to fix things then the next thing to try is replacing the pendulum assembly. This is the device that transfers drive from the reel motor to the cassette drive reels. It can become seized with age. Ultimately replace the reel motor bearing. This is moulded into the chassis and so replacement necessitates that the entire chassis be replaced.

One possible compromise is to fit a small piece of plastic into the bottom bearing to raise the rotor and allow it to move freely. Sony supply a repair kit for this purpose.

No head drum rotation

You may find that the head drum fails to rotate. This is due to a problem with the Hall effect device on the head drum motor which is secured by a blob of glue. Full details on how to fix this can be found on the head motor page.

Deck goes into rewind

This can be caused by failure of the tape end sensor. You can check this by unplugging the sensor at its socket on the PCB.

Failure to load a tape

This is usually caused by a damaged carriage. However, if you are lucky then it may just be the plastic arm which stops the tape being inserted correctly that has become lose. Look for a white plastic arm on the right hand side of the carriage.

Not recording sound

This is caused by failure of the bias oscillator. Symptoms to look out for are patterning on the video picture. Repair usually requires replacement of the bias transformer and associated capacitors.

Loading gear location

Loading gear and shaft repair

The loading shaft for the Sony has no official replacement but most times it can be repaired, unless the gear is in two pieces or a tooth has broken off. There are two approaches to solve this, either repair the broken gear, or purchase a similar gear and adapt it to fit.

Replacement part approach

In practice the diameter is sufficient and the reduced length does not really matter, just use super glue (or other suitable glue) to glue them to the shaft and protrude the shaft 1mm or so past the end. See: RC-Toy-Car gears

If you don't need 50 then look for a smaller quantity but the price does not seem to change much.

Repair of the existing gear

Remove the black plastic gear from the shaft and then clean the shaft of all plastic residue.
Take the gear and pass a hobby knife (or similar narrow blade) down through the gear crack to remove any plastic residue to allow it to close to its correct gap width.
Place the gear in a soft jaw vice such that you can apply pressure to close up the gear crack.
Now use a drill with the same diameter as the outside diameter of metal drive shafts knurled shaft end to clean out the rough plastic residue from the shaft hole.
Take the gear out of the vice. Take the shaft and put super-glue on it and press the shaft back into the gear.
Put the assembly back into the vice again (with the shaft facing down flat, gear top facing up) closing up the gap again but leaving about 3mm of the gear exposed above the top of the vice. (note the gap is facing across the vice gap).
Drill a hole the diameter of paper-clip metal through the top of the gear and shaft but just below the surface of the top disc.
Insert the wire from an unfolded paper clip and turn both ends hard up, then trim off the excess metal. This will lock the plastic gear to the shaft and should keep tension across the gear and not encourage the gap to open up again.
Remove any excess super glue from the cracked tooth gap. The repaired gear may make a slight noise in operation but that is only an issue when loading and unloading. It puts the machine back into normal operation versus a scrap heap or spares. The pin clears the drive gear by a good margin and prevents the plastic spinning on the shaft again.
Thanks go to Noel Higgins for this fix

Quick fault guide

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Fault	Solution
No head drum rotation, no picture (not even from tuner)	Check N20 ICP fuse where power supplies enter the main top PCB at the back left.
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