Betamax PALsite: SL-C20 : Technical

Technical

Inside view

SL-C20 rear connectors
Rear connectors

Pendulum

hall effect sensors with glue
Hall effect sensor attached with glue

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 cause is usually down to two contributing factors.

The first is the drive pendulum which can cease up and the second more serious factor, is wear on the reel motor bottom bearing. The pendulum is simple to replace but the bearing is moulded into the chassis and 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.
The diagram above shows the pendulum assembly

Stops in play, record and reverse/forward wind

Do not confuse this with the above symptoms. The problem in this case is due to lack of 4.43MHz colour sub-carrier reference in the servo circuit. This in turn is normally caused by failure of silicon fuse PS1 on the YC-25 board.

Patterning on picture

If you experience a patterning on the picture irrespective of whether you are viewing a tape or not then the most likely cause is that capacitor C319 in the power supply has dried up.

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.

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
Damaging tapes or tape looping during eject	Check resistor R420 (3.3ohms) then transistor Q334 2SC10613
Insufficient or excessive colour in playback	Check resistor R019 (56Kohms) on panel YC25.
Intermittently will not eject-cassette	Check connector CN608 on the SS16 panel for a poor contact.
No capstan servo lock in record	Suspect failure of transistor Q332 (2SC2458) on panel SS16.
No colour	Check the value of resistors R024 (56k ohms) & R278 (10k ohms) before suspecting capacitor C047, all on the YC25 panel.
No E-E picture after 15 seconds, records in black and white	Adjust RV009 for 4.3volts p-p at C104 on panel YC25.
No functions or display	Check 1C001 on the TP37 board
No play, FF or cue	Suspect failure of the forward sensor.
No take up / chewing tapes	Check condition & positioning of the pinch roller.
Not erasing previous recordings	Check for failure of Q503, Q501 then Q502
On/standby switch inoperative and corrupt display	Check diode D001 on the TM37 panel.
Program display showing -au-	Check resistors R063 (120Kohms) & R073 (4.7ohms) before suspecting failure of IC003 on panel TU39
Stopping in rewind / review mode	Suspect failure of transistor 0606 (2SC2458) on panel SS16
Switches off shortly after switch on	Check PSU. In particular regulator IC401 (STR1229]) on the PS23 panel
Will not eject cassettes, pinch solenoid not operating	Check resistor R654 on panel S516
Will not tune	Check for failure of IC004 on the TU39 board before suspecting the pre-set unit PS001 of failure
Intermittent squiggly patterning	Pattern is very similar to that caused by C319, but NOT applying to E-E loopthrough. Check Delay Line DL001 on video PCB.
tape not returning to cassette prior to eject	this is sometimes caused by the gears not fully engaging. press rewind stop then eject.
The machine only laces up half way	Check and clean Microswitch or replace if necessary.
Will not load / lace or eject tapes	When attempting to load a tape, the loading motor is seen to stall. This is due a fault in the loading / threading clutch assembly. (Located directly atop the loading motor). The clutch would jammed due to a cracked "sun" gear (the central gear in the clutch0. Replacing the gear cleared the fault.
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