UK Vintage Radio Repair and Restoration Discussion Forum

A common problem with the Sony SL-C9UB VTR is that the clock / channel display disappears and the tuner stops working. This often happens at switch-on from the mains, the machine having worked OK the last time it was used.

This happens when the DC-DC converter fails. It converts the +12V line from the main power supply (also a DC-DC converter, just a much bigger one) to +36V for the varicap tuner and -28V / 6.3V AC for the digitron display. The circuit inside is a resonant converter, a handful of transistors ring up a small ferroxcube transformer whose secondaries provide the required outputs. Basic regulation is applied by damping the oscillations should the output voltages rise excessively. Note that this isn’t a switch mode circuit, although it is often referred to as one.

The unit is sealed in a screening can and dissipates 1.2W, so heat builds up inside. As the years roll by, this takes its toll on the various parts inside, especially the electrolytic capacitors. Eventually the oscillations just won’t start and the output transistor remains conductive, placing the ringing transformer and its series coil connected across the +12V supply. This is the time to switch off, quick! Not only will the little DC-DC converter be ruined if the power is left on, the big one can suffer if the +12V gets shorted. The latter is very difficult / expensive to repair.

When these machines were current it was a simple matter of ordering a new DC-DC converter from Sony. They were expensive, but it made for a quick and certain repair. That option is now gone, so you have to repair what you have. The converter is located on the side of the main power supply (see photo 1, its next to the relay). The easiest way to remove it is to take the whole power supply unit out of the chassis and then remove the little PCB from the side, the converter unit can then be unsoldered.

The first stumbling block is that the can is soldered shut. It is difficult to open neatly but the best way is to use a big iron and start in one corner, prying up under the tabs you’ll find there and working around the edge. The bottom plate will probably get a bit bent up, but that can be straightened afterwards. Be careful not to damage the parts inside. Next, slide out the PCB and have a look. There is a large electrolytic capacitor laid across the other components. If this is ‘tried’ the circuit won’t start, if you’ve got this far replace it anyway. It is originally 330uF but a 470uF one from the Matsushita ‘FC’ series fits nicely in the space so that’s what I use. Whatever you choose, the capacitor must have a low ESR and be 105C rated. Next, remove and test the large (these things are relative...) transistor at the top left corner of the board and test it. It will probably be leaky if the converter has been left switched on but not functioning for any time. It is a 2SD789 and only this type works, there is no point messing about with substitutes. Langrex have them though, and they are not expensive. It the transistor has failed, have a look at the little barrel coil connected to its collector pin. If the outer wrap of this looks discoloured, remove the coil and rewind it. This isn’t as daunting as it sounds, simply cut away the wrap and spool the wire off. Find some wire of the same gauge, cut it to the same length as the original and wind it back on again. You will need to do it neatly or the coil won’t fit back into the space. After this, check the smaller electrolytics and replace any which are suspect.

A nice thing about this converter is that it will run happily with no load from a bench supply, so before re-assembling it connect a 12V supply to the pins marked ‘in’ and ‘com’. These marks are printed on the PCB that the converter is soldered to. If all is well the +36V and 28V supplies will appear. The correct current consumption is 100mA @ 12V, if it is higher than this there is still a problem. This may well be that the ringing transformer has shorted turns, it is tricky to rewind but it can be done. Photo 2 shows a repaired PCB ready to go back into the can. Test the unit at each stage of reassembly, making sure that the pins don’t short to the metalwork. Finally, re-assemble the machine and test – the clock display should make a welcome return!