UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 31st Aug 2018, 10:03 am   #1
Studio263
Octode
 
Studio263's Avatar
 
Join Date: Apr 2007
Location: Hampshire, UK.
Posts: 1,574
Default Sony SL-C9UB clock display repair

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!
Attached Thumbnails
Click image for larger version

Name:	C9 DC DC 1.jpg
Views:	201
Size:	93.7 KB
ID:	168421   Click image for larger version

Name:	C9 DC DC 2.jpg
Views:	204
Size:	80.1 KB
ID:	168422  
Studio263 is offline  
Old 7th Sep 2018, 3:52 pm   #2
dj_fivos_sak
Heptode
 
Join Date: Mar 2015
Location: Lefkada, Greece
Posts: 969
Default Re: Sony SL-C9UB clock display repair

Hi. Very nice post. I have a similar issue with my Sanyo VTC 9455. The VFD display goes out after leaving it plugged in for approx. 30 minutes. Then I have to leave it unplugged for several minutes to get it working again. If it doesn't go out, then the time can't be changed in clock mode and it won't switch to the tape counter. The clock PCB has a Texas Instruments IC. Maybe that's the fault? I can't see any DC-DC converter on mine.

Fivos
dj_fivos_sak is offline  
Old 12th Sep 2018, 1:00 pm   #3
Studio263
Octode
 
Studio263's Avatar
 
Join Date: Apr 2007
Location: Hampshire, UK.
Posts: 1,574
Default Re: Sony SL-C9UB clock display repair

That will more than likely be something completely different - check the supply lines, the reset pulse and the 50Hz clock timing input at the IC which runs the display before and after the numbers disappear, that will tell you where to look. A freezer aerosol is also useful for finding faults like this.

A DC-DC converter is an expensive component and only up-market machines where space inside was at a premium would have ever had such a thing - normally conventional (simple!) power supply techniques were used.
Studio263 is offline  
Old 12th Sep 2018, 7:09 pm   #4
Red to black
Nonode
 
Red to black's Avatar
 
Join Date: Mar 2010
Location: Sunderland, Tyne and Wear, UK.
Posts: 2,473
Default Re: Sony SL-C9UB clock display repair

Quote:
Originally Posted by Studio263 View Post
A DC-DC converter is an expensive component and only up-market machines where space inside was at a premium would have ever had such a thing.
I can only think of three machines off hand that used them as a complete component
Obviously the Sony SL-C9, also the Toshiba V209, and then the Ferguson 3V23 and its various clones.
I will probably have missed and/or forgotten some.
__________________
I don't suffer from Insanity. I enjoy every minute of it.
Red to black is offline  
Old 12th Sep 2018, 7:14 pm   #5
Mooly
Octode
 
Join Date: May 2011
Location: Lancashire, UK.
Posts: 1,349
Default Re: Sony SL-C9UB clock display repair

Nice thread. Those DC-DC convertors were troublesome back in the day, and later spares had holes drilled into the can to aid cooling, presumably while containing any RF emissions. I would advice replacing ALL the electrolytics in the can... it is just false economy not to do so once the can has been opened.

A real classic
Mooly is offline  
Old 18th Sep 2018, 1:53 pm   #6
af024
Octode
 
Join Date: Jan 2003
Location: Ashby-de-la-Zouch (it's not by the sea)
Posts: 1,254
Default Re: Sony SL-C9UB clock display repair

An interesting post Studio263,

I must confess to not even giving the coil a passing glance.

From this post ..
https://www.vintage-radio.net/forum/...ad.php?t=95103
I'm reminded that the 2SD774 also works (this seems to be an alternative that Sony used).

Regards,

Andy
__________________
Now where on earth did I remove that from?
af024 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:32 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.