SHARP 10P 18G - TV screen not turning on

Hi all,

haven't posted for a bit now... :-)

Recently I wanted to repair my Sharp 10P 18G TV radio cassette player, but only the cassette player because the other two functions were working completely fine. I had to replace the belt.

I have to say it was very very easy to get it disassembled, went like butter! Very well made. Unfortenetely I had to take apart all te boards before I could take out the radio and cassette player. I gave every board a good clean with air and a brush.

After I took the board out, I replaced the belts with an almost perfect fit, and sprayed some contact cleaner into the pots and switches.

I got it all assembled, and tried all the functions, radio worked as before, cassette player now works quite well (I will sort it out later), but the TV screen doesn't come on.

I don't know what happened, I didn't do anything to the PCB, didn't change any components.

I just cleaned it.

I can hear the oscillator of the tube come on, but there seems to be no anode voltage because my hair does not go up if I get my hand close to the screen. There seems to be no heating too, I can't see anything light up in the dark in the tube.

I have no idea, the service manual in't available for free, you have to pay, but I don't have intention to buy it.

Checked the fuses, all of them seem to be OK.

The audio of the TV works, there is something going on in the tube section. There doesn't semm to be high voltage comeing from the transformer for heating and the anode.

Any help appreciated

73s,
Patrik

Hi Patrik,

Nothing like posting a TV question up on a radio forum is there.
(I suppose it's got a radio receiver in it.)

Check your B+ or 12 Volt line....

The fuse next to the L.O.P.T. (Line Output Transformer) should carry 12 Volts for the HT/Flyback.

The heater element on the C.R.T. should also be at 12 Volt.

It's not unusual for old wiring to fail or PCB solder joints to crack (especially around the line output transformer) when pulling apart these old units.

As far as I remember several models carried the same TV tube and connectivity (maybe even similarities in the main circuitry) so worth looking at another model in the range to help out :-

https://elektrotanya.com/sharp_10p-28g_sm.pdf/download.html

That's about the best I can suggest because if that 12 volt is missing there'll be no heater filament power and/or H.T. voltage generated.
(Worth checking the CRT filament continuity as well whilst you're about it.)

Best of luck with it.  

73,
Victor

Hi Victor,

yes, it has a radio receiver ;-)

The general transformer (the one in the power supply which supplies every circuit) works perfect, as the radio and cassette player work good.

The 10p-28g i have the SM of, I'm not sure if the test points for the TV circuit are the same though.

I'll try looking at it again.

The 12V line works.

I'll try looking at the PCB, as far as I'm concerned, there are no VISIBLE cracks but there might be some, that I don't see.

Is the L.O.P.T the tansformer, which supplies the fillament and anode voltage?

As I said, the TV worked fine before I put it apart, and didn't do any modifications... It justb went out, and back in.

73s and thanks,

Patrik

Hi again Patrik,

Don't worry about the test points on the SM, they're probably not the same but the schematic should give you a good idea of the electronic functionality. As I mentioned the CRT is more than likely the same so you should be able to check the filament voltage at the end of the tube by comparing.

On many domestic mass produced electronics goods of the time the soldering would have been a 'wave soldering' process which wasn't exactly perfect. 

Any larger components like electrolytic capacitors would have had a glue to keep them on the board during this process and often misidentified decades later as capacitors 'leaking'. ( ) Similar woes happened with line output transformers where the solder was so thin and along with cheap paper substrate PCB's produced mechanically poor soldering. A flex or two of the PCB could see those solder joints fail (usually in a circular ring) so the transformer won't get power to it and the rest of that circuitry therefore wouldn't work.

As you've mentioned, all you did was pull it apart to replace the cassette belt so it'd be worth a good visual check around the rest of the unit to make sure something isn't amiss. 

It's all too easy to miss a disconnected wire, crack in a solder joint from flexing etc. and might be worth soldering up the joints on that line output transformer just in case.

Should be a fairly easy job to trace back the B+ or 12 Volt line back to the regulation transistor and make sure that circuitry is working fine also.

Sorry I can't offer more insight as repairing things from afar is not mine or many engineers/technicians forte but hopefully it turns out to be a simple fault. (The info already given is from past experience and know-how designing, manufacturing and repairing such items over the years.)

I hope you get sorted.  

(Do love a bit of 'retro' electronics as I'm pretty 'retro' aged myself! )



All the best,
Victor

Hi Victor,

thanks for all the help!

Turns out, when I assembled it back, I forgot to put a clip or something for ground onto the CRT, and the board that is on the CRT (the small one) wasn't connected to ground... :-) Foolish mistake.

I cut a wire and soldered it on the TV PCB and connected it to a faston clip on the CRT board.

All with some help of course :-).

73s and thank yo very much,
Patrik

That's great news there Patrik and very well done.  

(I must admit I've made the same error with the CRT board ground connection in the past. )

All I could do was offer some pointers but you yourself have managed to sort it out so give yourself a deserved pat on the back.

Well done again and all the best to you,
Victor