CTV 1976/1977 Sharp C-2051H - Linytron

It's puzzling why the lamp goes out after a few seconds. If it is the crowbar firing then it should stay lit until the power is removed I would have thought.

Lamp should flash brightly at switch on as the reservoir capacitors charge but not usually for that long. 

@michael-dranfield Yes, exactly. My mistake, I wasn't observing closely as I thought the lamp was still across F703.

Is it not just the degauss doing it’s thing, or has that been disconnected?

The Lamp Limiter bulb behaviour is really confusing to me too.

Anyway, it's still blowing F703, so according to the Sharp fault guide, possible candidates are Q709, T702, Modular PWM-M (wherever or whatever that is), or the protection circuit. Get the feeling I'm going round in circles here.

@lloyd everything was reconnected when I did the last test.

Progress, I found the symptom, of the fault, as, yet I don't know what to make of it or the cause, and the disjointed cct diagram does not help in this matter.

I have everything connected except the deguass, and I'm not using the Lamp limiter.  Powered on whilst trying to monitor the 150V rail on my logging meter via TP601, I saw a brief puff of smoke from behind the PWM-F module, where T702 is located, the fuse blew.

I put a new fuse in this time the timed delay version, prior to this I was using quick blow to be safe. While I watched intently, the back of PWM-F and T702, I powered on once again. Briefly I heard sound from the speaker, excellent! Followed by a puff of smoke as I literally watched R756 Glow lava red, fizz and go open, good news is F703 fuse did not blow.

So I need to figure what is killing R756, yes that is the same 10R 1W I originally found burnt out right at the start of this nightmare that I thought was due to C739 being super leaky, no, must be something else.

As I say, the cct diagram is a nightmare, I cannot make head nor tail of it in relation to the cct board on top of T702. It's just shown as a box on the diagram.

Hi, Chris I see where your going wrong now, first check the capacitor you have replaced is rated for 250 volts, don't put in a 160v like the service manual says, this board is the video output stage HT supply the cathode of D716 goes to the R469 on the tube base, it's the 185 v video output HT rail. 

The cap (c739) I replaced is rated for 450V, can you expand on where I've gone wrong?

Try this

See above reply, I didn't use 160V I used 450V

Posted by: @michael-dranfield

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this board is the video output stage HT supply the cathode of D716 goes to the R469 on the tube base, it's the 185 v video output HT rail. 

That makes sense now, I scoured the circuit for the other end of point Z, got it.

Posted by: @crustytv

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See above reply, I didn't use 160V I used 450V

Posted by: @jayceebee

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Posted by: @michael-dranfield

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this board is the video output stage HT supply the cathode of D716 goes to the R469 on the tube base, it's the 185 v video output HT rail. 

That makes sense now, I scoured the circuit for the other end of point Z, got it.

 

If there is no 185v rail or a short I now suspect D717 will conduct adding R757 into the gate circuit of the thrisytor firing the crowbar?

Posted by: @crustytv

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The cap (c739) I replaced is rated for 450V, can you expand on where I've gone wrong?

Posted by: @jayceebee

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If there is no 185v rail or a short I now suspect D717 will conduct adding R757 into the gate circuit of the thrisytor firing the crowbar?

Posted by: @michael-dranfield

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D716 goes to the R469 on the tube base

There aren't any 4xxx components on the CRT base on this set.

Posted by: @michael-dranfield

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check if the capacitor on the CRT base is sc then, it's very likely faulty. 

All components on the CRT base are 9XXX except for two 7xx components

There's only 4 caps on the CRT base C743, C983, C981 & C982. One large disc C983 130pF rated at 6kV, one little disc C743 .01uF and two fat film C981 & C982 both .1uF

So I'm as flummoxed as I was before, if not more now as to what on earth is causing R756 to die

Sorry Chris, my mistake, the cap is not on the CRT base but on the video output board PWM B.

I needed to take a break from this TV over the latter part of the weekend, less related to the TV, more due to that bloody circuit diagram.

Back at it today with the invaluable insight into this disjointed cct by Michael, especially with regard to T702 top PCB and where it was feeding, and also the protection cct feeds. I don't think I would have made any progress without it, as no matter how much I stared, it was not sinking in, personally the diagram caused my mind to blue-screen. Anyway, armed with the new knowledge and testing C436 a 4.7uF at 250V, shows it to be super poorly. Using the CRB-3 to feed it 200V of its rated 250V, the best it could manage was 1.2V and falling, leaky does not begin to describe it.

Finally, for the moment, the Sharp wakes up.

The rogue's gallery.

Result! Well done for sticking with it.