CTV 1976/1977 Sharp C-2051H - Linytron

Posted by: @jayceebee

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you mention that the purple lead from the PCB on top of T702 goes to the cathode of D404 which I see is connected to the service switch

It certainly is

Posted by: @jayceebee

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If that is correct I'm even more puzzled

Welcome to my world!

As it's also connected to the brightness control all I can think of is spot suppression. Still can't work out where pin11 of the PCB ends up.

Ignore that last post

Posted by: @crustytv

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Posted by: @michael-dranfield

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it's highly likely C739 is faulty. 

Not so sure, it seems OK to me.

An in circuit test of the 10uF 160V electro, shows it is only slightly higher in capacitance at 12uF with an ESR of 4.2R. A normal quality cap is expected to have an ESR of 7.7R and a high quality cap and ESR of 1.2R. 

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https://www.radios-tv.co.uk/vintage-electronics-blog-forum/typical-esr-values-for-electrolytic-capacitors/

I think the easiest test would be to eliminate the line output stage first by disconnecting R735 and switching on again with a 60 watt light bulb in place of Fuse F703. See what happens then. Leave the faulty resistor for the test. 

You need to remember though for this test you only have a split second though so won't be able to do any other tests because there won't be any chopper drive after C721 has charged

Posted by: @michael-dranfield

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I was wondering if it had a high leakage current

Absolutely yes!

I'm getting sloppy in my old age, there was a time when I would never just rely on the Peak ESR70. It's OK for reading capacitance and ESR but woefully inadequate for leakage tests, for that you need to dump rated voltage and a little extra into the device under test.

So I stopped being lazy, went deep diving into my store and got my leakage tester which allows me to feed up to 500V. First I applied 100V, then 200V, the neon strikes and remains lit, it leaks like a sieve, barely holding 20+ volts.

I normally use this for reforming caps, a good cap steadily attains the rated voltage being fed in, eventually the neon extinguishes as you reach the input voltage. You then proceed to the next level,  repeating the process until you reach rated voltage with no leakage. This cap is toast!

So it looks like as you rightly surmised C736 along with R756 is the culprit.

I think the easiest test would be to eliminate the line output stage first by disconnecting R735 and switching on again with a 60 watt light bulb in place of Fuse F703. See what happens then. Leave the faulty resistor for the test. 

To be honest I would just change C721, C722, C713 and the one you have just tested all for good measure, then look to see what faults you have left, incidentally I spoke with Jeremy a few weeks ago to see if he could add a leakage test on to the Peak, if this is possible though it would be limited to 50 volts so not much good for high voltage electrolytics. 

Just a short (excuse the pun) update to where I am on this, parts I needed turned up, and I set about changing the caps (were either open/leaky or very high esr) mentioned above and the resistor/cap on top of T702.

To be honest, I foolishly thought I'd now be in business now, but no. I powered up on the Lamp limiter and exactly the same behaviour, the fuse still blows, so something is still amiss.

The hunt continues..... I guess next I should follow Michael's sage advice from a couple of posts back (copied below).

Posted by: @michael-dranfield

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eliminate the line output stage first by disconnecting R735 and switching on again with a 60 watt light bulb in place of Fuse F703. See what happens then

As advised, disconnected R735 (1R/2W) to isolate the Line stage.  Removed F703 and strapped a 100W (no 60W in stock) across the holder, then powered up. The bulb lit up and then got brighter, brighter, brighter, switched off before I blinded myself, or it blew. OK, that's good, it at least eliminates the Line stage.

I need to go and study the cct again, as to what on earth is causing the protection cct to fire and I've still no fathomed what on earth this modular PWM-M is that is also listed as a possible cause of F703 dying.

Remove the lead to the chopper collector Q709 so it cannot start and note the effect. If the result is the same then it's possibly a faulty crowbar thyristor. As the chopper cannot run there will be nothing on the 150v rail and no current flowing through the current sense resistor R731 to turn on Q708 and fire the crowbar. 

If in this condition the bulb does not glow reconnect the collector and disconnect the base of Q709. If still no glow then the issue is most likely to be a drive or overvolts issues caused by a fault on the PWM-F board.

I've been all over the manual and I can't find the mysterious PWM-M either. 

Don't forget the chopper transformer is also generating 3 other secondary rails apart from the main 150 volt HT rail so a short on any of these could cause excessive current flow. 

Another point to note, there is another input to the crowbar Thyristor gate right at the bottom of the circuit, I can't for the life of me see on the diagram where it goes to though, you may just have to trace the source on your set. 

Posted by: @crustytv

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As advised, disconnected R735 (1R/2W) to isolate the Line stage.  Removed F703 and strapped a 100W (no 60W in stock) across the holder, then powered up. The bulb lit up and then got brighter, brighter, brighter, switched off before I blinded myself, or it blew. OK, that's good, it at least eliminates the Line stage.

I need to go and study the cct again, as to what on earth is causing the protection cct to fire and I've still no fathomed what on earth this modular PWM-M is that is also listed as a possible cause of F703 dying.

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Hi Michael, 

It does light up instantly bright, but then gets starts to get even brighter as I guess the current draw intensifies. I did test with another fuse and observed it fail, upon power on it's an instant spectacular failure with to me a bright green flash on the glass.

Posted by: @jayceebee

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Remove the lead to the chopper collector Q709 so it cannot start and note the effect. If the result is the same then it's possibly a faulty crowbar thyristor. As the chopper cannot run there will be nothing on the 150v rail and no current flowing through the current sense resistor R731 to turn on Q708 and fire the crowbar. 

If in this condition the bulb does not glow reconnect the collector and disconnect the base of Q709. If still no glow then the issue is most likely to be a drive or overvolts issues caused by a fault on the PWM-F board.

I've been all over the manual and I can't find the mysterious PWM-M either. 

 

Hi John, I did remove the thyristor a few days ago along with Q708 to test. The thyristor passed ok on the Peak scr tester, though I'm reminded of a tricky W621 thyristor fault on the Thorn 3k. This tested ok on the multimeter, it also tested ok on my homemade 9V latching tester. However when finally I hooked it up to my 30V bench supply, only then did it show that it would fail to latch at 30V. That was a 75V thyristor, maybe a similar scenario here, who knows.

I will try your suggested tests later this morning. 👍 

Ok John, here are the results...

Posted by: @jayceebee

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Remove the lead to the chopper collector Q709 so it cannot start and note the effect. If the result is the same then it's possibly a faulty crowbar thyristor. As the chopper cannot run there will be nothing on the 150v rail and no current flowing through the current sense resistor R731 to turn on Q708 and fire the crowbar. 

Removed the collector connection from Q709, the bulb does not light up. That would suggest indeed the Thyristor is OK.

Posted by: @jayceebee

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Reconnect the collector and disconnect the base of Q709. If still no glow, then the issue is most likely to be a drive or overvolts issues caused by a fault on the PWM-F board.

With the collector reconnected, and the base now removed from Q709, once again the bulb lights up!

Your suggestions did not anticipate what the likely cause might be should the bulb illuminate on the second test, only if it didn't.

What is it you once said to me....." You seem to attract sets that have obscure faults".

Ok, so yes the crowbar appears healthy. The fact that removing the chopper drive by disconnecting it's base still results in the lamp lighting providing the transistor and it's mica washer are blameless would lead to me give C719/20/36 more thorough checks.

This time last week, I removed and tested Q709 to be tested, it was OK, see here. Also at the time replacing the Mica washer and applying new heat sink compound.

I removed again today, in its entirety, Q709, its holder and attached caps C719, C21, C736. Tested the transistor and this time it was found to have an emitter collector short, really!

I replaced the 22nF caps with new as they had gone high, but left C719 as it was still spot on. Luckily, last week I also ordered a few more 2SC1578 as I had none in stock, so fitted a new one. Reconnected everything, reinstated R735 to establish the line timebase back circuit and fitted a new F703 fuse.

Connected the TV to the lamp limiter and this time here is the result, I really don't know what to make of this any more.