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Tv SMPS trips on start up with Main Smoothing cap in???

tvtech

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Hi Guys

OK. As the thread title says this particular set has me stumped..

Smoothing cap removed and set will start up and run (albeit with a humbar). As in an open Smoothing Cap scenario.

With Smoothing Cap in circuit on start up....set trips. And will not start up.

Take Smoothing Cap out....start set....and reconnect Smoothing Cap after a few seconds...and all is good.

Perfect picture, perfect PSU regulation....EVERYTHING.

Obviously, a start up issue with the Chopper IC. I have tried two. From different Manufacturers. And they both do the same thing. And checked every component on the Primary of the Chopper. All is good.

Anybody have any ideas here???

This is probably the only time I will encounter this problem ever on a SMPS. And I would really like to know what you Folks think.

Kind regards,
tvtech

Diagram below as an attatchment. Please go to page 65 out of 66 and look in the top left hand corner. That is where the PSU is.
 

tvtech

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I know how to get around this issue though..

Seeing that the PSU is good after start up without the Main Smoothing cap initial presence..

A simple Relay and maybe 5 Second timing for it to kick in the Smoothing Cap will work....I have done mods before with sets that nobody else could repair on line stages. Not PSU like this one....

Not one has ever bounced on me (as in failed/return job). These were mostly Samsung sets with Line Stage issues that nobody could fix. They are still out there and working. Unexplained stuff. After a few years.

PSU mods are new for me though. So I will see how it goes.

So I will see what you guys come up with tonight. If you cannot help...I have a backup plan :)

Regards,
tvtech
 
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alec_t

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Have you checked resistors R803, 804, 806, 806a ? They all seem to be involved somehow in the start-up. Not sure what the effect of them going high/low would be. Has smoothing cap (C806) been replaced?
 

debe

Active Member
I would replace R803 & R804, these feed pin4 on IC801 as a kick start for the powersupply. This is a common fault with this type of powersupply.
 

tvtech

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Most Helpful Member
I would replace C809 and C808, common faults, and also check the primary supply rectifiers aren't leaky (D 893, 4, 5 and 5), another common fault.
Hi Nigel

The first thing I did yesterday was to replace C808 and C809. They were both low in value as expected. I thought my problem was solved. No way.

I have replaced the two 1Meg resistors and checked every other component on the Primary. Opto Coupler has been changed too. Damn thing is still the same :confused:.

I can get it to start up with the Main Smoothing cap in by taking a Voltage reading with my YEW between Primary Earth and Pin 1 of the STRF. The set starts up immediately. Every time. I have checked for a hairline print crack maybe etc.. Everything tests 100%.

Regards,
tvtech
 

ben7

Member
Is the capacitor a low esr cap? Sometimes using a very low esr cap will cause problems.
 

alec_t

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I can get it to start up with the Main Smoothing cap in by taking a Voltage reading with my YEW between Primary Earth and Pin 1 of the STRF
That suggests pin 1 is too high, but is pulled low enough (surprisingly) by the meter resistance. What's the YEW's resistance?
 

tvtech

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Most Helpful Member
That suggests pin 1 is too high, but is pulled low enough (surprisingly) by the meter resistance. What's the YEW's resistance?
Hi Alec

100K Ohm per Volt. My Fluke has exactly the same effect (also starts set).

Regards,
tvtech
 

alec_t

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Hmm. So something needs to pull pin 1 down. Normally it would be R807/8. Could be R808 (assuming you've checked its resistance is ok) has a dry joint so isn't doing its job?
 

tvtech

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Hmm. So something needs to pull pin 1 down. Normally it would be R807/8. Could be R808 (assuming you've checked its resistance is ok) has a dry joint so isn't doing its job?
Hi

I checked R808 out of circuit...like all the other components. It is good.

Funny thing happened. I put a new Opto Coupler in this morning...switched on and set started straight away. So shouted Yipee!!

So I let it run for around half and hour and decided to put all back in the cabinet....and switched on...back to the same nonsense....

I have soldered the whole Primary for dry joints, have ensured that every pad does in fact come into contact with the component/s it is supposed to connect to. I have checked for hairline cracks....I don't believe I have ever been more thorough on a SMPS. Yet it will not start up without help.

This thing has me pulling my hair out.

We are a busy shop and I have so far spent far too much time on this set...so I am behind on a lot of repairs.

So I put the thing aside this Afternoon and cleaned up and caught up a bit. If all goes well, I will take it out again next Week and try again.

Thing is, this set has an absolutely stunning picture once the thing is working. If it was not for that, I would not have spent so much time on it.

Regards,
tvtech
 

alec_t

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Weird indeed. My bets are still on something associated with pin1.
Looking again at the circuit I see R806/6a don't quite connect to pin 1. Drawing error? Spark gap? What is their purpose anyway?

Edit: To answer my own last question I presume they're to protect against supply over-voltage. But any spikes ought to be suppressed before the bridge rectifier hence before they get to C806.
 
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tvtech

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Weird indeed. My bets are still on something associated with pin1.
Looking again at the circuit I see R806/6a don't quite connect to pin 1. Drawing error? Spark gap? What is their purpose anyway?
Hi Alec

Its a drawing error. They do connect.

My friend, not much involving CRT stumps me. Seriously.

In around probably 10000 or so sets. Return rate around close to zero. I fix a set properly. Or not at all.

This one has me by the nuts.

And it is not exactly a complicated PSU.

I am at a loss for explanation.

Cheers,
tvtech
 

tvtech

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Weird indeed. My bets are still on something associated with pin1.
Absolutely. you are on the right track....but what???

Keep thinking mate. That's the thing. I have run out of ideas though.

Cheers,
tvtech
 

tvtech

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And I tried stuff today. Everything I know, have been taught, or ever been shown as in SMPS. Still the same.

You know what though...as long as you are not blowing up spares and wasting...you are good to go.

This one has me for now. But I can't help thinking....something so basic that that I overlooked..

But what???

Regards,
tvtech
 
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alec_t

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Do you have a measurement of the pin 1 voltage when the SMPS fails to start? (It should be ~0.7V after start-up).
 

tvtech

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Do you have a measurement of the pin 1 voltage when the SMPS fails to start? (It should be ~0.7V after start-up).
Hi Alec

No I cannot get a reading on Pin 1 when it is battling to start up as both my meters start the set up. When running I get 2V on Pin 1 on both meters.

Regards,
tvtech
 

alec_t

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When running I get 2V on Pin 1 on both meters.
That's interesting. According to the datasheet for the regulator chip pin1 feeds a comparator with a 0.71V reference. I would have expected pin1 to be held to ~0.71V by the feedback loop. But then, my expectations are often different from reality :).

Another possibility: dielectric breakdown of the pcb material or of a coating (lacquer etc) around R806/6a.
Suppose there is 'tracking' such that above, say, ~250V there is effectively a few hundred kΩ in parallel with R806/a.
Without C806 in place the voltage across R806/a is <250V for much of each half cycle, pin1 voltage is below the comparator threshold for that time, giving the oscillator a chance to start up.
With C806 in place the voltage across R806/a (once C806 has charged) remains above 250V for all of each half cycle so there is continuous breakdown and pin1 volts stay above the comparator threshold, thus preventing the oscillator from starting.
Just a thought.
 

tvtech

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Most Helpful Member
That's interesting. According to the datasheet for the regulator chip pin1 feeds a comparator with a 0.71V reference. I would have expected pin1 to be held to ~0.71V by the feedback loop. But then, my expectations are often different from reality :).

Another possibility: dielectric breakdown of the pcb material or of a coating (lacquer etc) around R806/6a.
Suppose there is 'tracking' such that above, say, ~250V there is effectively a few hundred kΩ in parallel with R806/a.
Without C806 in place the voltage across R806/a is <250V for much of each half cycle, pin1 voltage is below the comparator threshold for that time, giving the oscillator a chance to start up.
With C806 in place the voltage across R806/a (once C806 has charged) remains above 250V for all of each half cycle so there is continuous breakdown and pin1 volts stay above the comparator threshold, thus preventing the oscillator from starting.
Just a thought.
You know what mate, you are exactly at where I was yesterday just before I took the set off my bench.

I thought to myself: let me thoroughly clean the PSU with Methys, hair dry it thoroughly and check to see if there are any signs of tracking etc on the PCB.
So I did and switched on. Same nonsense...

What you said above were my thoughts exactly. We are on the same wavelength.

Best regards,
tvtech
 

alec_t

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Most Helpful Member
We are on the same wavelength
And, like you, I'm now fresh out of ideas :) Either something that shouldn't is pulling pin 1 high, or something that should isn't pulling pin 1 low. Can't see any other possibility; unless the chip itself is flakey.
 

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