Power supplies in TVs, VCRs, etc

[ljcox]

Does anyone know where I can find a general description of how the power supply in modern TVs, VCRs, etc. works?

It operates at some tens of kHz and drives a transformer with rectifiers on the secondary side that feed the circuitry.

Both my TV and VCR use this type of supply and they sometimes don't start when the mains power is turned on.

So I assume that the feedback is sometimes not sufficiently positive to sustain oscillation.

As I don't have a storage oscilloscope, I can't see what happens when the power is turned on.

Any assistance will be appreciated.

Len

 

[Nigel Goodwin]

Does anyone know where I can find a general description of how the power supply in modern TVs, VCRs, etc. works?

It operates at some tens of kHz and drives a transformer with rectifiers on the secondary side that feed the circuitry.

Both my TV and VCR use this type of supply and they sometimes don't start when the mains power is turned on.

So I assume that the feedback is sometimes not sufficiently positive to sustain oscillation.

As I don't have a storage oscilloscope, I can't see what happens when the power is turned on.

Any assistance will be appreciated.

They are called 'switch mode' power supplies, the most common fault is electrolytic capacitors going high ESR - in your case probably the 'start up' capacitor, usually something like 47-100uF 25V in the primary side of the PSU. There may also be a number of high value 'start up' resistors in series as well (feeding the capacitor I mentioned), it's EXTREMELY bad practice putting resistors in series like this, and they commonly go high resistance.

Be VERY aware that the large electrolytic in the primary may well stay charged up, and can give you a substantial shock!.

 

[ljcox]

Thanks Nigel,
I have already tested and changed the electrolytics as they had high ESR, but still no better.

I did not check the resistors, will do that next.

Len

 

[Edmond]

I’d like to help you, but without schematic I have no chance. Write the manufacturer and type of the TV and VCR. I’ll try to look for schematics.
In general I can give you some standard tips. Disconnect the electronics from the PS. A shorted output can break down it. Check the primary part (fuse, rectifier and bypass cap). Check the switching device (transistor, FET, or IC). Check the secondary rectifier shotky diodes and the bypass caps. There is a start up signal from uC, but without that some outputs (ex. 5V) must be wake up. Check the loop-back part (capacitor or IR decoupler).

 

[jin29_neci]

switch mode power supplies are very complicated. If your tv wont start with a defective SMPs. You must first check for the B+ if you must measure at least 115v or depending upon the TV. In many cases 115 v is ok most especially in SONY TV's. If you have dont have a B+ try to check the ac voltages that coming inside of your SMPS. Then after this check for a leaky diodes (Rectifiers) or brunt out resistors and a defective capacitors. This is the possible troubleshooting can do or try to check your IC regulator. thats i can say but I will manage to post the schematic diagram we can probably help you. sory for my english. thanks :wink:

 

[zachtheterrible]

I did not check the resistors, will do that next.

You can't check resistors when they are in the circuit because you will get wrong readings. And taking them out to check them would be a major pain cuz theres so many of them.

 

[Edmond]

The resistor checking is very simple. Just look them, if they are not burned then are probably OK. Don’t measuring them. Usually in the SMPS the resistor is burning due to a mistake. But a very important thing: the fault is not caused by the burned component it is just a consequence. You must find the main faulty component, in other case the replaced burned resistor win burn again. I suggest you to replace the all caps (1uF…100uF) and the switching component and the PS will work.

 

[ThermalRunaway]

Just because the startup-resistors aren't burned doesn't mean they aren't faulty. Quite often switch mode power supply designs use two startup resistors in series and one of them can go high resistance, which means that the proper startup voltage is not applied and the charging circuit doesn't charge up enough to start the chopper transistor off. This can result in intermittent startup of the set which no doubt will eventually lead to a completely dead set. Sometimes one of the startup resistors can go completely O/C, but still show no visual signs of stress.

Intermittent startup of power supplies can of course also be caused by a multitiude of dry joints, but you seem pretty clued up so I assume you've already scouted around for those.

What make and model TV is it? Perhaps I've worked on it myself...

Brian

 

[ThermalRunaway]

By the way, you don't have to take the resistors all the way out to check them - just desolder one end of the resistor and lift it's leg up so that it doesn't make contact with the PCB anymore. Then you can do a proper check.

One thing though, and I can't stress this enough, MAKE SURE YOU DISCHARGE THE RESEVOIR CAP FIRST!

Otherwise as soon as you apply your soldering iron it'll go bang in your face ;-)

Brian

 

[Edmond]

Just because the startup-resistors aren't burned doesn't mean they aren't faulty.

Hi Brian! How many wrong resistors did you seen without external visual signs?

 

[ThermalRunaway]

I've seen many faulty resistors and other faulty components which show no visual signs of stress. Obviously there are cases when resistors and other components can be burnt or bulged or whatever which gives a visual sign that they're faulty, but this is by no means always the case.

I'm quite glad of the fact really, because if all component failures showed visual signs of their stress, you wouldn't need to employ skilled techs like myself to find the faults.

Brian

 

[Edmond]

I don’t sad that the all wrong component has a external visual signs, but in high power applications like SMPS the wrong components usually has a visual signs. In my practice (more than 15 years) I didn’t seen wrong resistor without visual signs.

 

[ThermalRunaway]

Yes I do agree with you - when something goes wrong in a switch mode PSU it can often have catastrophic results, especially if the fault is on the primary side or forms part of the feedback circuit on the secondary. It's not unusual for chopper transistors, driver transistors and surrounding circuitry to explode, which obviously results in visual signs of stress (unless they completely vapourise!)

I've also seen resistors and diodes etc burnt out in power supplies.

But I've also seen startup resistors go high resistance or o/c without any visual signs of stress. The failure of a startup resistor is not a major fault and is unlikely to cause a catastrophic event so usually there are no visual clues as to the cause of the fault when this occurs. Usually the reason the startup resistor goes high resistance or o/c is because of the amount of current it's been passing over the years. Slowly but surely the component wanders outside of it's tolerance and eventually the power supply has trouble starting up. Since this type of fault is not the result of a short circuit of any kind or major positive feedback in the secondary - primary feeback loop, it's unlikely to be catastrophic enough to show any visual signs of stress other than perhaps a little browning (which can be difficult to spot in amoungst all the dust inside the set) Sometimes they just go faulty over time.

Brian

 

[ljcox]

Thanks to everyone for their advice. I'll read through it and see how I go.

In 40 years of electronic design and repair, I have seen plenty of faulty resistors without any visible signs. Some go high, others o/c.

In this case, the problem is intermittent rather than catastrophic. Sometimes the oscillator starts at switch on, sometimes it does not.

Sometimes it will start after a few on/off cycles, somtimes I have to leave it for a day or 2. I can't recall if I heated the components to detect a thermal fault. But I have not done any select cooling. I may have to do this if there are no other clues.

Len

 

[ThermalRunaway]

Have you checked the elctrolytic caps on the secondary supplies of your power supply? If you've got one of those playing up then it may be for the first few minutes of the power supply trying to start one (or more) of your secondary supplies aren't coming up which could be causing the TV to revert to standby or go into some kind of protect mode.

When the fault happens, do any of your secondary supplies come up or is it the case that the chopper transistor isn't oscillating?

Brian

 

[ljcox]

I forgot to answer Brian's question.

It is a VCR of GE brand Model VGE 805AU made in China.

Len

 

[ThermalRunaway]

Ahhh it's a VCR we're talking about, I thought we were on about a TV. But all of the things I mentioned still apply.

I'm not familiar with that particular model number, but that doesn't mean I haven't worked on the chassis. I don't suppose you have a schematic do you? I might recognise the circuit or at least be able to point out some things for you to check

Brian

 

[Edmond]

If the PS is built with IC, I suggest you to replace it. How the PS work? It gets always the main voltage (115 or 230VAC) and it has a soft start (switching on by uC) or the main switch switching on the main voltage?

If you don’t have schematic, try to found the IC(s) datasheet. The datasheets usually containing a main application, which can be useful for understanding how is it works.

 

[ljcox]

Ahhh it's a VCR we're talking about, I thought we were on about a TV. But all of the things I mentioned still apply.

I'm not familiar with that particular model number, but that doesn't mean I haven't worked on the chassis. I don't suppose you have a schematic do you? I might recognise the circuit or at least be able to point out some things for you to check

Brian

Brian,
I only have a photo copy of the relevant section. I could possibly take a photo and post it but it may not be very readable.

It has 5 resistors of 220k each in series from the rectified mains to the MOSFET and a few other places. So I asume that this is the "starting resistor". I discharged the smoothing electro and put a short between the ends of the resistor string (to prevent the electro etc. affecting the measurement). I then measured across each of the 220k resistors with an Ohmmeter and they all measured about 176k as expected (220k // 880k).

I then measured a few other resistors in circuit and found a pair of 22R in parallel that measured about 15 Ohm. So one or both must be a bit high.

After I check the other resistors, I'll temporarily shunt the 22R resistors with a resistor to bring the result down to about 11R in order to see if that is the problem. 15R is almost 50% greater than the required 11R, so that may be sufficient to make the starting intermittent.

Thanks for the help.

Len

 

[ljcox]

Have you checked the elctrolytic caps on the secondary supplies of your power supply? If you've got one of those playing up then it may be for the first few minutes of the power supply trying to start one (or more) of your secondary supplies aren't coming up which could be causing the TV to revert to standby or go into some kind of protect mode.

When the fault happens, do any of your secondary supplies come up or is it the case that the chopper transistor isn't oscillating?

Brian

Brian,
Nearly missed this one. The fault is the latter.
Intermittently no oscillation.

After replacing the electros that had high ESR, I switched it on and it worked. Turned it off then on again a few seconds later, no problem. Switched it off again, waited a few more seconds, switched on, no start.

Did this a few more times, and sometimes it started, sometimes not.

Eventually, it refused to start at all. Went back the next day and it started.

So something is very marginal.

Len