TV Power Supply MOSFET replacement

radiohead

Joined May 28, 2009
514
I'd love to pick your brain regarding TVs. They all operate pretty much the same, right? Like a radio has the RF amp, mixer, oscillator, 1st IF amp and so on, some may be more complex than others, but the block diagram will be pretty much the same...
 

debe

Joined Sep 21, 2010
1,389
Just a thaught, perhaps radiohead should start a seperate thread on his problems so it dont get confusing here.
 

R!f@@

Joined Apr 2, 2009
9,918
Wonder what OP is up to.

And yes TV consists of An input ( tuner, which some has inbuilt IF amp and others don't
), Tuner is responsible for picking up different channels in the LF,VF & UHF + some decent one has Cable band too & most recent has DVB capability.
Tuner has all the local oscillator and mixer and so an so forth.
Tuner is sometimes serviceable for me that is. And it is RF shielded resulting in a metal box thingy

Then comes IF for some, which separates the Audio and Video plus the Syn pulses for Vertical & horizontal + the color burst for color syncing.

Audio path takes it's route to it's DSP ( if any ) and power amp.
Video path is a lot more complex. Which has secam, NTSC & PAL color DSP's. Then goes to Video amp which boosts the low video to high voltage to drive the CRT cathodes.

The vertical sycn goes to it's vertical amp and to deflection coil.
Horizontal takes a bit more complex path. Horizontal is responsible for generating the very much needed Higher voltage anode supply and other necessary auxiliary supplies and the horizontal deflection too.

And everything is powered by a standard PSU to generate the necessary start up and +B voltage.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
Wonder what OP is up to.
Hah, yes, and you are not the first. As I said, the project is some distance away and I have many other distractions, so it will be a while. Spent the afternoon jetboating yesterday!

I discovered that an old breadboard project of mine is actually very similar to the commercial ring tester for inductors such as flybacks and yokes. So I'm going to try making one.

The idea is to generate a 2% duty cycle, 10Hz square wave. The brief pulse resets a counter and sends a small (20mA or so) current into the inductor under test (ie. flyback), in parallel to a capacitor. When the current is cut off, the resulting LC ringing is detected (above a threshold) and squared up, and the peaks counted. The higher the count, the better the inductor. I already had a timer, counter and op-amp on the board ready to go, so I'm almost there.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
Finally back to it! Today I did some testing, all with power off and everything in-circuit. I could use some help (!!) understanding the results.

First, my DIY ring tester showed a full 10 rings (!!) on the LOPT primary (LL005 pins 1 & 5 or ground and 5). I got no more than 1 ring on every other pin pair combination. Inductor LL032/33 gave 6 rings (pins 1 and 3). The other side, pins 4 and 6 are tied ground and the base of the HOT, so no rings on that side.

The following transistor tests all used a cheap multimeter on the diode testing setting. I put the multimeter lead indicated on the transistor pin indicated and read the mV on the scale. As you can see in the table, I'm a bit lost interpreting all the tests. For instance, what happens with the bias resistors? I would have expected them to pull the mV very low, but I get "open" readings.
[update] Ah, just read that the results will usually appear "open". The meter current cannot overcome the bias resistor and the transistor doesn't turn on. Makes sense.



Here's the schematic again for the horizontal drive section under test.
 

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Thread Starter

wayneh

Joined Sep 9, 2010
17,496
I need suggestions for my next move. Since the transistors are not obviously bad, not shorted at least, I'm tempted to start removing the LOPT. I'd disconnect the secondary connections first. The primary looks good with my ring tester, but I suppose these flyback transformers usually fail on the high voltage side?
 

nomurphy

Joined Aug 8, 2005
567
I've had to repair just about every TV that I've owned for various failures; GE, Motorola, Panasonic, Hitachi.

That hissing sound on older sets was typically caused by the internally shorted flyback.

DARWIN WARNING:
One thing that I was taught for testing the HV ouput, and have done on older units, is to put the tip of a screwdiver with a well-insulated handle under the plastic hood of the HV cable that goes to the CRT. If you get arcing to the tip then the HV is good (it is conducting through the high impedance screwdriver handle and you).

A less macho way of doing this is to remove the HV cable from the CRT, and tape it an inch or more away from some grounded metal with NOTHING else nearby -- briefly turn on the power.

The least macho way is to rent/buy an HV probe.

Oh, by the way, make sure you discharge the HV output before placing your fingers in the area.
 
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Thread Starter

wayneh

Joined Sep 9, 2010
17,496
Since my set won't power on, I really can't try that. I don't think there would be HV at the tube before the set powers on? Let me know if I'm wrong on that.

I am very suspicious of the flyback transformer. The primary appears good. Technically speaking, my ring tests of the secondary showed it's "bad", but I think that's inconclusive while it's still in-circuit. Since it's "only" about $25 to replace and I didn't find any obviously bad (to my eye) transistors, I may go ahead and just do that. It's just the thought of it NOT working afterwards that is holding me back. :(
 

nomurphy

Joined Aug 8, 2005
567
Since my set won't power on, I really can't try that. I don't think there would be HV at the tube before the set powers on? Let me know if I'm wrong on that.
You're probably correct regarding an older set; newer ones have the instant-on feature where the power button goes into a time-dilation pre-circuit and the power comes on just before you actually touch the button -- most people are not aware of this.

I thought you had fixed primary-side functionality, and were therefore considering issues with the flyback output??

It seems to me, that part of self-sufficiency is knowing when it is costing more to be stubborn than it is in just lettting go. However, if you're doing it for fun and a sense of accomplishment, then damn the torpedos and have at it.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
Yeah, I'm pretty close to the end of the line with this set. But since the FBT is such a well known and documented cause of failure for this model, I'm reluctant to give up before I rule that out. I'd just like to a bit more sure before buying one.

You're correct, I do have the power supply working and have replaced the HOT. With the HOT removed, the set briefly powered on (front panel LED comes on) but then cycled off. I had PS voltages and hissing while "off" (no light). With the HOT in place, the front light never comes on. Should there be HV in the "off" state if the FBT is working? I'm a little reluctant to leave it plugged in very long while it's hissing.
 

nomurphy

Joined Aug 8, 2005
567
The FBT can store a charge, which is why one should always discharge the HV line before working on a TV with a CRT.

You could try disconnecting the HV line from the CRT (no external short or load), to see if that helps keep the primary-side running. But that may not work either, per other posters, if it's a set that incorporates certain aspects of fault-sensing.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
...if it's a set that incorporates certain aspects of fault-sensing.
It absolutely is.
With the power indicator off, but the set plugged in and hissing, would you expect HV on that big red cable? I'm not opposed to testing that as long as I can figure out how.
 

nomurphy

Joined Aug 8, 2005
567
No, I would not expect the horizontal or HV to be active with the TV off, or in standby mode.

You say the power indicator is off, but is the power actually off? If you haven't yet, try checking this. There were some TV's that used opto-isolator in the power on circuit -- typically these should fail to the off mode, but there maybe something else going on with yours.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
You say the power indicator is off, but is the power actually off? If you haven't yet, try checking this. There were some TV's that used opto-isolator in the power on circuit -- typically these should fail to the off mode, but there maybe something else going on with yours.
Well, even when the power light is off, the PS is still on whenever it's plugged in, and B+ appears at the HOT. The PS is under regulation feedback via an optoisolator, which may have failed, though I don't think this would prevent power-up. I gather it's meant to compensate for a sag in voltage at the tube. It was suggested before that I replace it, but I haven't yet. My bad.

BTW, I posted the service training manual and schematic earlier in this thread in post #26. This might address any questions you have better than I can.

Nuts, just realized I forgot to look at the voltages on the base of Hout as R!f@@ had suggested a while back. I wanted to get all the unplugged testing done first and completely overlooked any hot testing.
 
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R!f@@

Joined Apr 2, 2009
9,918
The Transistor readings does not make sense to me.

If you have done the voltage test I stated before, I could have known where ur fault lies wayneh.

I need you to go thru my post #51 and post ur results.

Voltage measurements tells fault area much faster than pulling out components to tst
 

R!f@@

Joined Apr 2, 2009
9,918
The transistor readings are not how it's suppose to be.

Any transistor without a R inside or not a darlington should not give any reverse reading in diode check mode. When checking CE, make sure the base is left alone. And any BC or BE reverse readings should show open readings from a DMM.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
I need you to go thru my post #51 and post ur results.
I've finally had a chance to complete this.

It wasn't very exciting: No AC or DC voltage on the base of TL031.

I used two meters jumpered together, one on the DC 2000mV scale and the other on the AC 200V scale. The black probe was jumpered to GNDL and the red probe held manually to the (tiny) base of TL031.

Helper #1 watched the meters while helper #2 plugged in the set. Up came the whine/hiss after a few seconds. No voltages on TL031. Hit the power button. No immediate effect. But a few seconds later, the power-on indicator light came on and the audio popped slightly. No voltages. Power light goes out seconds later. Repeat press of power button, exact same result. No voltages seen. Unplug. Hissing fades out seconds later.

Now what?
 
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R!f@@

Joined Apr 2, 2009
9,918
My PC just gave me BSOD and restarted, and I was done writing. :mad:

OK. No H drive will give u a hissing sound from the PSU. As it will be running at a lower frequency.

The PSU will sync with in ms at power on to the horizontal stage..

So no H drive results in squealing.

Wayneh, can u measure the DC voltage levels marked on the schematic deflection page.
Cross check with the red numbers marked around the TL035, TL033, TL031, TL061, TL062 and 8VOS line, V12 line & +VSUPPLY line.

During the DC voltage measurements, if some voltages are missing try cutting power, wait for around 10s and power up. Keep the meter connected and ur eye on the scale. Like to know if the voltage spikes or not.

Come on MSN.. I am here..and if you are near the TV, it would be easy.
 
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