CRT TV repair, power supply defect.

You say chopper transistor? Is this a Thyristor, Mosfet or a bipolar transistor?

In any case, it almost sounds like you have insufficient power being generated.

Did you use exact replacement parts or SUBS like NTE ?

Also, is the circuit board brown from over heat in that area?

Jamie

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Jamie

the chopper transistor is a MOSFET, The exact parts were used in the replacement.

but almost everything in the power supply circuit has been replaced. I suspect the mosfet has taken out one or two more components in the switching circuit that I'm missing or could there is a problem with the defection circuit?

"Vernon Paul"

** And using the sets own tuner - what happens ??

.... Phil

the set shuts down as well

"Vernon Paul" "Phil Allison" :

the set shuts down as well

** Suspect a problem associated with the deflection yoke.

Maybe a shorted cap, VDR or similar.

Or the dreaded yellow glue .......

.... Phil

Hmm...since there is picture for a few seconds before it shuts down I don't think its the yoke.

Sorry but what is VDR?

"Vernon Paul" "Phil Allison" "Vernon Paul"

Hmm...since there is picture for a few seconds before it shuts down I don't think its the yoke.

** You need to lose attitudes like that if your ever expect to fix anything.

Sorry but what is VDR?

** Voltage Dependant Resistor.

... Phil

On Wed, 14 Sep 2011 17:56:26 -0700 (PDT), Vernon Paul put finger to keyboard and composed:

I'm not familiar with that chassis, but you didn't say whether the B+ was OK.

Another thing to check would be the components in the line stage, eg the tuning capacitor.

I'd also verify the secondary loads on the FBT, perhaps by monitoring the current in the fusible resistors.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

Thanks Franc,

will check the B+

I think its the L7.3 A chasis since the service manual downloaded from the net seem to match, but there is no marking of the chasis number on the PCB.

It's possible.

On the flyback you most likely have a B+ or some may call it a C+ since this source is derived from a tap on the fly back.

It's common to monitor one of these sources for a quick shut down, because it covers the flyback area problem and supply going to it. How ever, caps do go bad here and the ripple being seen could be just enough to shut it down when the circuit gets loaded.

This is a good time to use your scope, if you have one. First you test the gate signal to the MosFet you replaced. My guess is it should be a PWM type signal and should vary when load varies. You want to see what the duty cycle is. what I have done in the past to test for this is to put a low load R on the output of the supply while I am watching the scope to see if the duty system increases to maintain output. Of course, you could also just simply watch the output with a DMM as you alternate your external load R.

While you're probing around with the scope, see how clean the signal is at the switching supply's output after rectification and filtering.

I've seen bad UF switching type diodes/rectifiers go bad, they test ok with a meter but have slow recovery issues under operation. Make sure you replace these types with equally fast units. A good indication of this is one operating hot.. This is a good time for a hand held IR meter. :)

Next, check the Secondary supplies coming from the flyback for any ripple around 15khz.. You shouldn't see much at all. if you do, it could be bad caps or slow recovering diodes.

P.S. since the original MosFet was shorted, have you checked to make sure the switching transformer leads are well soldered in? This was a common problem back then and depending where the lead was that had a cracked joint, it could cause intermitting back EMF on the MosFet and short it.

Also, you may want to test for the Drain voltage and signal via the scope to make sure you are not getting any over voltages that look like spikes.

That should give you enough to work on.

Jamie

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Wow Thanks Jamie,

very informative post. Will check the things you mentioned and let you know the outcome.

Vernon

The only thing that I would add is to be very VERY careful when you start waving a 'scope around the primary side circuitry of a switcher. Make very sure that you know where your ground reference is, as it is very likely that the ground lead on your probe will be common with the 'scope's chassis, which will in all likelihood, be bonded to the earth of the line power feeding it. The bang that this can produce, and the trail of catastrophe that ensues, can be spectacular ...

Arfa

"Arfa Daily"

** It is ESSENTIAL to use an isolation transformer in the AC supply for such work.

Then you can ground the negative side of the main filter caps and work as normal.

.... Phil

What happens if you use a video source that's portable, runs of batteries, and isn't connected to mains or anything else (such as a portable dvd player)?

off*

An update on my repair situation. I decided to follow all the instruction gathered here and some from Sam, beginning with the easiest first. This is what I got. By reducing the G2 and brightness all the way down the TV works fine, no shutdown. Even with a decent brightness and G2 cranked up and decent picture available, it does not shutdown. However when there is some bright scene there is signs of tendency to shutdown. When the the brightness and G2 is cranked up too much it shutdowns. The voltages from the power supply seems to be stable during the shutdown, not sure about the currents though.

Is the problem more likely in the power supply or deflection side?

On Wed, 21 Sep 2011 21:00:20 -0700 (PDT), Vernon Paul put finger to keyboard and composed:

Is there an ABL (automatic beam limiter) circuit? I have seen ABL failures that have caused shutdown.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

Sometimes called x ray protect.