NotTaR of Television Sets : Problems with SCR based regulators      
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Problems with SCR based regulators

Here are typical symptoms:

"Sharp TV has a short blast of high voltage and sound then shuts down. All components in regulator area test good. I have two of these sets." Is there a good sharp tech out there thats seen this problem?"

(From: Mr. Caldwell (jcaldwel@iquest.net).)

There is a bulletin from Sharp on troubleshooting ANY SCR regulated TV, this can easily be adapted to RCA, GE, Emerson and Panasonic sets that have similar circuits given a little thought but the technician.

You are going to need to figure part of this out as I no longer have the schematics available. All this will do is allow you to rule out either the regulator or the horizontal section.

Don't plug this in until you've read the whole list.

Figure out how to bypass the turn on circuit from the microprocessor (unless it's a manual one). This is usually just jumpering the relay but sometimes Sharp puts a horizontal Vcc turn on transistor that also must be jumpered.

Next jumper across the SCR anode to cathode.

Now using an *variable isolation transformer* turn the voltage on it down and plug the set in. Bring the voltage up slowly, if you can bring the AC up so that the DC on the jumper across the SCR is within the regulated voltage you should have a picture and this rules out the horizontal section as the culprit.

If the set shuts down prior to getting the DC up enough then you've got problems in the horizontal section. Either you have something wrong with the high voltage transformer or the tuning caps or there is a problem with the x-ray protect pick off voltage to the deflection IC.

If it's the horizontal section you can set the AC at approx. 25v and look at the waveforms in the horizontal output section for defects like ringing.

I've never gotten a good troubleshooting technique down for the regulator since it's an active circuit the waveforms and voltages are not stable when it's failed. A good diode, transistor and capacitor checker will help.

It would help to get the service manual for that set, the training manual for that chassis and the bulletin dealing with troubleshooting SCR regulators.

Also the training manual should have a good explanation of how this regulator works. In a nutshell the regulator is a switched mode circuit that uses a winding from the high voltage transformer to turn off the SCR. The regulator is always turned off at the same time by a pulse from the high voltage transformer. Regulation is achieved by controlling when the SCR is allowed to turn on.

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