Tuesday, March 15, 2016

Question and answer part2

Peter B. Mutale from Zambia
Dear Humphrey,
Thank you for your educative ebooks that I have been reading on repair of CRT televisions. Please I would like you to help me on:

1.How I can know the presence of base voltage on SOT base lead. Your book 'CRT Television Repair Course' states that this voltage could be about -1.38v. The question is - Is this DC or AC and which digital meter range can I use to measure this negative voltage. I have been trying to determine it with my digital meter but have failed to come up with the answer.


Actually these figures I got them from a working CRT television using transistor instead of ic for the switching the power supply.

If you look at the books written from the theory point of view this voltage is around 0.7 volts dc, this is normal voltage drop of a silicon diode, when testing a transistor with a meter consider it as diode and therefore expect a voltage drop across the p-n junction to be around 0.7 volts.

This is how it should be, but in my case I did the testing when the TV was okay and working and therefore means there was a lot of switching going on and therefore the meter is confused if it is dc or dc being tested.

So I can conclude that whenever you see some reading on your meter at the base of SOT transistor but not zero, then you are good to go.

In nutshell the actual figure is not important unless if you get the voltage is the same at the base and collector means the transistor is shorted C-E junction.

2.My CRT television can't give me any voltage on the secondary side, not even the standby light is on. However, when subjected to the current limiter, it indicates that there is no short as the light grows briefly and then goes off. This TV uses a transistor to switch the chopper transformer on and off. Upon testing about 297v is present at collector of SOT but it is difficult to know if -1.38v base voltage is also reaching its base for I do not know how to test it. I checked the SOT drive transistor
with analogue multimeter out of circuit board and it seemed ok. The HOT also seems to be ok upon testing.
Please help me. I do not know what to do next in order to receive the required voltages at the secondary side.

As I said won’t get actual -1.38 volts, if the TV is not working I expect at least some voltage at the base of switching transistor (S.O.T) and should be positive.
If you get zero volts here remove the base leg of this transistor and test from the circuit board where this transistor was installed at the base leg on the PCB track and if you now get voltage at that point means the SOT is shorted base-emitter junction which is very rare but do happen.
If the voltage is still low check for any component on the base line of this transistor and the ground which could also have shorted, here the biggest suspect is SOT drive transistor.
Not yet got the culprit now trace back from the base of the SOT what is blocking this voltage most likely an open component and the main suspect here is the startup resistors or track open.
You may also follow from the 300 vdc to the base of this transistor and this will help to pick up what is blocking the voltage to the base of the SOT.

Mphora Nkomo from South Africa…..

Hi Humphrey,
In your e-book CTR TV REPAIR Course under the subject EMI, you mention that
Capacitors allow high frequencies to pass but very hard for low frequencies.

Further on you say a capacitor which is connected in parallel with the power supply effectively shorting the radio frequencies to the ground hence allowing only the low supply frequencies to pass.

I am getting confused here and can you please clarify the concept and make it more simpler.

Kind regards,
Mphora Nkomo.


EMI refers to Electromagnetic interference/ radio frequency interference)

What does this mean, as you know the power supply in your house has a frequency of 50/60 Hz.

This means it is actually a signal just like audio signal, floating freely in the air around you are many frequencies like the mobile phones, television frequencies which are very high frequencies and they are generally referred to as radio frequencies.

These radio frequencies can easily get superimposed on the power supply outlet.

You don’t need these into your electronics gadget because they will appear as inferences or noise to the system (in our case Television set)
That is why EMI filter come in to take care of these un-wanted frequencies…

Looking at the diagram above on the right you will notice that the capacitor is placed in parallel to the supply line (one leg of the capacitor is sitting on the live line and the other on the neutral (ground) and therefore because capacitor pass AC (high frequency) and block DC (low frequency) then the high radio frequencies will find an easy entry at the capacitor only to realize that it was directly to the ground (what I can shorting to the ground)

At the same time the low frequencies of 50/60Hz will find it very hard to pass to the ground and therefore will continue with the journey together with a few more high frequencies until they come to an inductor, on the other hand inductor allow low frequencies to pass and very had for the high frequencies to pass since inductor is placed in series with the power supply then the 50/60 Hz frequencies with proceed and the high radio frequencies with be stopped from proceeding.

That is basically the working principles of EMI circuit.

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