CRT TV Repair Feedback and a follow up with a couple of questions

Hello Humphrey,
I wanted to let you know that I got the book. My friend Jean bought it for me but I'm the one attempting to understand CRT TV repair.

I've only gone through a couple of pages but I really think it's exactly what I needed. I hope you will allow me to send you any questions I have after I finish reading it twice (to make sure I don't waste your time).

Thank you very much.

Best regards, Nick

Hi Nick,
Thank you for the feedback Nick....sure i have already added you to my support list and therefore you are most welcome to come back to me any time you need my assistance.

Wishing you all the best in your future endeavors...

Kind regards Humphrey

Nick

Good morning...Wanted to follow up with you on a couple of points please.

1. If secondary diodes are after bridge rectifier and chopper DC transformer, why measure in AC setting? (see attached image 1) 

Good question Nick…First let me point out that switch mode transformer is basically used to provide line isolation and to generate multiple output voltages for the secondary circuits.

For the Transformer to generate output voltage it will need input voltage which in your case is 150 VDC which has come from the output of the bridge diode and filtered by the main capacitor…now we have a problem here …because Transformers are not designed to handle direct current(DC)

To care of this another component is brought in the equation and this is the switching transistor…as the name implies the work of this transistor is basically to switch (on/off) very fast and by so doing voltage is generated on the secondary side of the switching transformer(chopper).

Now the output of all transformer is alternating current that is why you need to test for AC immediately after the transformer but before the diode…To get back to direct current you need again a diode to rectify…that is why you have a diode and a filter capacitor on every secondary output line (please note that these diode are not ordinary diode but fast recovery diodes (FR) and therefore never replace them with ordinary diodes.

Another example of this scenario is the fly back transformer where we have the B+ Voltage (again DC) after passing through the primary coil of the FBT …..we have the horizontal output transistor (H.O.T) which is used to switch the B+ on/off  and by so doing the FBT generate various voltages on the secondary which includes..Screen voltage, focus, HV, heater etc

2 & 4. I have a board which measures 72 VDC at the main cap (we use 110VAC). The board uses 4 discrete diodes for rectification. What voltages should I be measuring on the output side of each of these diodes? I’m aware that 110AC equals 150VDC RMS but what values should each individual diode produce?

In your case (110V) outlet I expect you get 110Vac across the AC inputs legs of the bridge rectifier and 150 VDC across the main capacitor…please note that when measuring across the AC input pins your meter is set to AC range and when measuring the Voltage across the main capacitor you are now using DC range in your meter..OK...

If you have done that and still get 72 VDC across the main capacitor this is an indication that something is wrong on the supply and the main suspect is that actual capacitor ESR gone to the roof or one of the diode in the bridge is open…note I say open but now short because if any of the diode on the bridge short that will cause an instant destruction of the fuse (blow)

3. In image 3 you mention that Main DC is the 150VDC coming from the main capacitor through the primary winding of the chopper to the SOT or power regulator IC.

Correct

 I thought the main DC was switched on and off by the SOT/IC, meaning it had to go through them first and then to the chopper (same thing with the HOT-FBT circuit?)?

As I said earlier Nick, the work of the SOT/IC is to switch this Main DC on/off very fast and by so doing generate Voltage (AC) on the secondary side of the chopper transformer

Thank you for your patience and time. I really need to learn how to fix these TVs.

Thanks and most welcome Nick…all the best

Nick

Regards Humphrey