Need help with Philips TV 21CT9460/73S

[raticus]

Philips tv help please!

Hi

I have a philips chassis 21CT9460/73S with psu problem.

The +115v supply is only +/- 60v.
If I disconnect the flyback transistor it goes to +/- 300v.
Replacing the lopt made no difference.

Please assist by pointing me to a diagram,or maybe you have seen the fault before.

Your assistance will be appreciated.

Thanks
Raticus

 

[E-design]

Re: Philips tv help please!

Hi

This is a general method I used lots of time before:

Normally it is something loading the LOPT on one of the secondary windings. Either a leaky rectifier diode, shorted capacitor or some stage that is fed from these windings. Simple ohm-meter tests on the diodes, caps etc. should point to any faulty component on the secondary side. If that turns out to be OK, try disconnecting the feeds to the various sections, audio, vert etc. You may not have any picture coming up, only a line or something, but it will help identify the stage that consumes all the power causing the line stage to go in protection mode by allowing only the designed max power thru the LOPT. This is why the voltage is reduced, because something is drawing more current.

E

 

[raticus]

Re: Philips tv help please!

Thanks for the reply,E-DESIGN.

I have checked most of the diodes and semis and find nothing obvious.
There seems to be not enough current on the 115v supply to the flyback tranny.
The tranny doesn't even get warm.
I think the psu has some sort of current-limiter,which is maybe at fault?

 

[E-design]

Re: Philips tv help please!

I don't think that the limiter is at fault. The transistor running cool indicates that the limiter is working to protect the switching transistor. What happens is that the switching duty cycle is drastically reduced when the protection is active. Don't forget to look for faulty snubber components if any.

Another useful trick is to open the lines on the secondary supplies, one by one and insert a 12v or 24v automotive bulb in series of the connection.
You will need to solder wire leads on these bulbs. The voltage rating to use will depend on the max voltage expected in these supplies (looking at the voltage rating of the electrolytic caps in each line will be a good indicator). Now what will happen is: when a specific stage is drawing its normal current the bulb in series in the supply line will glow very faintly or not at all. If that stage however have some type of overload, the bulb will be much brighter, dropping most of the voltage across it. This bulb trick will allow the LOPT stage to start up without going into protection. Very simple but useful trick if you understand it.

E

 

[FANT]

Re: Philips tv help please!

Another tip.

Being the flyback circuit the one that draws most of the current, cut the connection ( or thrace ) between the capacitor after the rectifier and the HV generator circuit and replace the load wit a bulb lamp 230 V 40 or 60 W ( 40 w for 14 or 16 ", 60 W if higher ).
Turn On the TV and verify the voltage across the lamp.
If the voltage is norma ( the expected one, so 115 V ) the problem is after the PSU, otherwise it is on the PSU.
Also take care that this problem usually happens when one or more windings on the HV transformer are shorted, the starnge thing is that the PSU should autoprotect and you should hear an audible click every second or so.
Very strabge the voltage remains at 60 V.
Also check the current sensing resistor on the PSU ( should be a 1 W resistor resistence range from 0,56 to 1,5 Ohm, connected at the emitter or source of the switching transistor.

Mandi

 

[barrybear]

Re: Philips tv help please!

Another possibility is the power supply transformer itself. If you can replace this and then run the powersupply directly into a 240 volt 60 watt bulb and see if it PS strarts. Also the high value resistors around the powersupply are worth testing
Barrybear

 

[raticus]

Re: Philips tv help please!

Thanks for all the tips so far,guys.
I will check it out this weekend,and let you know what I found.

Cheers
Raticus

 

[raticus]

Re: Philips tv help please!

We have a liftoff!!

Thanks for your responses,guys.

The fault eventually turned out to be two transistors in the psu.
Numbers 7322(bc548) and 7323(bc558).
They measured ok , but replacing them solved the problem.
(low gain).

Cheers
Raticus