schematic attached, need help please.

[keeganJacobs]

Guys please see my attachment.

Sheet 1 is 115Vac coming in and going through a bridge rectifier and sheet 2 its Vdc. Please see my text writing where it starts failing.

Then thing is this , as soon as it fails from there , it goes to many other places on the different sheets. Is there a shortcut or what is the best possible way to break circuits to eliminate areas instead of measuring each component on each sheet that is failing.

Guys the pencil markings on the sheets are off a good board readings which are about 64 volts . The bad board coming out of pin 2 is 48 volts.
Thanks,

 

[Mikebits]

Sounds like one of the 17V zeners off of pin 3 is shorted. 63-17 = 46.

 

[Pommie]

I wondered about the zeners but got confused as there are 3 17.1V zeners from point 3 to ground and so point 3 should only be at 51V. Maybe the 63V isn't measured to Pri-Gnd. I would certainly check that all the zeners have around 17V across them.

Mike.

 

[keeganJacobs]

Is there a way to check if its not on a different sheet? Is it possible we cut the circuit ? I ask because if you look in sheet 2 , top right corner , 66v is going out to a different sheet and even there its measuring 48 v which it should be around 66v.

The failure does start on sheet 2 and its on pin 3 of CR66 but what if its actually from a different sheet ?

Is there a way i can break a circuit so i know for sure its only on this sheet ? Thanks guys

 

[Mikebits]

Did you check the zeners? You could lift pin 2 of CR66, this would disconnect everything past it. See if pin 3 reads right voltage with pin 2 lifted.

 

[keeganJacobs]

Did you check the zeners? You could lift pin 2 of CR66, this would disconnect everything past it. See if pin 3 reads right voltage with pin 2 lifted.

Thanks for the help...I checked the zeners and VR24, and VR17 werent reading what they should be but replaced them and didnt fix the issue. Vr17 on a good board is reading 3.5 volts but on the bad board , it was reading 0 volts so i thought that was it but no.

Is there any other place i can cut the board so i can trap this problem ?

 

[Mikebits]

Lift pin 2 of CR66. If good then reconnect, work down the path. Start pulling parts along path to isolate. CR64 for example.

 

[keeganJacobs]

Lift pin 2 of CR66. If good then reconnect, work down the path. Start pulling parts along path to isolate. CR64 for example.

Mike, Pins #1 and Pin# 3 are measuring correct. Are you saying lift pin #2 and measure the pad of pin 2 on cr66?

Other pins on cr 66 ( 1, and 3 ) are measuring correct even when pin 2 is touching pad so i was just a bit confused why i would lift pin 2.

I will lift pin 2 on CR49 to break circuit on 66v going to different sheet.

 

[Mikebits]

Now I am confused. The voltage drop across pins 2 and 3 of CR66 should only be .7v Are you saying Pin 3 reads normal but pin 2 reads incorrectly? Put your meter across pin 2 and 3 of CR66 voltage read should br around .7V

 

[keeganJacobs]

Now I am confused. The voltage drop across pins 2 and 3 of CR66 should only be .7v Are you saying Pin 3 reads normal but pin 2 reads incorrectly? Put your meter across pin 2 and 3 of CR66 voltage read should br around .7V

I have my DMM Gnd on PRI-GND. Positive measuring each pin with VOLTAGE applied to board. 115Vac coming in the board from sheet 1 , and the after rectifier the voltage is switched to DC and when it reaches CR66...

CR66 pin 1 and 3 are measuring around 48 volts which is good. Pin 2 on good board should measure around 63 volts but on bad board it measures 46v.

When diode testing without voltage applied to board , cr66 is reading ok but its just when i apply voltage to the board is when i get those other readings above.

 

[kchriste]

CR66 pin 1 and 3 are measuring around 48 volts which is good. Pin 2 on good board should measure around 63 volts but on bad board it measures 46v.

The 63 Volts on pin 2 of CR66 doesn't come from the stacked zeners via pin 1 or 3 of CR66. It comes from the CR64/C142/C143 rectifier and filter combo which gets it's "AC" from the L9/Q22 flyback circuit.

Vr17 on a good board is reading 3.5 volts but on the bad board , it was reading 0 volts so i thought that was it but no.

This confirms my suspicions about the fact that Q22 (SEPIC FET) isn't switching or maybe being driven at all and thus there is no AC on the coil (L9?) to drive the CR64/C142/C143 rectifier and filter combo and thus produce your 63 volts on pin 2 of CR66.

 

[Mikebits]

Hmm, sounds like a may have misled you some. Sorry about that

 

[keeganJacobs]

The 63 Volts on pin 2 of CR66 doesn't come from the stacked zeners via pin 1 or 3 of CR66. It comes from the CR64/C142/C143 rectifier and filter combo which gets it's "AC" from the L9/Q22 flyback circuit.

This confirms my suspicions about the fact that Q22 (SEPIC FET) isn't switching or maybe being driven at all and thus there is no AC on the coil (L9?) to drive the CR64/C142/C143 rectifier and filter combo and thus produce your 63 volts on pin 2 of CR66.

CR66 off of pin 2 , R630 , and R631 in parallel were reading way off on a bad board so i replaced them and now the resistance is measuring as they should but pin #3 off of CR66 is still reading around 48.

L9 out of pin 7 and 12 should read around 60 Vac but its only pushing through 8Vac. The coil before that L12 input is reading 8VAC also which should be reading around 60Vac. I dont know whats bringing this down.

When you guys take readings for resistance and voltage , do you most of the time have the - Ground of the DMM on PRI-GND ?

Just need some help trapping where the problem is at and where its not so i dont waste my time on other sheets. But to me it looks like its coming from the q22 because no voltage is going out to the zener and there should be like 3 volts atleast.

 

[kchriste]

CR66 off of pin 2 , R630 , and R631 in parallel were reading way off on a bad board so i replaced them and now the resistance is measuring as they should but pin #3 off of CR66 is still reading around 48.

Q20? (The FET on the right hand side; 62.2V marked next to it's drain) could be defective or have been turned on more than it should by it's control circuit, so check it and VR21 also.

When you guys take readings for resistance and voltage , do you most of the time have the - Ground of the DMM on PRI-GND ?

In this case you'll be measuring voltages relative to PRI-GND because the circuitry of interest is on the primary side of the transformer. The dotted line shows the separation between the low voltage secondary side and the high voltage primary side.

Just need some help trapping where the problem is at and where its not so i dont waste my time on other sheets. But to me it looks like its coming from the q22 because no voltage is going out to the zener and there should be like 3 volts atleast.

It could be Q22, but it also could be U23 which drives it. You may still have to "waste your time on other sheets" if U23 is not being driven.
The way this circuit works is that the two 5K 3watt resistors, along with CR66 and the zeners (VR21,22 & 23) provide a bootstrap voltage of apx 51V to get the switching regulator started. Once it starts, you'll get your AC voltage on the drain of Q22 which when rectified by CR64 gives you your 63V that you are looking for.