Magnavox 40me313v/f7 tv PSU

[glentz]

Need help. Have a Magnovox TV, 40me313v with a bad PSU, on the hot side I believe, and I can't figure out what's going on. I've read through prior threads covering repairs of basically identical PSU's such as https://www.badcaps.net/forum/showthread.php?t=71919 and threads referenced there.I have the service manual.

Originally the TV just stopped working. Initial examination showed the PSU fuse was fine but Q601 was shorted or otherwise not working and D653 on the cold side was bad. I replaced Q601 and D653 and that's when the problems really started. As soon as I powered the board (not installed in TV) there was a nice spark and Q601 literally broke into pieces. Fuse 602 did not blow. Well I started troubleshooting further and found Q621, Q602, R611, D609A, R609, D608, D607A, R621 were all suspect or bad. I replaced all of those and repowered the board without Q601 installed. No sparks or blown fuses. Testing voltages at Q601 pads I got ~170Vdc D-S (or D-gnd) and ~25Vdc G-S (or D-gnd). The 25v value was high compared to what the schematic shows (10.3) and the 170v is low (190v) but within specs for ratings of Q601 and I (wrongly) thought maybe without Q601 installed, these values were OK. So I installed a second new Q601. Blammo, same blow up of Q601 and it took out some of the other components I replaced and now it looks like R614 is showing somewhat low resistance. F602 is intact. It seems obvious current/voltages at Q601 are not right. Voltage at the output of the D601-D604 rectifier bridge is the same ~170V.

What do I need to look for? At this point I'm at somewhat of a loss. Do I need to pull legs of D601-D604 to make sure they are OK? What part of the circuit drops the voltage going to the Gate of Q601? Is it D607A only or do R614, R603 and R604 (all 560k smd's) play a role? I did check capacitance of C614 and C601 and they were spot on. I don't have an ESR meter. IC601 seems ok. I broke down after the first failed repair and bought a working PSU. Non powered in-circuit comparisons of the SMD's (resistors, diodes, transistors) and other components between the two boards helped me figure out those other failed components discussed above.

To summarize, at this point without Q601 installed I can power the board. Installing Q601 causes Q601 to blow up and take out related components as well as some components in the switching control and overvoltage control section.

Sincerely Greg Lentz

[dick_barton]

What is the power supply board number?

Did you replace Q601 with the idential part number?

Have you checked the diodes in the secondary in the event of a short circuit in one of them.

[R_J]

You should at least be using a 60~100w incandescent lamp in series with the a/c line input (or a variac) This will limit the current and may save a few components. These are hard power supplies to work on, Make sure D605 & D623 are ok, these are zener diodes so a diode test will only tell if they are shorted or open, not what voltage they zener at. Without Q601 the circuit will do nothing, and if the circuit does not oscillate, Q601 will be turned on and take 160vdc straight to hot ground.
I suspect R611 was open and D608 was shorted originally, and when you replaced Q601 everything else went as well, Replace ic601 as well as it is likely damaged.
When Q601 shorts it usually supplies 160vdc from D to G & S so everything connected to the gate and source will be damaged.

[glentz]

Originally Posted by R_J

You should at least be using a 60~100w incandescent lamp in series with the a/c line input (or a variac) This will limit the current and may save a few components. These are hard power supplies to work on, Make sure D605 & D623 are ok, these are zener diodes so a diode test will only tell if they are shorted or open, not what voltage they zener at. Without Q601 the circuit will do nothing, and if the circuit does not oscillate, Q601 will be turned on and take 160vdc straight to hot ground.
I suspect R611 was open and D608 was shorted originally, and when you replaced Q601 everything else went as well, Replace ic601 as well as it is likely damaged.
When Q601 shorts it usually supplies 160vdc from D to G & S so everything connected to the gate and source will be damaged.

Thank you for the advice. Agreed that R611 and/or D608 were probably bad when I tried to repair first time. Agreed that I am only able to do standard diode check so even though zener D605/D623 do not appear to be shorted or open, they could be bad. I will replace those along with D608(again) and D607A(again). Is there any way to test IC601? Pins 1-2 showed readings indicating the "diode" was ok. I don't remember off hand what pins 3-4 showed but I know it wasn't a direct short. I do have replacement already in hand.

Greg

[glentz]

Originally Posted by dick_barton

What is the power supply board number?

Did you replace Q601 with the idential part number?

Have you checked the diodes in the secondary in the event of a short circuit in one of them.

Board number is BA3AT0F0102-2. Yes Q601 was replaced with same part, TK10A50D.

Greg

[glentz]

Originally Posted by glentz

Board number is BA3AT0F0102-2. Yes Q601 was replaced with same part, TK10A50D.

Greg

P.S. I did check the secondary (cold side) diodes and found one bad one during my initial inspection. I've rechecked and all seem fine.

Greg

[glentz]

Is there a way to isolate the hot side from the cold side? with the problems I'm having I'm hoping the cold side is still ok and would hate to cause more issues because of the problems I'm having on the hot side.

Greg Lentz

[glentz]

Originally Posted by glentz

Thank you for the advice. Agreed that R611 and/or D608 were probably bad when I tried to repair first time. Agreed that I am only able to do standard diode check so even though zener D605/D623 do not appear to be shorted or open, they could be bad. I will replace those along with D608(again) and D607A(again). Is there any way to test IC601? Pins 1-2 showed readings indicating the "diode" was ok. I don't remember off hand what pins 3-4 showed but I know it wasn't a direct short. I do have replacement already in hand.

Greg

I've heard of the light bulb trick. What is the principle/theory behind that? Is the light bulb like a very low amperage fuse or what?

Greg

[R_J]

The light bulb will limit the current, for an example: the power supply has a dead short, the lamp would light at full brightness. If the power supply is trying to work, the lamp might pulse, if the supply is working, usually the lamp will light for half a second then go dim. This is used to check the power supply under light load, once the power supply is repaired and connected to the tv the light bulb should be removed from the a/c input.
The cold side provides feedback via the optoisolator to control the primary, this is how the voltage is regulated.
Check that D654 & D652 are not shorted

[neilc6]

I believe this is a similar power supply.

https://www.badcaps.net/forum/showthread.php?t=72916

[glentz]

Originally Posted by neilc6

I believe this is a similar power supply.

https://www.badcaps.net/forum/showthread.php?t=72916

Yes that thread is one of the ones I've looked at. I need to figure out why Q601 continues to fry itself, taking out components in the protection and switching control circuit. While voltages at the pads for Q601 seem within spec, it's possible the current isn't being limited properly or it's not oscillating which would point to something in the Q602 part of the circuit, possibly D605.

Greg

[glentz]

Originally Posted by R_J

The light bulb will limit the current, for an example: the power supply has a dead short, the lamp would light at full brightness. If the power supply is trying to work, the lamp might pulse, if the supply is working, usually the lamp will light for half a second then go dim. This is used to check the power supply under light load, once the power supply is repaired and connected to the tv the light bulb should be removed from the a/c input.
The cold side provides feedback via the optoisolator to control the primary, this is how the voltage is regulated.
Check that D654 & D652 are not shorted

Ok thanks for the additional information. I had originally checked D654/D652 but I'll check again.

Re: prior post about the two zener diodes, D605 and D623. Exact parts are not available, at least not in single piece quantities. It looks like the parts are 2% tolerance and the only parts I can find are 5% at best. Is 5% tolerance OK? If not, do you have a source for suitable replacements?

Greg

[R_J]

I'm sure 5% tolerance will be ok for the zener's

[glentz]

Originally Posted by R_J

I'm sure 5% tolerance will be ok for the zener's

Ok, thanks. I did find a 2% tolerance for one of them BUT it is only 0.35W power dissipation whereas the original ZD is 0.5watt. What do you think???

Greg

[glentz]

Originally Posted by R_J

You should at least be using a 60~100w incandescent lamp in series with the a/c line input (or a variac) This will limit the current and may save a few components. These are hard power supplies to work on, Make sure D605 & D623 are ok, these are zener diodes so a diode test will only tell if they are shorted or open, not what voltage they zener at. Without Q601 the circuit will do nothing, and if the circuit does not oscillate, Q601 will be turned on and take 160vdc straight to hot ground.
I suspect R611 was open and D608 was shorted originally, and when you replaced Q601 everything else went as well, Replace ic601 as well as it is likely damaged.
When Q601 shorts it usually supplies 160vdc from D to G & S so everything connected to the gate and source will be damaged.

.

Question about the oscillation. Without Q601 installed, will there be oscillation? I assume you mean the 160vdc will oscillate??? Is that something I can see happen with a multimeter or do I need a scope?

Greg