Magnavox 40me313v/f7 tv PSU
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
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
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