32MF301B/F7 No power, several bad parts on PSU

[johnboy1313]

I posted in this thread, but it seems like it's not getting any attention, so I'll post again here.

https://www.badcaps.net/forum/showthread.php?t=25477

I picked up a Magnavox 32MF301B/F7 with the same no power issue and same PSU. Unlike the original poster, my fuse wasn't blown. R602 and R611 (labeled R601 right next to R602 in the hand drawn schematic) were both failed open. I'm guessing that based on the hand drawn schematic provided by budm in the link below, the open resistors saved the fuse. Being that the fuse is good, I'm wondering if the lightbulb trick would work in this case.

https://www.badcaps.net/forum/attach...9&d=1358581659

I also found the following failed components.

Q600 shorted
D604 shorted
D607 shorted
D610 shorted
D640 shorted
R608 open

R605 and R606 are both giving weird readings in the KOhms out of the board, so I'm replacing those as well.

Once those are replaced, if any other component is shorted, the lightbulb trick should then work, correct?

[budm]

Re: 32MF301B/F7 No power, several bad parts on PSU

https://www.badcaps.net/forum/showth...t=25477&page=4 see post 73.
You may also want to read Kevin comments:
https://www.badcaps.net/forum/showth...t=25477&page=6
Its been awhile so I do not remember all the details. Use the lamp when you install all those new parts.

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

Thanks Bud

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

I replaced what I found to be bad components but the lamp still lights. I found that with D610 removed, I still see a dead short between the two holes that the diode solders into. It looks like the cathode of D610 connects to R608 and Q600, the anode connects to HOT ground. Q600 and R608 both test good. What should I look at next?

Schematic
https://www.badcaps.net/forum/attach...4&d=1361833980

[tom66]

Re: 32MF301B/F7 No power, several bad parts on PSU

D610 will read short, it has 0.2 ohm in parallel.
The diodes on the output aren't shorted?

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

I'm not really sure which diodes are the output diodes (D605 and D609???). I've checked all of the diodes on the hot side and none show shorted.

[tom66]

Re: 32MF301B/F7 No power, several bad parts on PSU

The output diodes are the ones on the secondary/cold side.

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

I've tested every diode on the board. I can't find anything shorted. I'm at a loss here.

[tom66]

Re: 32MF301B/F7 No power, several bad parts on PSU

Permanent glow from lamp or just flashing? To try to narrow down the short, with the bulb installed, you can check for voltages across components. Any power components with near zero volts across them could be shorted. Try testing across one of C602, C603 or C604. What do you measure? Also, try seeing if you get any output - try isolating the main board in case of a short circuit too.

Also, if Q601 is bad, the transistor will never turn off, so the circuit will act short continuously. Could have been damaged if Q600 main power FET blew.

See theory of operation described elsewhere:

This is a self-oscillating PSU (ringing-choke converter)

* The initial startup pulse comes from the 220K Rs R603, R604, R600.
* Once the MOSFET begins conducting (Vgs>2V) current flows through T600 pins 3-5 building a field up in the transformer.
* The sudden increase in primary current causes a reflected current across pins 7~8 of T600 which drives the gate fully into saturation (about 20V) limited by zener D604.
* Once the primary current hits a certain limit (defined by approx 0.7V/R608), the transistor Q601 will start discharging the MOSFET gate.
* As the MOSFET gate discharges below the threshold the primary current begins to drop and this causes a large negative pulse across pins 7~8 of T600 which rapidly discharges the FET gate, improving efficiency.

It's quite a clever circuit, but it has no intelligence. It'll do it's best to drive a dead short, like a shorted output diode, quickly resulting in the destruction of the MOSFET due to overcurrent or overstress.

https://www.badcaps.net/forum/showth...t=36180&page=3

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

Permanent glow from lamp. C603 and C604 are not installed. I'll check C602, I'll have to solder leads to the cap to be able to test it.

Thanks for the idea with the main. It seems obvious now, but I probably wouldn't have ever thought to try that.

I've tested Q601 out of the board twice. It is not shorted.

[tom66]

Re: 32MF301B/F7 No power, several bad parts on PSU

If Q601 is OPEN it would also cause a problem... Use a transistor tester or just plain replace it as it's a 5¢ component.

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

Replaced Q601 and still have the lamp on. Not sure where to go from here. I may just put it under the bench.

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

Well, I finally got back to this one. It's been sitting since May. I decided to throw the fuse back in and see what would blow. I plugged it in and it came to life. I just finished watching Back to the Future on it. It's wierd because substituting a lamp for the fuse would cause the bulb to glow. I thought that meant that something was still shorted and that the fuse would blow.

[budm]

Re: 32MF301B/F7 No power, several bad parts on PSU

What Wattage of the lamp did you use? 100W will be fine if the main power is not activated and just in standby mode, otherwise you will have to use more than 200~300W LAMP.
You have to play with wattage of the lamps to get the ideas what size lamp you will need when you work on the stuff.

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

Ahh, I don't think it was even a 100W lamp. Thanks for the tip.

[johnboy1313]

Re: 32MF301B/F7 No power, several bad parts on PSU

Just fixed another one of these. It needed...

Q600
Q601
R608
D607
D610
D640

Fortunately, these parts were all spares. I bought two of everything last time.