Badcaps.net Forum
Go Back   Badcaps Forums > Troubleshooting Hardware & Devices and Electronics Theory > Troubleshooting Power Supplies and Power Supply Design
Register FAQ Calendar Search Today's Posts Mark Forums Read

 
Thread Tools Display Modes
Old 10-25-2017, 09:15 AM   #21
budm
Badcaps Veteran
 
budm's Avatar
 
Join Date: Feb 2010
City & State: S.F. Bay area
My Country: USA
Line Voltage: 120V 60Hz
I'm a: Knowledge Seeker
Posts: 33,865
Default Re: Magnavox DRV power supply SMPS

Must be really bad lamp for it to blow the filament that is made to handle 120V 60W.
__________________
Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
budm is offline   Reply With Quote
Old 10-25-2017, 09:17 AM   #22
budm
Badcaps Veteran
 
budm's Avatar
 
Join Date: Feb 2010
City & State: S.F. Bay area
My Country: USA
Line Voltage: 120V 60Hz
I'm a: Knowledge Seeker
Posts: 33,865
Default Re: Magnavox DRV power supply SMPS

Quote:
Originally Posted by johnfin View Post
in that older schematic attached in this thread under the problem flow chart it says if the fuse is blown check the "primary component". Like what would that be?
Rectifiers, MOSFET, Transistor, Diodes, caps, etc.
budm is offline   Reply With Quote
Old 10-25-2017, 06:16 PM   #23
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Re: Magnavox DRV power supply SMPS

So the light bulb is on. Will this allow me to check voltages?
johnfin is offline   Reply With Quote
Old 10-25-2017, 07:12 PM   #24
budm
Badcaps Veteran
 
budm's Avatar
 
Join Date: Feb 2010
City & State: S.F. Bay area
My Country: USA
Line Voltage: 120V 60Hz
I'm a: Knowledge Seeker
Posts: 33,865
Default Re: Magnavox DRV power supply SMPS

Is it on full bright or dimly lit? If it is on full bright then it means you still have shorts circuit.
budm is offline   Reply With Quote
Old 10-26-2017, 06:50 AM   #25
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Re: Magnavox DRV power supply SMPS

I started out looking for voltage drops while plugged in and the light is on but I guess that's not the best way to do it. Can someone explain to me (limited electronics tech experience) the best way to look for a short in a circuit like I have. Thanks
johnfin is offline   Reply With Quote
Old 10-26-2017, 11:34 AM   #26
momaka
Badcaps Veteran
 
momaka's Avatar
 
Join Date: May 2008
City & State: VA (NoVA)
My Country: U.S.A.
Line Voltage: 120 VAC, 60 Hz
I'm a: Hobbyist Tech
Posts: 8,380
Default Re: Magnavox DRV power supply SMPS

Quote:
Originally Posted by johnfin View Post
Can someone explain to me (limited electronics tech experience) the best way to look for a short in a circuit like I have. Thanks
Take your multimeter and set it to the lowest resistance scale (if it's a manual-ranging multimeter). Then start testing components.

Diodes and MOSFETs tend to go short-circuit / low resistance when they go bad. For MOSFETs, first look for a short-circuit / low resistance between Gate and Drain or Gate and Source. If that passes, look for short-circuit between Drain and Source.

Bipolar transistors (BJTs) usually tend to go short-circuit / low resistance between Base-Emitter or all three pins, but can sometimes also go open-circuit. Use diode-test on your multimeter to test the transistor.
For NPN -type transistors, you should get a diode reading (400-800 mV or 0.4-0.8V, depending on your meter) between Base-Emitter and Base-Collector pins, with the red multimeter probe connected to Base, and black multimeter probe connected to either Emitter or Collector pins.
For PNP -type transistors, you should get a diode reading (400-800 mV or 0.4-0.8V, depending on your meter) between Base-Emitter and Base-Collector pins, with the *black* multimeter probe connected to Base, and *red* multimeter probe connected to either Emitter or Collector pins.

Resistors like to go open-circuit or high-resistance when they go bad. If a resistor is showing lower resistance in circuit than what it actually is, then it's *probably* okay.

Inductors and transformers, especially ones used in SMPS, will show short-circuit / low resistance when measured both in and out of circuit. This is *normal*. That said, inductors and transformers *almost* never go bad (save for inverter transformers on LCDs and flybacks in CRTs). Thus, don't waste your time removing any transformers and inductors to check them. 99.99% of the time, they won't be the issue.

Electrolytic capacitors you cannot test in circuit, but if there is a short-circuit or low resistance across one (very rare), check if there are any low-resistance resistor connected it accross it. If none, you should remove the electrolytic cap from the circuit board and test it for short-circuit with the multimeter. If there is no short-circuit, also check it with an ESR meter (if you have one) to verify ESR and capacitance.

That said, I typically define short-circuit as a resistance of 10 Ohms or less. A *possible* short-circuit could be up to 30 Ohms and in rare cases, up to 50 Ohms.

Before starting to measure resistance with your multimeter, first touch both probes together and see what resistance you get. It should be less than 1 Ohms ideally, though cheap multimeter can sometimes show up to 3 Ohms with probes touching. This test shows you the lowest resistance your multimeter can measure. When you get that resistance across some component, that means you probably have a short-circuit. But again, for inductors and transformers, this is normal.

Another suggestion: DO NOT try testing the power supply with the transformer removed, as it will tell you nothing. You need everything to be connected when you apply power.

And continue using the incandescent bulb in place of the fuse until you get the power supply fixed. Otherwise, you will just keep blowing up parts every time and not certainly knowing what blew up.
momaka is offline   Reply With Quote
Old 10-26-2017, 12:22 PM   #27
goontron
5000!
 
goontron's Avatar
 
Join Date: Dec 2011
City & State: South Greeley, Wyoming
My Country: US
Line Voltage: 13.9kv HT service and some 240v center tap oddity.
I'm a: Professional Tech
Posts: 3,853
Default Re: Magnavox DRV power supply SMPS

^ Or blowing parts that were otherwise OK. Say, gate drivers ICs.
__________________
Things I've fixed: anything from semis to crappy Chinese $2 radios.

"Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

My "Extremest" views: I will spread my knowledge to whoever will receive it. It is their prerogative if they do bad with it.

My backup powerplant doesn't have a microcontroller.

Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore
goontron is offline   Reply With Quote
Old 10-26-2017, 02:20 PM   #28
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Re: Magnavox DRV power supply SMPS

Should i check all of those tiny parts on the bottom of the board?
johnfin is offline   Reply With Quote
Old 10-26-2017, 02:31 PM   #29
momaka
Badcaps Veteran
 
momaka's Avatar
 
Join Date: May 2008
City & State: VA (NoVA)
My Country: U.S.A.
Line Voltage: 120 VAC, 60 Hz
I'm a: Hobbyist Tech
Posts: 8,380
Default Re: Magnavox DRV power supply SMPS

Quote:
Originally Posted by johnfin View Post
Should i check all of those tiny parts on the bottom of the board?
It wouldn't be a bad idea.

But I suggest starting with the main switching transistor on the primary side first (the one attached to the heatsink) and also everything directly connected to it. The fact that your light bulb stays lit means either that transistor is faulty or whatever is driving it is faulty, making it stuck fully turned on all the time when it shouldn't be (it should be switching at a high frequency.... hence the name switch mode power supply or SMPS for short).

If you provide bigger pictures (ones where we can actually see the board designators for all of the parts), we may actually be able to give you more detailed instructions what to check and how to check it.

Last edited by momaka; 10-26-2017 at 02:33 PM..
momaka is offline   Reply With Quote
Old 10-26-2017, 06:48 PM   #30
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Re: Magnavox DRV power supply SMPS

MOSFET is fine. Must be a tiny part on bottom or tranny because I have checked all of the other parts.
Attached Images
File Type: jpg IMG_20171026_202007650_006.JPG (73.7 KB, 19 views)
File Type: jpg IMG_20171026_202027152_007.JPG (79.4 KB, 16 views)
File Type: jpg IMG_20171026_202034108_008.JPG (74.1 KB, 16 views)
File Type: jpg IMG_20171026_202043397_009.JPG (98.1 KB, 23 views)
johnfin is offline   Reply With Quote
Old 10-27-2017, 06:12 PM   #31
momaka
Badcaps Veteran
 
momaka's Avatar
 
Join Date: May 2008
City & State: VA (NoVA)
My Country: U.S.A.
Line Voltage: 120 VAC, 60 Hz
I'm a: Hobbyist Tech
Posts: 8,380
Default Re: Magnavox DRV power supply SMPS

Quote:
Originally Posted by johnfin View Post
MOSFET is fine. Must be a tiny part on bottom or tranny because I have checked all of the other parts.
If the transformer was blown, that board of yours would have looked like a Kingsford briquet.
No seriously, it takes a lot of current to blow a transformer. The only time you can blow it is if you either incorrectly replace the fuse with one of much higher current rating or install a jumper in its place. And even then, you're more likely to see the input choke burn up or your house's breaker trip.

So you have a fault somewhere else, that is for sure. Now the pictures you provided are still way too small for me to be able to trace anything, so I can't tell you if that 2-transistor oscillator circuit actually has self-limiting circuit to prevent the main transistor from getting stuck ON. Thus, I suggest replacing the optocoupler and 431 shunt regulator on the secondary side if you have some spare ones.
momaka is offline   Reply With Quote
Old 10-28-2017, 12:12 PM   #32
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Re: Magnavox DRV power supply SMPS

Funny you should mention that, did it, opti and all 431's. Pulled all caps and put them on an esr and sprague tel-ohm. Both the mosfet and NPN are fine. Next to the diodes again. Been down this road once and again the last part left is the tranny.
johnfin is offline   Reply With Quote
Old 10-28-2017, 06:33 PM   #33
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Good news

Possible good news. I think I found the bad part. Fingers crossed. It was one of the last transistors in the primary side. to-92 case. C3032. It tested ok before with a b&k transistor checker but I dont trust the machine now. I used the multimeter and he transistor is no good.
johnfin is offline   Reply With Quote
Old 10-31-2017, 06:57 PM   #34
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Re: Magnavox DRV power supply SMPS

Well, no luck, the new transistor did not work. Bulb still bright. It either the tranny or the small parts on the bottom.
johnfin is offline   Reply With Quote
Old 11-01-2017, 07:52 PM   #35
momaka
Badcaps Veteran
 
momaka's Avatar
 
Join Date: May 2008
City & State: VA (NoVA)
My Country: U.S.A.
Line Voltage: 120 VAC, 60 Hz
I'm a: Hobbyist Tech
Posts: 8,380
Default Re: Magnavox DRV power supply SMPS

Quote:
Originally Posted by johnfin View Post
Well, no luck, the new transistor did not work. Bulb still bright. It either the tranny or the small parts on the bottom.
Well, you only have like 15 SMD parts on the bottom side, and most of them appear to be resistors with a few diodes and ceramic caps, so that shouldn't be too hard to test. Ceramic caps and diodes you test for short-circuit (bad) and resistor test for open-circuit or abnormally high resistance (may have to remove some of them for the measurement, though).

Again, I don't think it'd be the transformer. But hey, at this point, you might as well take it out again and actually disassemble it. Just peel the tape around it first. Then separate the two core pieces. Should be either two "E" core pieces glued together or an "E" and an "I" core piece. After that, it's just a matter or removing more tape to expose the wire in the windings.
momaka is offline   Reply With Quote
Old 11-02-2017, 10:12 AM   #36
Th3_uN1Qu3
Believe in
 
Join Date: Jul 2010
City & State: Bucharest
My Country: Romania
Line Voltage: 230VAC 50Hz
I'm a: Professional Tech
Posts: 5,352
Default Re: Magnavox DRV power supply SMPS

Definitely not the transformer at this point. Check the power semiconductors and the PWM controller again.
__________________
Quote:
Originally Posted by PeteS in CA View Post
Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
A working TV? How boring!
Th3_uN1Qu3 is offline   Reply With Quote
Old 11-02-2017, 11:31 AM   #37
momaka
Badcaps Veteran
 
momaka's Avatar
 
Join Date: May 2008
City & State: VA (NoVA)
My Country: U.S.A.
Line Voltage: 120 VAC, 60 Hz
I'm a: Hobbyist Tech
Posts: 8,380
Default Re: Magnavox DRV power supply SMPS

^ No PWM controller on this one. It's an oldschool 2-transistor self-oscillator.
momaka is offline   Reply With Quote
Old 11-02-2017, 02:13 PM   #38
budm
Badcaps Veteran
 
budm's Avatar
 
Join Date: Feb 2010
City & State: S.F. Bay area
My Country: USA
Line Voltage: 120V 60Hz
I'm a: Knowledge Seeker
Posts: 33,865
Default Re: Magnavox DRV power supply SMPS

See post 12 and 15 diagram for the topology being used in this power supply, if the switching control transistor is stuck in the OFF mode then the main power MOSFET will be stuck on.
budm is offline   Reply With Quote
Old 11-02-2017, 06:14 PM   #39
johnfin
Senior Member
 
Join Date: Oct 2017
City & State: tampa, fl
My Country: united states of america
I'm a: Knowledge Seeker
Posts: 110
Default Re: Magnavox DRV power supply SMPS

Its closer to the mdr515 schematic, not exact though. I need some help diagnosing this thing. Can I put a signal tracer on the transistors, and mosfet to see if it is switching? I would like to check it out when the power is on and bulb on. As far as the small parts on the back of the board, some are micro transistors, not just resistors and diodes. I was told not to remove the transformer yet when I did the short went away so how about this, cut some of the output lines on the transformer that dont return to the primary side so I can try to isolate the problem.
johnfin is offline   Reply With Quote
Old 11-02-2017, 10:27 PM   #40
budm
Badcaps Veteran
 
budm's Avatar
 
Join Date: Feb 2010
City & State: S.F. Bay area
My Country: USA
Line Voltage: 120V 60Hz
I'm a: Knowledge Seeker
Posts: 33,865
Default Re: Magnavox DRV power supply SMPS

What resistance do you get by measuring between the two legs of the main filter cap?
budm is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump



Badcaps.net Technical Forums 2003 - 2018
Powered by vBulletin ®
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
All times are GMT -6. The time now is 01:31 AM.

Did you find this forum helpful?