Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

[johnboy1313]

I've gone through several threads regarding this model television. I replaced four capacitors (one of which was obviously bad) with no change in the problem. I pulled the diode referenced in the picture below. With my meter set on diode setting, I get ~160 (not sure of the units) on the meter regardless of which direction I test it. Does this indicate a bad diode?

[tom66]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

John, of all the people, you can upload a good picture right?
I can't see where the diode is on that board... I doubt anyone else can.

It might not even be a diode you removed?

Almost always a diode conducting in both ways is bad, but it's quite rare for them to fail leaky like that, and not dead short (000 both ways.)

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

I know, crappy picture. I ran home for lunch and forgot my camera/phone at work. I'll get good pictures when I get off in a couple hours. It is a diode, a zener diode. For now, it looks just like this. I'll look online for a good picture of that area of the board.

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

Check this out.

http://video.circuitlab.org/2012/06/...-715t2056.html

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

I'm not sure how to determine which one I need.

http://www.digikey.com/product-searc...-C9V1&x=4&y=23

[budm]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

What is the TV chassis number, the board numbers so we can try looking for the service manual?

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

Here's the link to the service manual. The diode is listed on page 128, down towards the bottom.



I'll get pictures of my actual PSU and the PSU number when I get home.

[tom66]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

Seems like you may be on to something then. There's a schematic of that PSU here.
http://www.tgohome.com/ServiceManual...&Search=Submit

in case the zener/diode is different on your model. Page 110 onwards.

Not all diodes that look like that are zeners, and Philips has an annoying habit of using four digit numbers instead of letter-number.

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

That schematic is for PSU 313815864961, my PSU is 313815868291, just slightly different.

313815864961 http://www.shopjimmy.com/philips-313...wer-supply.htm

313815868291 http://www.shopjimmy.com/philips-313...upply-unit.htm

If you look close, you can just see the ends of the diode when you mouse over the upper left corner of the picture on the 8291 PSU.

[budm]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

I look at the board layout (tom sch) on page 111 (313815868291 your board) and look at the picture at Shopjimmy, I do not see that diode you shown in post 1.

[retiredcaps]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

With my meter set on diode setting, I get ~160 (not sure of the units) on the meter regardless of which direction I test it.

For most multimeters, when it is set to diode mode, the unit is always volts. In this case, it is 160mV or 0.160V.

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

I look at the board layout (tom sch) on page 111 (313815868291 your board) and look at the picture at Shopjimmy, I do not see that diode you shown in post 1.

Bud, if you zoom in on connector 1089 in the upper left corner of the pic, you can see the anode lead of the diode between the label for pin1 and the connector name 1089. The diode is not in the schematic because the schematic is for the other PSU available for this set.

Here's a pic of my PSU.

[tom66]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

It certainly looks burnt under the board there.

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

It certainly looks burnt under the board there.

Yeah, the little blob of hardened burnt PCB right at the top of the 0 in 6079 caught my eye. It wasn't until I pulled the diode that I realized how burnt the board was.

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

I swapped in a 1n914 diode from an electonics kit that i had from school per the first pic. It's working now.

[tom66]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

You replaced a zener diode, which has a 9.1V breakdown voltage, with a 1N914, which has 75V breakdown voltage.

Obviously the circuit works fine, but you'll probably find that you've lost some over-voltage protection or regulation circuitry.

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

That's kinda what I figured. I haven't buttoned it up. From the link in post #5, which one looks to be correct? Specifically, out of the two that can be ordered one at a time. I might as well order all of the small caps and replace them while I have the thing opened.

[budm]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

Is that a the 5V line that the Cathode of the Diode is connected to?

[johnboy1313]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

Honestly, without looking, I believe it's a 16V line. I know the diode is only rated at 9.1V, but I don't think this power supply has a 5V line and the anode is connected directly to both of the 16V pins on connector 1089.

[tom66]

Re: Magnavox 37MF321D/37 No Power, No LED. Testing a diode.

I think it's the 18V diode for the 16V line. The 16V line might rise if 24V becomes unloaded (inverter failure) or if the backlight is dimmed to a low level. The supply is group regulated on 5V(?) and 24V only.
i.e. 9.1V is wrong.