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

**johnboy1313** · 05-09-2013, 04:30 PM

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

With the bad diode in, no load on the 24V rail and the PSU plugged in, I measured ~19V on the 16V pins. Don't know if it means anything, just putting it out there.

**tom66** · 05-09-2013, 04:30 PM

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

What do you measure on 24V and 5V?

**johnboy1313** · 05-09-2013, 04:34 PM

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

According to the schematic, there is no 5V supplied by the PSU, just 24V and 16V. The voltage at pin 9 comes from the main board.

Attached Files

1089.pdf (64.0 KB, 43 views)

**budm** · 05-09-2013, 04:42 PM

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

Pin 9 is the Control line for Power supply on/off (PS_ON). the Diode is connected to that line and the ground?

**johnboy1313** · 05-09-2013, 04:57 PM

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

Originally posted by johnboy1313

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.

Are you asking or telling? Did you see this?

**tom66** · 05-09-2013, 05:03 PM

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

John, you're talking to yourself again...

**johnboy1313** · 05-09-2013, 05:07 PM

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

I think you mean still.

**johnboy1313** · 05-09-2013, 05:08 PM

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

Let's try this again then.

Originally posted by budm

Pin 9 is the Control line for Power supply on/off (PS_ON). the Diode is connected to that line and the ground?

Bud, are you asking about the diode connections or telling me about them? DId you see this?

Originally posted by johnboy1313

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.

**budm** · 05-09-2013, 05:23 PM

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

"Honestly, without looking, I believe it's a 16V line" That is why I ask to verify which pin the Cathode of the Diode is connected to so we can estimate the value of the Zener voltage. This Zener is used as a crowbar to limit the output voltage if the regulation failed. You see them use 6.2V Zener on the 5V line for example.
The power supply will still work without it, but at first turn on the spike can be higher thane the normal regulated voltage required. Most cheaper power supply will not have these Zener Diodes on the output.

**tom66** · 05-09-2013, 05:26 PM

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

A little 0.3W zener diode against the might of a 5V, 3A output (probably?) It's not going to stand a chance... They may hold it for some time but they usually fail. Hopefully they fail short but sometimes they go open.

**johnboy1313** · 05-09-2013, 05:35 PM

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

I might be wrong but it looks to me like the cathode is connected to the 24V rail via a 2.2Ohm .125W resistor (3045).

Attached Files

**budm** · 05-09-2013, 06:40 PM

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

When that Diode crowbar and shorted itself out, the current sensing on the Source resistor on the primary side will just shutdown, then you will hear ticking sound from the power supply trying to start/shutdown. It is not 100% protection circuits against output voltage runs away failure mode, better than nothing.

**budm** · 05-09-2013, 06:48 PM

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

It looks to me like it is not for crowbar, you can see the rectifier D6071 Cathode output feeding C2074, the Pi Inuductor L50074, Pi Output filter cap C2075 then goes out to pin 1 and 2 of the connector. The Anode of that Diode is connected to the junctionS of the inductor L5074/C2075, the Cathode is the output that feed the resistor then the trace goes from there.
I cannot see in fine details due to the Drawings, But that is what I am seeing.

**johnboy1313** · 05-09-2013, 08:16 PM

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

So, one more time.

http://www.digikey.com/product-detai...9-1-ND/2531096

Or

http://www.digikey.com/product-detai...8-1-ND/2762648

**budm** · 05-09-2013, 09:37 PM

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

If the Anode is connected to the +16v then it is forward bias on, it will not act as Zener since Zener is used in reverse bias mode. Can you trace and see where the trace of that resistor that is connected to the CATHODE to see where it goes? So far the power supply in the diagram does not show this diode, it does have Zener Diode D6076 with its Cathode connected to pin 1, 2 (+16V output pin) which is used to feed the ref diode that will cause the output voltage to drop if the 16V gets too high (it has to be high enough to bias on the Zener D6076), that Ref diode also sense the 24V output line, see page 110.
Your board has that extra diode which is not on the Shopjimmy board either.

**johnboy1313** · 05-10-2013, 05:48 AM

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

How far up the chain do you need to see?

**budm** · 05-10-2013, 08:46 AM

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

Can you take good clear pictures of the whole bottom side, then 1/4 section close up so I can trace out the copper traces. Same for the top side.
So far I cannot locate the circuit of your board version that has that diode and the resistor in that location yet.

**johnboy1313** · 05-10-2013, 09:05 AM

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

On page 128 of the SM, the spare parts list shows this diode to be added for a "Power Board Change". 6079 = DIO REG BZX79-C9V1 A (PHSE) A. I searched Digi-Key for "BZX79-C9V1" and it gave me the choices I posted above. It looks like they are alternates of each other.

It turns out that my PSU is labeled 313815864961 after all. I will get pictures for you this afternoon when I get home. Thanks for taking the time to help out.