Dynex DX-32L150A11 Standby Blink

[ohmmy]

This Dynex just gives a blinking standby light.

Measured ouput voltages from psu and only one 5v rail labled 5V1 has a reading, a very low wavering reading. Highest it reaches is 1v

When I disconnect psu from main board and remeasure voltages the same line "5V1" now gives a 5.20v reading, still nothing from the others.

I then tried loading that line with a 12v car lamp and the wavering voltages mentioned originally came back (not sure if the load was too much for it?)

No bulged caps, everything looks fine visually. I've attached some pics, if you need any others of specific areas please let me know. What should I do next?

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

I can also hear a very faint ticking sound from the psu. Any suggestions? Would like to get this up and running.

I've ESR'd the caps on the standby rail coming from the smaller transformer and they all tested good. As well as the diode on that rail.

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

That is indicative of several possible faults:
- shorted secondary diode
- bad startup capacitor (not necessarily bulging!)
- bad startup charge ckt

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

That is indicative of several possible faults:
- shorted secondary diode
- bad startup capacitor (not necessarily bulging!)
- bad startup charge ckt

I've tested all secondary diodes (in circuit) and they all seemed to test fine.

I've direct replaced what seemed like the startup capacitor, c525 47uf 50v with no change. (in attached picture next to N501)

Where would the startup charge circuit be?

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

Check the voltage across that capacitor in the fault condition and when working (main b/d unplugged). What do you get? You may have a short on main board. A 12V car bulb may draw too much (60W headlamp or 12W brakelight? Those are at 13.5V, but for 5V, will still be about 1/4 power, maybe more.)

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

Check the voltage across that capacitor in the fault condition and when working (main b/d unplugged). What do you get? You may have a short on main board. A 12V car bulb may draw too much (60W headlamp or 12W brakelight? Those are at 13.5V, but for 5V, will still be about 1/4 power, maybe more.)

It was headlamp bulb.

voltage at cap with main board unplugged: 13.6v
voltage at cap with main board plugged in: jumps around from 8-15v

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

OK, do you have a spare brakelight or similar? May work better as load for 5V. Looks like main board has short on it.

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

OK, do you have a spare brakelight or similar? May work better as load for 5V. Looks like main board has short on it.

I don't, anything else I could maybe use? What about a small computer fan?

Also, how would I go about finding a short on the main board?

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

I used a 12v 220ma CPU fan on that 5v rail and it powered up, voltage measured a steady 5.20v

Should I try with a higher watt load or safe to say there's a problem on mainboard?

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

PC fan ~100mA is fine for a standby type load, yes it indicates a short on the main board.

What you need to do is either try tracing for shorts using an ohmmeter, or try my favourite method which involves freeze spray. You freeze spray parts of a board whilst passing a small current (around 1 amp) through it. The parts that melt the freeze spray likely have the short.

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

PC fan ~100mA is fine for a standby type load, yes it indicates a short on the main board.

What you need to do is either try tracing for shorts using an ohmmeter, or try my favourite method which involves freeze spray. You freeze spray parts of a board whilst passing a small current (around 1 amp) through it. The parts that melt the freeze spray likely have the short.

How would I trace for shorts using an ohmmeter?

When I have mainboard plugged in, every component seems cold to the touch, in the past I'd at least notice some warmth on a faulty component.

Wouldn't know how to exactly only pass 1amp to the entire board while freeze spraying

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

Shorts are traced by testing resistance along the board, looking for low readings, it can be very difficult on a meter with only one decimal point which is why I suggest the current method, but that requires a constant-current constant-voltage lab power supply.

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

Would the short be on that 5v line or anywhere? I could try tracing it on the mainboard

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

It would be on that 5V line.

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

It would be on that 5V line.

I traced it to a couple smt capacitors and a couple voltage regulators. Do I need to check them off board for shorts? They don't test low ohms as of now, neither do the caps

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

What resistance did you measure at the 5V terminal directly on the main board, compared to those at the caps/Vreg?

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

I'll have to retest and get back to you. Just left my house. What should I be looking for exactly?

[tom66]

Re: Dynex DX-32L150A11 Standby Blink

Low resistance: generally less than 10 ohms is a concern.

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

What resistance did you measure at the 5V terminal directly on the main board, compared to those at the caps/Vreg?

So I'm looking for the same reading I get from the 5v terminal to ground when checking others components to ground?

[ohmmy]

Re: Dynex DX-32L150A11 Standby Blink

From the 5v terminal to ground I get a reading of 134ohms. I found the same exact measurement on a smt cap on both ends. It's a shade of grey. No clue what the rating would be. Could this mean it's shorted?