Phillips 22 inch no power high pitch transformer noise

[Tonymon1]

Please help. Phillips 22PFL3504D/F7 mfd 11/09. No power. Power supply makes high pitched noise. Caps look OK. No bulges. No leaks. R603 to R606 approx 470 ohms (analog meter). No voltage out. Please help me troubleshoot this. Thank you.

[Mr Bill]

Re: Phillips 22 inch no power high pitch transformer noise

I would start checking all the diodes. Have repaired a couple sylvania's & emersons that had a high pitch nosie and completly dead. Turned out to be a shorted diode.

[RON_CET]

Re: Phillips 22 inch no power high pitch transformer noise

check these two diodes and let us know what your readings are D607 and D668 use the Diode scale on your DMM if have

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

I checked them both with my analog meter. D607 and D668 both have continuity in both directions. Is there a specific readng I am looking for? I can get a DMM. Actually, it's time I got my own.

[eppidrummer]

Re: Phillips 22 inch no power high pitch transformer noise

I have the same exact issue, same tv only D607 is the only diode on the whole board that does have continuity. Does that mean its bad? Found some specs on that diode but cannot find the voltage?

[kevinm34232]

Re: Phillips 22 inch no power high pitch transformer noise

You should have a reading in only one direction, pull one leg of diode to confirm if it's bad.

[RON_CET]

Re: Phillips 22 inch no power high pitch transformer noise

The diagram below shows how to connect a digital meter to test the diode. There are a number of things to remember:

• Make sure you are using the diode range.
• Using a digital meter, connect the black lead to the cathode and red to the anode (forward bias - around 1kΩ).
• Reverse the meter connections (reverse bias - infinity reading).

* Note some times depending on how diode is wired into circuit may need
to remove or lift one side for more accurate reading.

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

Interesting results. I did unsolder one side of each diode to check them out of the circuit. When I put the meter on diode check, it said 0l. So I assume that is an open circuit.

D607 gave readings of 0l in one direction and 707 in the other. No sound. Diode D668 read 1s in both directions and the meter made a continuous sound.

[Mr Bill]

Re: Phillips 22 inch no power high pitch transformer noise

Diode D668 read 1s in both directions and the meter made a continuous sound.

This would indicate it is bad. Is this the only diode that is suspect? Is there a part number on the diode?

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

The part number is listed as NDTB020BST26. Right now it is the only one that is suspect.

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

The service manual says that D668 is a Zener diode 20BSB-T26 with a part number of NDTB020BST26. Does that match to a real number?

[Mr Bill]

Re: Phillips 22 inch no power high pitch transformer noise

Did a google search for 20BSB-T26 and came across a thread that the poster was looking for the exact diode you need. It was said that it's a 1W 20V zener diode. Did a simple seach at digikey for a 1W 20V zener and came up with this.
http://www.digikey.com/product-detai...DICT-ND/190282

Does this look like the diode you have (size)? The part number wasn't helping either. Maybe someone with alot more experience in this department will chime in.

[kevinm34232]

Re: Phillips 22 inch no power high pitch transformer noise

Had similar issue with Philips model 42PFL5432D/37. Problem: ZD906, called transient voltage suppressor. Purchased this as replacement:

http://shop.vetcosurplus.com/catalog...tbq9rj2dgg2j13

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

The digikey looks just like it. Should this be something I might be able to obtain locally?

[Mr Bill]

Re: Phillips 22 inch no power high pitch transformer noise

You are probably not going to find it locally. I usually will wait for other projects to make 1 big order but digikeys shipping starts at about $2

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

I replaced D668 and the unit powered up nicely, twice. When I moved it to it's normal location it wouldn't turn on and the same noise came from the power supply transformer.

[Mr Bill]

Re: Phillips 22 inch no power high pitch transformer noise

so you are back to square one? Did the replacement diode short out?

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

I have not reopened the set yet, but based on the transformer sound, I strongly assume so.

[Tonymon1]

Re: Phillips 22 inch no power high pitch transformer noise

Assuming the new diode shorted out, what would the next step be?

[Th3_uN1Qu3]

Re: Phillips 22 inch no power high pitch transformer noise

Check the circuit that diode is on, and replace the capacitors. Even if they don't look bad. The zener is there to protect from overvoltage - if it shorts, it means the voltage on that output is higher than it should be, which usually happens as a result of bad caps. When the ESR of a cap increases in a regulated power supply circuit, the power supply increases the duty cycle to keep voltage in regulation. However, with bad caps, while the average voltage remains the same as required (since this is set by the feedback circuit), the peak voltage increases, sometimes dramatically so.

An ideal DC source would have peak voltage equal to average voltage, however, in practice, there's always a small amount of AC ripple on top of the DC voltage, it's not quite a straight line as seen in textbooks, it's got peaks and dips, hence its peak voltage is a little bit higher than its average. The better the caps, the straighter the line and the smaller this difference. As they go bad, the peaks and dips in the voltage become larger and your device isn't happy anymore.

Without the zener to short first, a lot of stuff would fry.