[torbjorn]

Check whether the 50 Hz interference is syncronous to the mains or to the video signal, that will give a clue of where to look for the problem.

A method of checking it is to connect an oscilloscope to the loudspeaker output and adjust it so you see the interference. Then try trigger the scope from the mains or from the video signal and see when the interference gets stationary.

If the problem really is due to crosstalk from the video signal to the sound circuitry, double check that all ground connections from the CRT board, CRT coating etc are correctly fitted, and also check that no CRT related cabling is located near sound related cabling in the set.

[netblues]

Back here again..

The bu and fet change worked well and the set is stable since repair.
However lately I hear 50Hz interference from speakers, the more intense and white the screen is, the more 50Hz I hear.

I know it must be some capacitor somewhere dried-out..
Question is WHERE?

[vtech]

Counterfeit semiconductors are indeed a major problem and some are rather hard to detect. (one reason I do my best to nuke any "Chinese" ad here). Of course the best weapon is to try to stick with known supplier(s). Another way is to compare the logo on the device. On many occasions it is a dead giveaway. Here is a logo source
http://freespace.virgin.net/matt.waite/resource/logos/index.htm.

Test equipment such as a curve tracer can be used to look at the performance signature but surely not too practical when trying to repair something.
Also, here is some good information
http://sound.westhost.com/counterfeit.htm

[torbjorn]

Counterfeit power transistors seem to be a raising problem. Some years ago, i had trouble with a Sharp tv using a BUH515 in the line output stage. The original transistor was manufactured by ST and i got replacements with a Philips brand name from a, as i thought, reliable supplier. However, some research in the web shows that Philips never has manufactured the BUH515, so those parts i got have to be counterfeit. Probably re-labeled BU508 from a no-name manufacturer.

[Westronix]

Thanks for your reply Gary. I had already bypassed the connector on the scan coils by soldering the wires directly onto the tags, and the connector on the main board is ok.

I think (hope) I have solved it. Looks like dodgy BU2520D transistors...I have today ftted a new genuine Philips Phillipines transistor (and another IRF..) and the set has been running for two hours so far, with the heatsink barely warm.

The other transistors (still marked BU2520D but with 'PHL' above) have a slightly different case style and were apparently made in China. The heatsink reached a much higher temperature in only a few minutes.

The original problem was definitely the line output transformer, I think it had a failed rectifier in the eht section as it was loading the power supply but not sufficiently to cause a permanent trip. If I had kept the original BU2520D in there would probably have been no problems but I replaced it for good measure along with the transformer. Then the problems started....

Thanks again for the suggestion.

Brian.

[gary]

scan coil connector often goes dry jointed on those sets, will cause intermittent loptr failure

[westronix]

Time to resurrect his thread again!

This set is a 29PT8413/05.

Original symptoms green LED flashing on/off about once every two seconds, PSU tripping slowly. No dead short across line output stage supply. BU measured ok out of circuit.

Replaced LOPT, and BU for good measure. Set worked for 5 to 10 minutes then went off, red LED flashing and s/c across supply to line output stage. BU now s/c and IRF leaky.

Replaced BU and IRF again, checked C2418 on component bridge (good) resoldered possible dry-joint on output of driver transformer and checked entire circuit. No faults found. Scan coil circuit ok (no poor connections on plugs etc).

Set worked again for about ten minutes then BU s/c and IRF leaky again. Heatsink quite warm but not scorching. Picture fine until point of failure.

Any more ideas please???

[netblues]

Well, have replaced BU and fet again and the set is working as expected.
No faulty parts found (as much as I could test).
Found some old mica on the heater, this could be the curlpit.
After leaving the set on for 3-4 hours bu temperature is less than 50 Celcius

I suppose this could have been the issue for the second burnout.

I'll know in a week...
Thanks for the info

[torbjorn]

Check the flyback capacitor(s) and any capacitor in series with the deflection coils. Replace if any of them has any bulges or cracks in the casing or if the value is more than 5 % off. Check the entire line output and driver area very carefully for bad solder joints etc, also make sure that there are no signs of overheating in the connector to the deflection coils. Check all electrolytic caps in the driver stage, in particular if the set does not have a driver transformer but R-C coupling between the driver and output stage.
When you get the set running, check the waveform at the output transistor’s base with a scope, and check that the transistor does not get particularly hot.

[netblues]

I'll resurrect the thread.
TV had all the symptoms mentioned above.
Changed IRF, changed BU changed capacitor and tv worked for a few hours
with no issues.
Then BU burned again (new mica AND heating paste applied when replaced)

I have been through the chassis pdf's but not sure if it is an exact match

Exact Model is 32pw9763/32

Any ideas what else to look for (apart from replacing bu and irf again)

Many thanks