More Bang and Olufsen woes

[justanotherhigh]

Need the advice of you chaps again, i'm not exactly great at electronic diagnosis but i can manage changing parts properly and safely.
Now i just picked up an LX2500 that won't power on, it goes into standby, you press the remote or channel button, standby lamp goes out, the set sits there making a bit of high pitched static-y noise, then drops back into stanby. Nothing shows on the screen.

So, all fuses are OK, have changed all the signal boards off a donor but made no difference so presume fault is on the power board. Have changed c28 and tested out c29 no issues there.. have tested all the obvious previous repairs on this board, seem okay? On the back of the set is a sticker from a repair shop detailing an 'on + off fault' so would presume it has happened before.

Seems like a tough problem to try and track down. Any pointers for an enthusiastic idiot like myself?

[steve1010uk]

Have you checked the line output transistor TR11 for shorts is this the set with the screened EHT lead that gives lots of trouble.

Steve

[justanotherhigh]

Yeah, that checks out fine and this set has had the mod put in to solve that problem..

[ben]

Why did you change those capacitors?

You need to let us know what measurements you've made.
As Steve said, have you checked the line transistor for short? Any HT volts to the line stage? Series bulb in HT line?
If voltage present, any shorts on any secondary loads (audio, RGB, frame, etc) coming from the line o/p transformer?
Is the PSU outputting? Any shorted semiconductors on its secondary?
Soldering around line, line drive and PSU stages OK?

Until you start with these basic checks we're working in the dark or on guess work and so whatever we speculate will probably not get you very far. You need to narrow this down.

[julie_m]

It sounds as though the power supply is detecting a fault condition, and shutting down. The fact that it squeals suggests that the switching transistor is O.K, at least. This could be due to a problem with the set; or it could be a problem with the power supply itself.

Depending on the speed of your multimeter, or if you have a digital storage oscilloscope, you might be able to measure the voltages just before the power supply cuts out. If one of them is much lower than it should be, that might give you a clue what could be faulty -- you can eliminate any stage not using that particular voltage rail.

If you have an ESR meter, test the PSU capacitors. If one of them has gone faulty, and consequently is not able to hold its charge long enough to charge the next capacitor downstream, the voltage on that output would sag; and the protection circuit would detect this, assume an overload and shut down. (If you haven't got an ESR meter, weigh the price against a full set of replacement electrolytic capacitors for the set, which you might otherwise end up having to buy. They are an invaluable tool for working on PSUs and many other things. I might actually even break something on purpose, just to justify buying my own ESR meter.)

If it's not a duff capacitor, suspect a Zener diode, or even one of the rectifier diodes (usually a bridge on the input, and half-wave rectifiers on the outputs -- note, these must be replaced with ultra-fast recovery types. 1N4007s from your parts drawer will not be up to the job, not even for a quick test). Also test the secondary windings for continuity (much easier in the case of half-wave rectification).

[steve1010uk]

Perhaps start by removing the line output transistor and switching on, do you then have HT,
you could also connect a 40w lamp between HT line and earth to check operation of the power supply.

The 16 volt electrolytics on this chassis did have a habit of going short circuit