Reading the 'how to repair the MX5500 guide" I quickly repaired my own MX,  C20 & C25 were dried out.

With that in mind I bought a second one, for the children. Same problem I tought - when plugged in, no red led, TV stays dead.

So I replaced the C20, C25 and later when that did not solve the problem also C13, C10, C11, C15, C18. Still nothing.

Fuse is not blown and the 300V DC is present. The BUT12 looks and measures normal. No hot spots near C5

The only thing I hear when the unit is plugged into the mains is a high pitched noise (squeek).

Any suggestions?

thanks.

Yes... Standby 8.5V are missing.

Check C16 2200uf , there must be 8V in Standby.The same on C15, 1000uf, 8V.

Both caps are on the power supply board.

oke, thanks.

checked it, only 3V on C16, C15 0.7V

Converter is not working?

Also removed connector P2 from tunerboard still not powering up.

So I'm assuming that there no problem in the powersection of the tuner board.

OK, what is the voltage over C20 and C25 ?

C20 = +5V

C25 = - 5V

You can also remove the tuner board complete. The MX will not power up without it, but the LED should work without the tuner. As long as the 8V Standby are missing, there is no function of the cpu.

You should also measure D20, D28, D29, maybe they failed and the transformer works.

Very nice are defects on the solderings of L5, the very small transformer in front of TR1. There are usually cold spots.

Checked the PSU on the bench and 8V is present. Other voltages not but there is no load attached so maybe that's not so strange.

Found a defect diode in the mainboard, D6 is a short and I suspect C12 and TR8 also.

I think that explains the PSU shutting down?

Henkd

That would be a very good reason... but:

If TR8 is dead, C12 and D6... wow, that means a high voltage stroke direct into the HV deflection or probably a cold spot on one of these blue/white plastic caps or a defect high voltage transformer.

Then you should also replace D3, D4, TR4, C14, TR5, TR6 and TR7 + C9(!, bad cap...).

Sorry I should describe it better, C12 was not blown but serverly leaking, corroded.

Therefore TR8 was defect, D6 was not. I have replaced all three of them.

I'm a bit confused about the position of C12, according to the schematics the plus is connected to the base of TR8. The silkscreen marking on the PCB says that it must be placed otherwise. Can you tell me how to place C12. Minus right facing TR8 or not?

Hi

My mother has a LS5500 Type 3911 who died a few days ago. Some weeks before it happened sometimes the tuner switched to approx. 300 MHz and was standing there, it was not possible to change the program or to tune to another frequency, everything else seemed normal. You could then unplug it and wait a few minutes and plug it in again and it work again.
It is now complete dead but when unplug it from the outlet, I can hear a small piv a few seconds, I think it is the power supply switch frequency when discharging the capacitor. It is from 1990 so it's not new but she is happy with it. It is connected to a V8000 VCR and a Beosystem 4500 with a link room. A new Philips or Sony is somewhat more affordable for a pensioner but it can not talk to the rest, so she would prefer to have it repaired.
I have some experience with digital and radio electronics, but only theoretical experience with television. I am electrician and have experience with high voltage. I have searched Beoworlds Workbench and read what I could find. I removed the back cover and measured the standby voltage, it was at 2.2 volts. I changed all capacitors in the power supply with the exception of C5, this one was not in stock and the soldering and the capacitor looks nice, but still no life. I then tried the power supply alone, then there were 8 volts. So I measured the current in the connector P81-3 / H284 which was 660 mA and P81-6 / H282 was 245 mA, it would indicate a dead TR8 / BU508, it was dead and C12 appears to have leaked on the board.
I would be grateful if there are some who have experiences that will help me with good advice,
on what has caused it, and whether there is anything else that normally is been damaged, and if there is anything else that is a good idea too change now I have it all disassembled, beside what is described here or in Die_Bogener fantastic Workshop.

Thanks in advance
Peter Høyer


PS: hope you can read my English, it is not too good so I've had good help from Google.