Schaub Lorenz tv no go

[Blackbiro]

Hi All
Need help here please.
I have a Schaub Lorenz 26inch LCD tv Model VMD2600PA build date May 2006, that won't start up has no standby/power led lit at all.
These tv's were originally German but were later made in Korea.
Mr Google search tells me that 90% of problems are in the SMPS board and I am assuming that my fault is there also, no Service Manuals were found for download that I could find.
Searched Austech archives for info but none found.

The SMPS board is LCD-PSU200 and all components look in as-new condition.
The ceramic mains fuse is not blown.
No semiconductors or diodes read short circuit and junction resistance appears normal (15 ohms for silicon junction reading on my VOM.
Electro' caps are not bulging or shorted and give a charge indication on the VOM.
The whole SMPS board has been re-soldered around most components in case of cracked or dry solder joints.
On power the Mains section of the board is working up to a point just before the two Fets Q801 & Q802, nothing works after that, the Fets are not oscillating.

I get the impression that there is no kick/start up pulse generated to start up the power supply at switch on.
I have also re-soldered the left and right scan oscillator transformers just in case of faulty solder joints there.
I have also removed the other two processing/input boards for inspection but all looks ok.

So now i'm asking for help if someone has had this problem with this model or similar models before.
Sorry for the long post.
TNX in advance.

[Reschs]

Whilst not familiar with that brand.

There should be a 5V Standby Voltage present at all times.
This appears to be missing.
Look on the power supply board for a STBY5V connector.
Follow the circuitry and look for resistors gone high in value or faulty caps in series with these resistors.

[Skepticist]

Not familiar with this device but I have had a similar problem with other power supplies a number of times.
The standby supply is produced by a small low power separate inverter on the PCB. Pretty much universally 5V, 1A or less and it supplies the voltage to the 'on' pushbutton logic circuit and a led or 2, may be marked something like '5VSB' on the PCB and if that 5V is not present the main section of the supply can not start up. You can usually identify it by the very small transformer (compared to the main transformer) that straddles the insulation barrier between the primary and secondary sides of the PCB and an optoisolator that feeds the 'startup' signal back to the main inverter primary side when the power button is pressed. I've had failures of the standby inverter and the feedback circuit at different times - in one case it was a 1Mohm resistor that had gone open circuit which was very hard to detect in-circuit (only found it when I resorted to lifting components to test them individually).

I can't stress enough on how hazardous it is to go probing the PCB live (or even not live unless the mains side capacitors have been carefully discharged first).

A schematic would be immensely useful too naturally if you can get one.

ETA: Reschs was faster

[Blackbiro]

Hi team
I haven't fixed the tv as yet. I thought I found the fault when a 3 watt safety resistor on the p/s board measured 30K ohms when it should have been 2.2K 5% (red-red-red-gold).
I replaced it with a 2k 3watt I had on hand but still the power supply is dead as a dodo.
Absolutely no secondary start up voltages anywhere although the 240v mans section is working and has 345v ac on the large cap.
Cant measure anything that could be faulty on the power supply board.
Circuit or service manual is not available on line, have spent hours looking.
So now I am asking anyone anywhere that may have the circuit or link to the service manual for this tv tohelp out.
I an including pics of the power supply board and user manual.
Tnx in advance.

[Blackbiro]

Hi All

Well I have fixed the Schaub Lorenz tv power supply and the tv works again, (see post No 1 and picture in Post No 4)

The problem was one open cct electro cap 22uf/450v dc seen on the bottom edge of pcb in the centre and an open circuit 2K2 5% safety resistor shown just right of that cap'.
Also most of the remaining electro caps (3x1000uf/35v) shown at right end top of the pcb had reduced capacities as they had dried out due to age and also from being in proximity to the hot heatsink shown above the yellow transformer. These caps were being heated by the heatsinks convection and conduction through the copper.

Two other caps (1000uf/25 and 22uf/50v) were also replaced due to low capacitance. Tested on my C-Bridge and VOM meters.
The pcb was inspected for cracked solder joints and re soldered accordingly.

I never located a circuit diagram or service manual for this tv but I suppose one does exist.

I am going to modify the heatsink (the one above the yellow transformer that gets very hot) by shaping up a piece of 1.2mm think aluminium plate and by drilling and screwing it to the top of the existing heatsink so as to increase the cooling surface area and hopefully decrease the heating effect into the caps. Also it should extend the life of the rectifier screwed to that heatsink.
This tv was manufactured in 2005 in Korea so it is reasonably old by now.

So thats it, the main fault was faulty electro caps.
Hope this might help someone else in the future.

[tristen]

It's good to hear that you have successfully repaired the TV, Blackbiro.

Thanks for the update.