Samsung PS-50A466 probably bad mainboard

[PowerAmpFreak]

The standby supply acts overloaded, the output voltages is pulsating at about 2Hz. When I pull CN801, supply to mainboard, the standby supply is stable, so I believe the mainboard is overloading the standby supply. But I'm not sure if the standby supply is faulty and cannot provide appropriate output power or if the mainboard is loading the supply too hard.
The psu schematic is found at:


Anyone had a mainboard overloading the standby supply?

[ben7]

Re: Samsung PS-50A466 probably bad mainboard

Try loading the standby supply with a dummy load (I.E. high-power resistors).

I'm sorry, I can't be bothered with downloading it [yet ], so, can you tell me what the output voltage and power rating is for the standby supply?

-Ben

[tom66]

Re: Samsung PS-50A466 probably bad mainboard

That actually sounds more like a bad startup cap or startup network (diode, resistor, or very rarely, a transformer.)

[PowerAmpFreak]

Re: Samsung PS-50A466 probably bad mainboard

I'm running the standby supply on my bench and obviously it's a failure in the standby supply itself. Monitoring primary flyback pulses on transformer reveals the supply is operating in short burst mode, like in overcurrent hickup mode. Though no short is there to find. Transformer primary has a good 350uH inductance so luckily the transformer seems OK. I've ordered the switching controller, FQS0765R and a few diodes and will check.

[ben7]

Re: Samsung PS-50A466 probably bad mainboard

I'm running the standby supply on my bench and obviously it's a failure in the standby supply itself. Monitoring primary flyback pulses on transformer reveals the supply is operating in short burst mode, like in overcurrent hickup mode. Though no short is there to find. Transformer primary has a good 350uH inductance so luckily the transformer seems OK. I've ordered the switching controller, FQS0765R and a few diodes and will check.

Have you checked the parts in the primary snubber circuit?

Have you checked that the primary input voltage (~165v for US, ~330v for EU, ~400v if it has PFC) isn't fluctuating? If it is, and the supply has PFC, that could be at fault, or it could be a bad NTC.

Also, did you try powering the mainboard with an external standby supply, and then still using the main PSU to try and run the TV?

[PowerAmpFreak]

Re: Samsung PS-50A466 probably bad mainboard

I doubt the mainboard is actually bad. The standby PSU is a combined unit with several output voltages and it runs without the PFC in standby mode, and from the PFC during operation. The primary snubber measures OK and according to my digital scope, the flyback waveform at transformer primary "looks normal" during the short period. This might be a secondary regulation problem forcing the PWM controller into some kind of protection, so I will replace the TL431 error-amp and the optocoupler aswell.