I have built the PA03 and it has worked fine for me a long time producing good sound, but now it has a fault. The resistor R27 became to warm and shortly became broken. None of the other components look broken. Can you provide me with some fault finding tips, please? I don't understand the purpose of this resistor, since there seems to be a ground point at both sides of this resistor. When it comes to fault finding: I don't have a variable transformer to be able to vary the incoming voltage to the transformer. Can I use something else? I saw a tip to connect a 100W bulb in series with the incoming voltage to the transformer.
Thanks & best regards Franz
Please, check first if you have severe problems in your signal vs. power ground. If you have a voltage difference then you have fried resistor. If you fix the resistor and the switch on the amp with disconnected inputs and experience smoke, then the the LM4780 is broken if you have normal supply voltage. How many volts DC do you have? Near the 42 V max level?
Do you have the T or TF (fully insulated) type?
A recommendation from pavel is to have a tranzil in parallel in order to save the resistor in case of ground problems.
"Please tell to Franz following: better and quite safe is to have a parallel connection to this resistor low voltage symmetrical transil ( 5V6 / 0,6 W )...."
Thanks for the reply. I have measured the voltage over the resistor R27 mounting holes, without having a new resistor mounted. I have 0V DC & about 8-12V AC. This has been measured with an ordinary multimeter. The LM4780 is marked PM36AF & L4780TA. The LM4780 & the +-12V circuits are mounted on the same aluminium cooler, but with insulation between the cooler & the circuits. There are also insulation shims mounted on the screws. I have measured the resistance between the metal plate on the +-12V circuits and ground and there is no connection. I have about +-26.1V DC supply voltage, measured between D1 and ground & D3 and ground.
Do you have 12 VAC even with nothing is connected to the inputs?
Yes. Nothing connected to the inputs.
I vote for a broken LM4780 and/or faulty insulation.
Put the voltmeter on the power ground and try go identify in which points you have AC voltage.