Output transistors sockets SWR SM-400

[Enzo]

PK then, if it really needs to bias hotter, then explore the bias transistor and surrounding resistors. You may have to alter a resistor value.

[MarkusBass]

unfortunately I do have sm-400, years back I had to rebuilt power supply in this amp
which is bad design from factory, this voltage dropping resistors are overheating
surrounding diodes and caps, and indeed this amp should be fan cooled,

This is exactly what I noticed. 20-30 years ago all manufacturers (Ampeg, Eden, Marshall, Fender) did this in the same way. I already replaced +/-15V power supply resistors with 4x330 Ohms/5W and I will finally install a fan in this amp.

now my sm-400 (which is the PITA to work on it) has dc offset to high on one side and needs service again
slaw

This is a good place to get it fixed. The amp is not that difficult to work on. You just take the power amp module out of the amp and connect it to the amp with longer wires.

Mark

[MarkusBass]

PK then, if it really needs to bias hotter, then explore the bias transistor and surrounding resistors. You may have to alter a resistor value.

Thanks. You were right (as always ). Someone changed 1.8k resistors in the bias circuit to 1.5k. After changing it back to 1.8k everything (with the power amp) is OK. I still have some hum from the preamp. It seems that the amp was modified before the failure, e.g. 820 Ohms resistor in the bias circuit was changed to 470 Ohms. It is possible that the resistors were adjusted to Hfe coefficient of the temperature sensing transistor.

Mark

[MarkusBass]

now my sm-400 (which is the PITA to work on it) has dc offset to high on one side and needs service again

The amp that I have has 7 mV on one side and 30 mV on the other. I think that this is caused by the differential input stage (not matched transistors). What offset do you have?

Mark

[mozwell]

this may help

[Enzo]

That's a good file, but he does have the older circuit, the one using TO3s.

[MarkusBass]

The amp is already fixed. The problem with hum on one input was caused by the jack that did not have a good contact to the ground. I also had to clean all potentiometers. Now I'm waiting for test on stage.

Mark

[slawkowski]

The amp that I have has 7 mV on one side and 30 mV on the other. I think that this is caused by the differential input stage (not matched transistors). What offset do you have?

Mark

good to hear is fixed

dc offset on one side 190mV and 42mV on other
I'm going to order bunch of MPSA56 and match them for hfe