Should just add its not just 'thin plastic', it is a material that is designed to transmit heat as easily as possible from transistor to heatsink.
The pads looked fine. I'll let them as is for the moment. But I'll replace them if I replace the transistors later.
Now I will compare the new to me 306 to my faithful 303.
Now I will compare the new to me 306 to my faithful 303.
Thank you!
I'm surprised how warm this 306 comes after a few hours even at low level use. Is it due to the current dumping circuit? My 303 heatsink has never got warm or so little.
I'm surprised how warm this 306 comes after a few hours even at low level use. Is it due to the current dumping circuit? My 303 heatsink has never got warm or so little.
It will be normal. The 303 is a traditional type output stage and runs at low bias current which means little heat is generated. I think the rail is 70 volts or so on the 303 and around 10 milliamp flows in the outputs so that is just under 0.75w dissipation.
The 306 has higher rails (about 80 volt total) but crucially it has a Class A output stage (the driver stage) biased to provide 1 watt in to 8 ohms in Class A. That means there is more heat generation, around 2.3 watt per channel in the upper NPN driver transistor and similar in the lower PNP driver stage and its resistors.
So there is a big difference between the two.
The 306 has higher rails (about 80 volt total) but crucially it has a Class A output stage (the driver stage) biased to provide 1 watt in to 8 ohms in Class A. That means there is more heat generation, around 2.3 watt per channel in the upper NPN driver transistor and similar in the lower PNP driver stage and its resistors.
So there is a big difference between the two.
I've had this problem of driver transistor leads breaking on two 306 models now and posted here, as it's come up in the past, where there was bouncing up and down and severe flexing of the PCB when the amplifier was moved about or in shipment - as with cargo flights and delivery vans where the rides tend to be rough and personnel uncaring.
This wouldn't have been a problem when new but after 35 years or so, polyurethane foam has usually turned to mush and the trafo with PCB attached, bounces about with little restraint, so the driver transistor legs obviously could and did get quite a workout. Both amps actually continued to work but with a strange ringing sound effect which made it difficult to track down until I started pushing and prodding the board at the edges where the drivers are located. The rest you know but this design flaw is so basic and easily remedied that you might consider cutting out and fitting some more substantial but still insulating and somewhat heat resistant pads between the case bottom and PCB and the trafo and top of the case, as shown in the service manual and some pics on the 'net.
This wouldn't have been a problem when new but after 35 years or so, polyurethane foam has usually turned to mush and the trafo with PCB attached, bounces about with little restraint, so the driver transistor legs obviously could and did get quite a workout. Both amps actually continued to work but with a strange ringing sound effect which made it difficult to track down until I started pushing and prodding the board at the edges where the drivers are located. The rest you know but this design flaw is so basic and easily remedied that you might consider cutting out and fitting some more substantial but still insulating and somewhat heat resistant pads between the case bottom and PCB and the trafo and top of the case, as shown in the service manual and some pics on the 'net.
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BTW, "Sil Pads" are a relatively new innovation. They are soft, thin silicone rubber sheet material which also has a powdered alumina (or similar) filler material blended into the rubber. Alumina is an insulating mineral that conducts heat quite well, so it's ideal for the purpose. Rubber is used because it conforms to the imperfect metal surfaces for best heat transfer without needing any messy silicone/alumina grease. The final result isn't as good as mica/grease but few applications require the maximum possible thermal transfer anyway.