So I'm working on one of these that had blown fuses, this appears to be, not supprisingly that the 4 main output transistors were installed without insulating sleeves on the securing bolts. So all I have to do now is sort out the left chennel centre voltage. Now for the good bit, the PCB has no markings to show what resistors are what, only the caps and diodes are marked on are on the board, great!! 😱 Anyone else ever had this?
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Yes it didn't take long to work that one out. It's the lack of board detail that's strange and not helpful 🙁 still I'm sure I'll get it sorted
Don't you have the service manual?
That's the first thing you pick up before a soldering iron.
You won’t always have the luxury of a service manual. You need to learn to trace a circuit. Start with where the supply voltages come in. Cap or rectifier polarity will tell you this. Then whether is it’s the collector or emitter of the output transistors that drive the speaker. That tells you what polarity is used in which position and if they are same sex or complementary. There are only 3 possibilities that are typically used that don’t require a driver transformer or have flying rails (EF, CFP, or QC). Each has a unique identifiable connection. I’m pretty sure the 3020 is just an EF2.
yes i have the service manual, what i was trying to get at was you cant see any numbers on the board, so you dont know which one is which instantly
the numbers could be underneath,but thay might as well not be there
great though arnt they, all hand painted.
Attachments
I think you'll find the resistor numbers are all marked, but under the actual resistors. You can also just use the 'parts location diagram' in the SM.
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yeah cheers goldie.The version i had was unreadable, but ive managed to find one that can be read, cheers
These old school amplifiers are realy easy to fix.
Replace 2N3055s with 2N3055s. Installing mica washers and silicon grease, remove the solder bridge on each side and apply low AC voltage from your Variac to monitor the voltage drop across the load resistors.
After setting the quiescent current, replace the solder bridge on both sides.
Check the centre voltage is in the centre and that is it, oh yes, after repairing the other problems with switching and other lovely thing amps of this age have.
Replace 2N3055s with 2N3055s. Installing mica washers and silicon grease, remove the solder bridge on each side and apply low AC voltage from your Variac to monitor the voltage drop across the load resistors.
After setting the quiescent current, replace the solder bridge on both sides.
Check the centre voltage is in the centre and that is it, oh yes, after repairing the other problems with switching and other lovely thing amps of this age have.