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Re: Troubleshooting a 250 model 1 Bass amp [message #27401 is a reply to message #27393] Thu, 27 June 2019 15:47 Go to previous messageGo to previous message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
There shouldn't be -10 volts on the output of the preamp board. The cause could be a bad opamp or a power supply problem. Have you tested to see that both plus and minus 12 volt supplies are working correctly?

The 12 volt supplies are created by Zener diodes and large dropping resistors that are located in the middle of the chassis. The resistors are black wirewound types that are bolted to the chassis and the two Zener diodes are connected to a small terminal block that has wires that go to the preamp boards. You should measure about 12 plus and minus at the two terminal points.

If the two supplies are there and working, then the next step would be to test the output chip. If you look at the schematic for the 5066 board, you will see two triangles marked I1A and I1B. This is the output chip. There are two sections to it, thus the A and B designations. You will need to carefully measure the voltage on each of the input and output pins of the chip. The output of the board comes from pin #1 so I'd start there. The input pins for that section of the chip are at pins #5 and #6. Pin #5 is grounded, so there should not be any voltage there, and there should be little or no voltage on pin #6. If you get voltage readings on either pin 1 or 6, the chip is probably bad.

As for the solder question, the traces will conduct with or without being "tinned" with solder. Unless there is a sign of major corrosion to the copper trace there is nothing to worry about.

Good luck.

[Updated on: Thu, 27 June 2019 15:50]

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