My bass player still owns the same Gallien-Krueger 400RB bass amp he used back when we both lived in Los Angeles in 1989. It had been a while since he had used it last so it didn't work when he powered it on. The power came on, but with no sound at the output.
Everything looked good inside. It was super clean. But, the pots were very hard to turn. I suspected that the old pot grease had hardened and was getting between the wiper contact and the carbon element so I decided to clean all of the pots with Deoxit Fader Lube.
This stuff is amazing. It works so much better than the old can of Shack tuner cleaner I used in the past. A simple spray inside the element and shaft and then rotate back and forth a few times. Repeat the process until they feel new again.
When repairing any electronic equipment make sure to pay attention to what parts will have voltage potentials on them. Notice the two TO3 transistors at the upper right of the next image.
They are part of the power amplifier circuit. One transistor has about +65VDC on it. The other about -65VDC. Let me tell you that +65V and -65V do not add up to 0V. Across those transistors is about 130VDC of silent killing potential. We must be very careful and respectful when working on electronic equipment.
Thanks for reading,