Had a member ask me about the meter light flickering on a Palomar 300A, and the tube heaters going dark. Turns out I'm too wordy for the forum's chat feature. I saved the text, so here's what to check if you see this symptom:
The meter light and tube heaters are probably pointing to oxidized blades on the transformer's 8-pin Jones plug. The contacts inside the mating socket are also suspect. The 12-Volt heater circuit has a high current level, and will show the first signs of a poor connection. A nearly 50 year-old amplifier might have more than one fault. Odds are that it does, but I'll guess that someone has tried to 'clean' the blades on that plug with a pocket knife. This removes the shiny plated surface that's supposed to resist oxidation. This exposes the base metal of the plug. Makes it necessary to apply cleaning solvent to the plug every few weeks to keep up with it. A new plug and socket are the usual fix for this. Any arcs around the power-supply board underneath may have to do with loose mount screws. They provide ground connections. I should also point out that the 1000uf axial electrolytic on the board with the two HV filter caps is prone to fail as a short. When it does, the single rectifier diode feeding it will fail as a short, and AC voltage now goes to work on the keying and preamp circuits. Some versions will see circuit-board foil traces burn like fuse elements. Not pretty. All factory-original electrolytic caps should get changed before they can damage other stuff. Much cheaper that way. Just don't bother testing 1970s-era electrolytic caps. They fail from age alone, and tend to cause damage when they do.
73
The meter light and tube heaters are probably pointing to oxidized blades on the transformer's 8-pin Jones plug. The contacts inside the mating socket are also suspect. The 12-Volt heater circuit has a high current level, and will show the first signs of a poor connection. A nearly 50 year-old amplifier might have more than one fault. Odds are that it does, but I'll guess that someone has tried to 'clean' the blades on that plug with a pocket knife. This removes the shiny plated surface that's supposed to resist oxidation. This exposes the base metal of the plug. Makes it necessary to apply cleaning solvent to the plug every few weeks to keep up with it. A new plug and socket are the usual fix for this. Any arcs around the power-supply board underneath may have to do with loose mount screws. They provide ground connections. I should also point out that the 1000uf axial electrolytic on the board with the two HV filter caps is prone to fail as a short. When it does, the single rectifier diode feeding it will fail as a short, and AC voltage now goes to work on the keying and preamp circuits. Some versions will see circuit-board foil traces burn like fuse elements. Not pretty. All factory-original electrolytic caps should get changed before they can damage other stuff. Much cheaper that way. Just don't bother testing 1970s-era electrolytic caps. They fail from age alone, and tend to cause damage when they do.
73
