Get nothing when playing records.
What I've done:
1) recapped 7F7 ckt (all tubes tested good by TV shop)
2) replaced all brittle wiring in harness
3) rebuilt mech side of CM, spins good
4) tested Red/Green needle coil for poss. open, had indication of very low resistance. Gave thin coat of vaseline to replace lost factory goo.
Ideas? Would like to check the osc. ckt, but don't know how without O-scope. Whic pins what voltages?
Thanks much, we're making progress! AlexB
Check to see if there is a trimmer condenser on the Cobra oscillator. Some models have one and some don't. This adjusts the oscillator and affects the tone quality. If it is turned too far one way or another, the oscillator stops working. Make sure that the coil on the cartridge is not open (resistance of a few ohms across cartridge terminals). If you replaced all parts in the oscillator circuit with exact values, as said before, suspect the tube. If you replace the oscillator tube, try to get one that isn't microphonic, or you'll hear ringing all the time, and it may start howling. Check the RF choke in the oscillator circuit. Look for any bad connections in the tone arm socket. If you replaced the wiring in the tone arm, you may not be able to get the oscillator to oscillate. You must use the original style wire in the tone arm, which puts a lot of insulation between the central wire and the shield. If the wires are closer, the oscillator will do all sorts of things, or perhaps nothing at all.
When you get the Cobra working, it's always interesting when you're listening to your favorite records and all of a sudden Morse code starts pounding through. Doesn't happen often, but it can. Sometimes you'll hear people talking.
Thomas
:Pin 5 of the 6SB7 should be about minus 4volts if the osc. is working. If not. Try another tube, check associated resistors for out of spec.
:Jim
:Careful with the Vaseline. This can mush up your sound. Just a very small amount will do. Vaseline does not affect the acoustics of the cartridge in the same way as the original factory goo.
:
:Check to see if there is a trimmer condenser on the Cobra oscillator. Some models have one and some don't. This adjusts the oscillator and affects the tone quality. If it is turned too far one way or another, the oscillator stops working. Make sure that the coil on the cartridge is not open (resistance of a few ohms across cartridge terminals). If you replaced all parts in the oscillator circuit with exact values, as said before, suspect the tube. If you replace the oscillator tube, try to get one that isn't microphonic, or you'll hear ringing all the time, and it may start howling. Check the RF choke in the oscillator circuit. Look for any bad connections in the tone arm socket. If you replaced the wiring in the tone arm, you may not be able to get the oscillator to oscillate. You must use the original style wire in the tone arm, which puts a lot of insulation between the central wire and the shield. If the wires are closer, the oscillator will do all sorts of things, or perhaps nothing at all.
:
:When you get the Cobra working, it's always interesting when you're listening to your favorite records and all of a sudden Morse code starts pounding through. Doesn't happen often, but it can. Sometimes you'll hear people talking.
:
:Thomas
:
:
::Pin 5 of the 6SB7 should be about minus 4volts if the osc. is working. If not. Try another tube, check associated resistors for out of spec.
::Jim