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maco 75 problems
- Thread starter 552fl
- Start date
The amplifier is oscillating. Kinda like the audio feedback you get with the mike too close to the speaker of a radio with talkback. The "howl" you get with the mike too close to the speaker is audio feedback.
If the output from a RF amplifier leaks back into the input circuit, you get RF feedback. The RF carrier this generates will cause the keying circuit to think that the radio is still keyed up. Generic term for this behavior is "instability". An amplifier that's stable only amplifies, doesn't oscillate.
If you'll have a look at the coax that leads from the Load control on the front panel to the relay in the rear, you'll notice something odd.
The outer braid shield of this coax is not connected to anything. On either end.
We simply replace it with a length of small teflon coax, with a ground wire to the Load control in front, and to one screw of the Antenna socket at the other end. Using teflon lets me solder the shield braid without melting the center insulation.
You could try stripping some of the jacket from each end of the factory coax, and lap-solder a ground wire to each end.
More trouble to do it that way, but if you don't have any small coax on hand, it ought to work.
The shield of that coax needs to be grounded at each end. The factory setup turns that coax into a short transmitting antenna.
Inside the amplifier.
Causes feedback, from the output signal leaking back into the tube's input circuit.
We have seen other causes of instability in the Maco Brute amplifiers, but this one causes most of the trouble.
If it still acts wrong with the output coax shield grounded at both ends, check back in.
73
If the output from a RF amplifier leaks back into the input circuit, you get RF feedback. The RF carrier this generates will cause the keying circuit to think that the radio is still keyed up. Generic term for this behavior is "instability". An amplifier that's stable only amplifies, doesn't oscillate.
If you'll have a look at the coax that leads from the Load control on the front panel to the relay in the rear, you'll notice something odd.
The outer braid shield of this coax is not connected to anything. On either end.
We simply replace it with a length of small teflon coax, with a ground wire to the Load control in front, and to one screw of the Antenna socket at the other end. Using teflon lets me solder the shield braid without melting the center insulation.
You could try stripping some of the jacket from each end of the factory coax, and lap-solder a ground wire to each end.
More trouble to do it that way, but if you don't have any small coax on hand, it ought to work.
The shield of that coax needs to be grounded at each end. The factory setup turns that coax into a short transmitting antenna.
Inside the amplifier.
Causes feedback, from the output signal leaking back into the tube's input circuit.
We have seen other causes of instability in the Maco Brute amplifiers, but this one causes most of the trouble.
If it still acts wrong with the output coax shield grounded at both ends, check back in.
73
First and foremost check the parasitic suppressor and make sure its good before you look elsewhere.