| Author |
Message |
    
Silvereagle
Junior Member
Username: Silvereagle
Post Number: 11
Registered: 4-2003
| | Posted on Friday, January 05, 2007 - 4:43 pm: | 
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I have a 10 tube D&A amp.I put 3 watts into it and as I just start to talk the amp unkeys.I have checked all the tubes in it and when working properly I can put out 700 watts.I am at a loss as to where to start looking for the unkeying problem |
Bc910
Advanced Member
Username: Bc910
Post Number: 776
Registered: 7-2004
| | Posted on Friday, January 05, 2007 - 5:49 pm: |
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May have ABSOLUTELY nothing to do with your probllem, but with those old amps it is a MUST to replace the electrolytic caps!!!
There should be a bunch of old cardboard ones on the bottom side.
BC |
2600
Advanced Member
Username: 2600
Post Number: 580
Registered: 7-2002
| | Posted on Saturday, January 06, 2007 - 3:11 am: |
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The keying circuit in the oldest ones used a 6AQ5 tube. It would sense the radio's RF and close a relay with a 110-VOlt DC coil.
It works this tiny tube pretty hard. If it gets weak, you could see this.
And like the man said, if it's full of 30 year-old electrolytic filter capacitors they can fail and make the power supply appear "weak". The factory parts really, really SHOULD be bad by now, even if it has only 1000 original miles on it. Even if they got replaced when it was ten years old, those parts may have reached the end of the line.
If they already look bad, waiting for one of them to explode could be costlier than taking the initiative. We recommend adding a 270k (more or less) 2 watt resistor across each new filter. They serve to bleed the stored charge when the power is removed, AND to keep the voltage divided evenly across the one set of two and two sets of three. The factory got away with leaving them out, but I don't recommend it. It's the reason this model has a reputation for being a shock hazard long after it's turned off. They skimped on bleeder resistors.
One last thought: Some types of "swing" modification, especially to Galaxy/RCI type radios will vastly boost your negative modulation peaks. Gives the radio that gritty "large" sound, but gives that old keying circuit a bad headache. Radio's just weren't built that way 30-odd years ago when it was built.
If you have a tube-type CB on hand, and it holds the Phantom keyed steadily, that might point to a radio that's too "modern" causing the hiccup.
73 |
Silvereagle
Junior Member
Username: Silvereagle
Post Number: 13
Registered: 4-2003
| | Posted on Saturday, January 06, 2007 - 6:31 am: |
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Many thanks,will replace old filters.They are the original ones but had no a/c leakage on any of them. |
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