| Author |
Message |
    
Pinger
| | Posted on Friday, February 21, 2003 - 2:09 am: | 
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Does anyone know how to "add negative feedback" to an amplifier to keep it from staying keyed? |
Pinger
| | Posted on Saturday, February 22, 2003 - 1:40 pm: |
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or are there any other things that could be corrected to solve this problem? |
2600
| | Posted on Sunday, February 23, 2003 - 12:02 am: |
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Staying keyed???? Sounds like a blown keying transistor. If the wattmeter falls ALL THE WAY to zero when the radio unkeys, look there first.
If this is an amplifier you have built from scratch, and it shows wattmeter power AFTER you release the mike switch, it needs more than negative feedback. In a solid-state amplifier, negative feedback is used to improve the drive characteristics of the transistors, NOT to keep them from "feeding back" and holding the relay keyed in.
If this is a Texas Star amplifier, the blown keying transistor will keep happening every time it gets replaced until a small (4.7 to 22 uF) capacitor is added in parallel with the keying transistor. I'll skip the reasons why, it just works. If it's not a Texas Star, who knows?
73 |
Pinger
| | Posted on Sunday, February 23, 2003 - 11:34 pm: |
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The amp that's having this problem is a Boomer 600. I've checked the small keying transistors, and they test good. |
Ss8541
| | Posted on Monday, February 24, 2003 - 7:32 pm: |
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the boomer 600 is notorious for this. the 1st thing to do is to reduce the input carrier driving the amp to around 1-1.5w. then gnd the case of the amp with the shortest piece of wire possible. coax braid works well here. you may even have to change coax lengths between radio and amp. once you get these amps 'pampered' they will usually work pretty well.
when overdriven these amps are very dirty and will easily go into parasitic oscillations. if this is the case, your smeter in your radio should be pegged out after you let off the key. |
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