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Author Topic: Swan 700  (Read 439 times)
AC0FA
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Posts: 298




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« on: June 19, 2007, 07:58:57 AM »

The old Swan 700cx runs beautiful. Really no complaints.
There is just one thing that makes me go hmmm what the heck was that. With my other old swan rigs tuned to the same signal as the 700.
No birdies present on the other rigs, but I get some intermittent clicking cracking and popping on the 700CX sounds similar to atmospheric but I have verified internal. What the heck would cause that?
John          
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W5JI
Member

Posts: 146




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« Reply #1 on: June 19, 2007, 09:09:23 AM »

Assuming that you hear these noises with the antenna input shorted or connected to a dummy load, the likely causes are: (1)marginal tube, (2) old, failing resistor, (3)old, failing capacitor. Be sure to check the power supply to make sure it is not the noise source unless it is the same supply you are using with the other Swans.

It is difficult to find the source without an oscilloscope. If you are comfortable working inside tube-based gear, then you can try to isolate the source by progressively shorting the grid of each tube to ground, starting at the input RF amplifier stage and progressing towards the audio output amplifier stage. For example, if you are at the second IF amplifier stage and shorting the grid to ground kills the noise, you have determined that the noise is generated prior to that point. Then you can back up one stage at a time looking for the source.

With an oscilloscope, you can "see" the noise and may be able to isolate it faster. Without a scope, you are into the mode of substituting components or trying to isolate the bad or marginal component by tapping on it or spraying a coolant on it to see if the noise is affected. Don't forget to examine all the solder joints to see if any are poorly soldered or making marginal connection. Such connections will also cause the noises you described.

One other possible cause is if the transmitter output tubes are not fully cut off during receive due to improper bias or dirty relay contacts. If the tubes are not fully cut off, they can generate the noises you are describing while in receive mode.

Be careful and good luck......Jim  W5JI
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AC0FA
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Posts: 298




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« Reply #2 on: June 19, 2007, 10:34:20 AM »

Thanks Jim
I now have a plan and a procedure.
John
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