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Author Topic: Problem with TL-922  (Read 2735 times)
DJ9AO
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Posts: 8




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« on: August 04, 2004, 08:44:41 AM »

Hi to all!
We have a problem with our TL-922 amplifier here. It produces less than specified power and causes the exciter (TS-140) to reduce its output power, with or without ALC connected, probably because the 140 sees a bad SWR. (Grid-current does not exceed specifications on any band).
The owner took the amp for a repair, but the guy returned it, couldn't find anything bad in the input circuitry that could cause the drop in exciter power and just said the tubes seemed to be a "little flat". Is there anything else we could check before investing in new tubes?  

Thanks for help,

Oliver  DJ9AO
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W7DJM
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Posts: 1




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« Reply #1 on: August 04, 2004, 12:35:19 PM »

Well, if you actually know for sure that the exciter is rolling back it's power because of and that you KNOW that the input swr is high, that is a clue to your answer right there.

Either something in the input circuit is seriously out of line, there has been a component failure to CAUSE the change, or, I spose, the tubes could be seriously flat.

Just what will it output?

What kind of readings/metering do you see?

Plate color?

One thing (temporarily) you could try is putting a tuner BETWEEN the exciter and the amplifier to "help" the input swr, and see if that affects the problem.

Obviously, if you can get enough drive  (grid current) and the thing STILL won't put out rated power, you have problems.

Make SURE you are testing with

Proper line voltage  (Is it low?)

A good quality dummy load  (Cantennas can change value,  CHECK IT, and they can change up and down with heat)

A good quality, accurate wattmeter
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DJ9AO
Member

Posts: 8




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« Reply #2 on: August 04, 2004, 04:51:06 PM »

Thanks for your suggestion. Putting a tuner between TRX and PA seems like a really good idea.
Just let me rephrase my question: Is it possible that the tubes alone could cause the bad SWR or must there be other faulty parts?

73,  Oliver
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W8JI
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Posts: 9748


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« Reply #3 on: August 14, 2004, 03:49:03 AM »

Tubes have to be REALLY flat before input impedance changes. They have to be much less than half normal peak-emission.

I'd suggest the person who tested the amp did so on an exciter less critical to SWR. The 922 is noted for having somewhat poor input tuning from the factory.

I'd try an external input tuner before replacing tubes, and see if it corrects the problem. If it does, repair the input circuit.

73 Tom
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WA4OBJ
Member

Posts: 57




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« Reply #4 on: September 27, 2004, 09:33:58 PM »

Before you do anything, just a warning.  There are extremely high voltages present in this amplifier that can easy be lethal.  A wrong move can cost you your life if you aren't careful.  If you aren't familiar with and comfortable working around these voltages, send it to Kenwood or some other repair facility to get serviced.

Check the grid chokes L7 and L8 to see if they are open.  I had a similar problem and found one of the chokes opened because of an internal arc in one of the tubes.

You didn't mention if the grid and plate currents are normal.  If the zener diode (D2, 7.5 v)in the cathode has a high resistance, it could cause a similar problem.  Check for the 7.5 volt drop across the diode with a voltmeter. You probably should check RL-2-1 to make sure it is removing the -100 volts from the cathode while you are at it.

The service manual has you adjust the input tuning coils for minimum VSWR when the amp is being driven with 80 watts and the output toerminated in a dummy load.  Have you checked the input VSWR?  If you have a problem with tuning, I would expect it to be bad on one or two bands, not all of them.  You might also check the S6 bandswitch for arcs and burns.  It is possilbe that the switch has a path to multiple taps on the output coil causing it to be detuned.  Under certain conditions, it is possible for the antenna relay to disconnect the antenna before the input power is removed causing sometimes spectacular arcs and sparks not to mention burned switch contacts.
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