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Author Topic: TYT-9800 no power output all bands  (Read 1073 times)
K2OWK
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Posts: 1267




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« on: October 03, 2017, 03:25:21 PM »

Hello I have a TYT-9800 quad band transceiver. I have had this unit for many years (it is the original one). I have had no problems until now; all of a sudden there is no output on any bands. I have re-soldered the output FET. I checked the transmit/receive relays they are working normally. When I key the mike the front panel display indicates as it should. The output meter shows power out at all levels.  The repeater frequency shift shows the proper frequency. The receiver mutes as it should, but my power meter shows zero power out (the UHF/VHF power meter was checked with another transceiver and operates as it should). I checked the power output FET “Mitsubishi RD70HVF1” with a voltmeter with the following readings to ground. Key up one side reads 12 V DC (power supply voltage) the other side reads 0 V DC. Key down one side reads 9 V DC the other side reads 3.8 V DC. I would like to know if this is normal operation for this FET. I would also like to know how to test this transistor? This transistor is available for about $20+, but I would hate to buy this transistor replace it and find I still have the same problem. Any help I can receive would be greatly appreciated.
Note: The receiver works normally, with no problems
73s

K2OWK
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K4JJL
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Posts: 794




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« Reply #1 on: October 03, 2017, 03:52:56 PM »

Normally when they go bad, they short out (drain to source).
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K2OWK
Member

Posts: 1267




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« Reply #2 on: October 03, 2017, 11:42:38 PM »

Thanks for your answer. I am an old guy who used to work with tubes. My knowledge of FETS is very limited. This FET reads open except from one side to ground reads a charging cap one way and about 350 ohms the other way. Not sure which is the drain and the source. Two of the transistor pads are soldered on each side to the circuit board. The body of the FET is soldered to ground.

Any other advice would be appreciated, before I purchase a replacement FET.

73s

K2OWK
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K5LXP
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Posts: 5245


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« Reply #3 on: October 04, 2017, 03:09:07 AM »

Do you have any test equipment?  Knowing the drive into the transistor will tell you a lot.  Even an inexpensive SWR/power meter might tell you enough.  That being said, $20 doesn't sound like a lot to risk for a shotgun repair.  If it fixes it you've given the radio another lease on life it wouldn't have otherwise had, as bench time would put this radio soundly into the BER category.

Mark K5LXP
Albuquerque, NM
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K2OWK
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Posts: 1267




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« Reply #4 on: October 04, 2017, 11:28:07 PM »

Mark, Thanks for the comeback. I appreciate it.  I have at my disposal a complete electronics lab with most test equipment, including a spectrum analyzer, a Tektronix scope, a frequency counter and a ton of other test equipment. I am just not familiar with testing a FET.

73s

Barry  K2OWK
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