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Author Topic: Yeasu FT620B internal swr problem?  (Read 399 times)
KG9DH
Member

Posts: 11




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« on: June 18, 2017, 07:49:29 AM »

Hello all,
I have a Yeasu FT620B that has 0 output unless I disable the AFP (automatic final protection) circuit. Then I get about half the wattage I should and the final transistor, 2sc1307, gets very hot in a few seconds. The driver, 2sc1306 remains at room temperature. I've confirmed the dummy load swr is 1:1 using an antenna analyzer. I've checked for open and shorts in the finals circuit to no avail. I removed the relay cover and cleaned with Deoxit. An ohm meter indicates 0 resistance across the relay contacts and from the antenna connector to the L502 inductor in the finals circuit when the relay is in the transmit position. Voltages at the final transistor, q502, when in transmit is .69 volts at the base and 12.96 volts on the collector when in transmit. The same for the driver transistor, q501. I've checked the 2sc1307 and 2sc1306 with a Micronta transistor checker and also with a curve tracer and oscilloscope and they appear to be good. I pulled a lead from the capacitors in the final circuit and checked with a capacitor checker. They seem good as well.  I've peaked the tuned circuits for the transmit according to the user manual. It seems to me that the swr within the radio is high causing the AFP to fire. I'm stumped. Any suggestions would be appreciated. My skills are limited but I want to learn and six meters sounds  like a lot of fun. Receive in this radio works as it should.

Thanks,
Ed
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WB6BYU
Member

Posts: 16571




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« Reply #1 on: June 19, 2017, 10:29:39 AM »

One possible explanation is that the final stage is oscillating.  If it oscillates on a different frequency
than the one that the output filtering is designed for, it will see a high SWR for the oscillation
frequency
and the fold-back will operate.

You really need a spectrum analyzer (or at least the loan of one) to detect that condition, though
checking the collector current with the driver stage disabled might give you a clue.
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KG9DH
Member

Posts: 11




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« Reply #2 on: June 19, 2017, 05:09:32 PM »

Thank you for the suggestion. I had not thought of that. When I get a chance, I'll check the collector current and see where that is at.
Thanks again.
Ed
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