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Author Topic: Yeasu FT620B internal swr problem?  (Read 1007 times)
KG9DH
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Posts: 12




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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: 16772




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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: 12




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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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W6WUH
Member

Posts: 24




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« Reply #3 on: July 02, 2017, 12:03:35 PM »

check the bias for the final against the listed value.
check all the components which couple the final to the output
and the driver to the final.

if you cant get a spectrum analyzer.. use a wide range reciver
with a sniffer wire close to the driver / final.

is there a voltage chart available..
do you have the service manual.. ?

high swr is code for a shorted/lossy component, trace or wiring.
assume nothing.  check that your cables and even connectors are really good.

I think this radio has an actual t/r relay.. open it up and inspect it.
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KG9DH
Member

Posts: 12




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« Reply #4 on: July 08, 2017, 05:52:45 AM »

Thanks for the additional suggestions. I had to go out of town for a couple weeks so I haven't been able to work on this. I hope to get back on this next week.

 I have searched but have not found any service manual for this radio. I do have the schematic but it only lists supply voltages from the power supply.

 I did remove the cover on the relay and sprayed the contacts with DeOxIt. Did resistance measurements across the contacts and all connectors, which showed zero resistance. I also reflowed some solder that looked questionable. Disconnected the receive trap circuit which is connected to the antenna through a small cap. Made sure the grounds were all good. Still have the same problem.

I will try to see if the final is oscillating this week. May have access to a spectrum analyzer.

Thanks.
Ed
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W4KVW
Member

Posts: 68




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« Reply #5 on: July 09, 2017, 12:17:07 PM »

Pretty sure it's a Yaesu NOT a Yeasu. LOL  Grin

Clayton
W4KVW
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