Dual-band mobile w/ poor 2m RX - ModsAndRepairs Forum at eHam.net

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eHam.net Forum : ModsAndRepairs : Dual-band mobile w/ poor 2m RX

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Dual-band mobile w/ poor 2m RX	Reply
by K6IHC on October 27, 2009	Mail this to a friend!
I just got this used Kenwood TM-G707A 2m/440 dual-band mobile. It has very poor receive sensitivity on 2m, but transmit on both bands and 440 receive are fine. I have a service manual copy. Just seeking info on a place to start...

RE: Dual-band mobile w/ poor 2m RX	Reply
by KE3WD on October 27, 2009	Mail this to a friend!
The 2 mtr receiver's RF input transistor. First transistor after the antenna. If it is a FET of any kind, it is especially suspect, try replacing it. FETs can get "holes" in their substrate over time, they will still work, but at decreased gain. Static, lightning, things like that can take their toll on that first RF stage's transistor junction.

RE: Dual-band mobile w/ poor 2m RX	Reply
by WB2WIK on October 27, 2009	Mail this to a friend!
Was it like that from the time you got it, or is this something that happened recently?

RE: Dual-band mobile w/ poor 2m RX	Reply
by K6IHC on October 28, 2009	Mail this to a friend!
I just got this radio through the QTH.COM classified. The unit is in great condition, physically. It was a backup station radio for a person in rural Alaska. He knew of no problem with it, but they only have one 2m rptr in the area. As this particular radio is difficult to find these days, I'd like to fix it; maybe work out a partial refund from the seller. I'm looking at replacing the VHF input FET and the FET protection diode and see how it goes.

RE: Dual-band mobile w/ poor 2m RX	Reply
by WB2WIK on October 28, 2009	Mail this to a friend!
Before changing anything, I'd quantify exactly what I have first. The G707A has only one antenna connector. Have you confirmed the 2m antenna you're using is okay? Does the same antenna receive signals much more strongly on a different receiver, using the same coax and everything else? Is the AIP function turned "off?" (AIP engaged greatly reduces receiver sensitivity, purposely.) Do you have a way to measure RX sensitivity so if you make a change you can evaluate what the change did?

RE: Dual-band mobile w/ poor 2m RX	Reply
by WB6BYU on October 28, 2009	Mail this to a friend!
Once you've completed the checks that Steve recommends, check the schematic to see if there is a switching diode or similar ahead of the first transistor. I've had a couple of 2m rigs where a shunt diode had shorted (probably put there to protect the input transistor from strong signals.) Replacing the diode restored sensitivity. But there are a number of things that can go wrong in the front end of a receiver, and it isn't a good plan to just start replacing parts until you have done some trouble- shooting to try to find the cause of the problem.

RE: Dual-band mobile w/ poor 2m RX	Reply
by K6IHC on October 28, 2009	Mail this to a friend!
The radio will barely receive a strong local repeater signal on 2m, with a known good antenna. But an H/T across the room on 2 watts will full-scale the S-meter and come across loud/clear. Reducing the H/T power to 50 mW results in a half-scale on the meter and not as loud audio. And I know about the AIP, hard reset, etc. BYU - In the radio, there is a Schottky diode (SBD) just before the MOSFET VHF RX preamp (3SK239A). It is there to protect the FET. I figure that the diode went bad protecting the FET. I've read a few things about testing an SBD, and intend to do that. The seller has offered $75 toward repair at the Kenwood Service Center - West. I'm going to call the center and find out their rates. If it is close to $75-$85, I will probably take him up on it. This is a very clean unit, and the TM-G707A is discontinued and fairly difficult to find on the used market. I've been running a G707A mobile nearly daily for over seven years, with no issues. I just wanted another one for my other vehicle. I'm pretty sure this is a diode and/or FET issue. The parts are cheap, but a repair at the service center should also get a post-repair testing and alignment.

RE: Dual-band mobile w/ poor 2m RX	Reply
by K6IHC on November 1, 2009	Mail this to a friend!
Well... I contacted the Kenwood-West service center. They charge $35 for a diagnosis. Repair rate is $75/hr., plus parts. Shipping cost there and back is on me. Sounds like at least $150. PacParts has the MA742 diode and 3SK239A FET for less than $20, including tax and shipping. I asked the seller for a $20 refund, to cover the cost of the parts, and he accepted. Looks like I'll be firing up the iron for this task, soon...

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