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Author Topic: Mfj 1026 receive signal on main antenna is lost when switching power on the 1026  (Read 5502 times)
M0NJA
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« on: March 25, 2013, 12:58:07 PM »

Hi I seem to have killed this mfj 1026, d3 was blown changed that, but still loose signal on receive the moment I switch the red button on the left power button on the mfj, I hear the relay click then all signal is lost.
Any Ideas where to look?
73 m0nja
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AJ8MH
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« Reply #1 on: March 26, 2013, 06:51:19 AM »

D3 is used to detect transmitted RF energy causing Q3 collector to go low.  This will cause the relay to switch to “pass through” mode and protect components in the receive path during transmit.

You can manually place a low at Q3 collector by shorting the “CTRL” input to ground.  When this is done, the relay will also go into “pass through” mode.  This will allow the receive signal to pass through the relay, so you should hear signals again.  If you don’t hear receive signals, you still have a problem with the RF sensing and switching circuit or the relay is bad.  I suspect that this test WILL work, and your real problem is in the receive RF path.

The receive signal enters the 1026 at the “MAIN” input through C8 to R9 to Q5 up to the base of Q4 through C6 and out the “TRANS.”  Note that the “AUX” receive signal also travels a portion of this path.

I’m going to assume that using your “AUX” sense antenna doesn’t produce a receive signal, either.  This would point to a problem with Q6, Q5, Q4, D1 or D5 in the receive path.  Protection Lamp2 on the schematic is not in my unit.

If you're lucky, D1 or D5 is shorted to ground.
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M0NJA
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« Reply #2 on: March 27, 2013, 02:11:21 PM »

Hi thanks for the reply yes I get receive again when grounding the ctrl. So I am assuming I have killed Q3?
Many thanks.
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AJ8MH
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« Reply #3 on: March 27, 2013, 07:59:08 PM »

What you have done is simply test the switching of the relay by manually placing a low on the collector of Q3 through the “CTRL” input.  The only safe way to test Q3 is to test it with a multimeter.  You could connect the 1026 as you would for normal use, and key the transmitter, but you run the risk of pushing RF into the 1026 receive amplification circuit and damaging it.

But, I'm afraid that may have already happened.  Especially if you don't have any receive signal when turning up the sense or main antenna controls.

You have to remember that D3 (which you replaced) rectifies the transmit signal allowing Q3 to switch the relay.  When this relay switches, transmitted energy goes to the antenna.  If D3 is bad, transmit energy is fed into the receive amplification circuit damaging it.

I suspect that D1 and/or D5 are bad.  Q4 may also be bad.  All this is because the relay failed to protect these components when D3 failed.

If you don't have troubleshooting experience, working with surface mounted components may prove to be too much.
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M0NJA
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« Reply #4 on: April 03, 2013, 02:02:00 PM »

Hi thanks for your help it was q4 replaced that and now use the proper keying method using ptt instead of relying on rf sensing.
73 M0nja
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AJ8MH
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« Reply #5 on: April 04, 2013, 05:42:41 PM »

Great job!  Good idea to use the control input with PTT.  Glad to help.
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