Re: {Collins} 75A-4 help


Do you have the capability of injecting a well controled signal on 20 meters? Try injecting a 20 meter signal at the output of the RF amp first stage. The stage gain should be about Av=10. I suspect that the RF input coil on 20 meters is dinged. A common problem. You need to start doing a little more measuring and injecting and signal tracing to find where this is going bad and less "trying" things (shot gunning). In general, good diagnostic tracing and injecting will lead you right to the problem. If you need help with this, give me a call.

By the way, while the calibrator can be used as a part of the diagnostic practices sometimes, losing the cal signal is the same as losing your input signal since the injection point is before the RF input coil. No help there right now.


Bill Carns, N7OTQ
K0CXX, Trustee MRCCC
Editor Signal Magazine
CCA Acting President & Board Member
CCA # 94-00009, Founding Board Member
Wimberley, Texas
512 847 7010
512 618 2762 (Cell)

----- Original Message ----- From: <w0vmp@xxxxxxx>
To: <collins@xxxxxxxxxxxxx>
Sent: Monday, February 01, 2010 9:21 PM
Subject: {Collins} 75A-4 help

I still have a couple of problems with my 75A-4 (SN 1788)...20 meters does not work and all receive signals are being attenuated when the antenna is connected to the 50 ohm coaxial input (signals are normal when the antenna is connected to the antenna terminal strip, shield to ground and center conductor to screw terminal #2).

In trying to fix the 20 meter receive problem I have tried a new crystal (Y-4) and have ruled out any problems with C-12, C-27 and C-42 by substituting a new mica capacitor...still nothing on 20 meters. I have placed a jumper across each set of contacts used on the band switch in the 20 meter position, one at a time, trying to find a bad connection but still nothing.

I have found that the crystal calibrator works normal on all bands except 20 meters...could this be a clue to my problem?

All of the recommended capacitors have been replaced. All of the other bands align normal and the receiver sounds great.

Looking for ideas, 73,

Omaha NE

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