Examining the Johnson Matchbox ATU

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ke7trp:
I thought you could run a test for us. But if you dont own them, I guess you cant.

C

K5UJ:
<<<RX Matching
On the 30K station, I do run the 75A1 through the Matchbox for receive, does seem to help the signal level, suspect its more of a matter of impedance matching with the front end of the Collins receiver. I've read that you can change the tap point on the inductor for the rx input, have not changed mine, seems to work fine.>>>

I think the stock matchbox had two taps on the link, one for tx and one in another point a few turns away for rx which was of course switched in by the stock TR relay.   Many (including mine) were modded at some point with the relay and rx tap removed.    I guess that rx tap was for older rx like the A1 that had 300 or 600 ohm antenna terminals on the back.  There was a KW MB on eBay a few weeks ago that still had the TR relay so a few survive I guess.

Rob 

ke7trp:
Yes. Exactly.  Mine, Is straight through.  If you use the TR, on RX, It will match to the older 300 ohm recievers. This is why some are missing performance on RX using 50 ohm recievers.

C

KD6VXI:
I have the original 275 matchbox owned at one time by W6PIG.

It's been modified so that the TR switch is still in service, but the receiver tap was moved for 50 ohms.

It has the instructions on how to move it BACK to 300 ohms written in pencil.

I can pop it open and count turns if anyone wants one designed to have 50 ohms on the receiver tap.


--Shane

ke7trp:
That might be good info to have posted.  You can use an MFJ to figure it out. Recently, I have been experimenting with matching the Rxer to the antenna. It makes a big difference for weak signal. But I fully realize why most people dont bother. I normaly talk AM when the band is in. If I guy is showing 40 over to me, It wont make any difference in the copy.

Good to see you posting shane.

C

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