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RE: [TowerTalk] MFJ-1025/1026

To: <>, <>,<>
Subject: RE: [TowerTalk] MFJ-1025/1026
From: "Tod - MN" <>
Date: Sat, 22 Jan 2005 22:05:31 -0600
List-post: <>
Quite a while back, when this reflector was discussing subjects somewhat
close to antennas, towers and receiving HF signals, Bob wrote the following,

> 4.To protect the Aux Antenna input, it employs a internal, 
> soldered in, 12v dial lamp as a "fuse". If you are the 
> careless kind you may want to modify this  to avoid having to 
> remove the unit from the case, unsolder the lamp and resolder 
>  in a new one if that terminal gets large input voltage from 
> xmtr. Time consuming  if you are contesting or just DXing and 
> the unit is the only reason you are  hearng the DX.

I happen to have an MFJ-1026 and happen to have replaced the 12v dial light
bulb fuse several times. Finally, I decided I should ground the AUX input
whenever I was transmitting. I won't bore folks with the details, but I
found that one can control an FET which handles the relay that does the
grounding from the circuit point where the FET on the 1026 board controls
the TX/RX relay for the MAIN inputs. Using an FET is necessary so that you
don't load the existing circuit too much. Put the FET and the relay on a
small prototype board and mount it near the AUX input. Wire length for the
relay and FET is unimportant because you are dealing with control lines not
RF lines. The leads from the relay to the AUX input and ground as best made

One of the small, fast 12 volt SPST NC relays from DigiKey or others
(costing about $3 + shipping) is quite adequate. To be really safe one wants
to have the relay closed (grounded) when transmitting and also closed when
there is no power to the MFJ-1026. Use the relay to ground the AUX input at
the connector (leave the bulb fuse in place)and you will protect the LC
filter at the AUX input.

Folks who have this unit will have the schematic and can figure out what I
wrote. Everyone else should simply hit the delete key.



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