Zetagi MB+5 issues

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Zetagi MB+5 issues

Post by johnkyrle »

Hello All,
Having just purchased a Zetagi MB+5,I am having a hell of a time trying to set it up with my 148 GTL-dx.
The trouble I am getting is that with the Mic gain on the Zetagi set at 5 (for example) and the Cobra at 3/4 gain,I get a 'modulation' signal even though there is nothing there (on ssb,obviously).I have tried setting both to roughly the same level and it works fine on the lower part of the band but then,further up,I have to readjust the gain,either on the Cobra or the Zetagi to stop it.If you can imagine a Power meter in line,then,obviously,it 'swings' when I talk on,say,26.285 but when further up the band on 27.600as soon as the Mic is keyed,the meter shows an output when there is no noise,talking etc and I have to adjust the gain on either the Zetagi or Cobra to stop it.This is more apparent when running higher power,say for example,100 watts.
This is the second mic I have had from the supplier as the first one was faulty.I can appreciate that the 148 may well have been 'tweaked' in the past,it is nearly 40 years old after all,but,there must be a way to set this up properly?
Equipment layout is the Zetagi into the 148 to a KL300 to a Zetago HP201 and onto an Imax 2000.
Any help greatly appreciated
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Re: Zetagi MB+5 issues

Post by ch25 »

Seem like common mode current issues, RF in the shack. So typical for Imax :lolno: :lolno:
You can't have too many antennas...
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Re: Zetagi MB+5 issues

Post by wa10 »

it does sound like common mode on the coax, you have a few options to try,

best option is to install the imax in a way that should minimise common mode on mast & coax, minimise reception of local noise & improve low angle gain a little, properly isolate the imax from the mast add radials & a choke,

try a choke without the other mods, it may work but its very much hit & miss,

change the electrical length & common mode impedance of your coax by shortening it or adding a long patch lead & trimming in increments to see if you find a length that has low enough cmc to be useable,

many desk mics have poor cables, even kenwood & adonis mics have **** cables making them much more sensitive to rfi than they can be,
replacing the cable with a fully foil shielded cable wired to shield all wires & make the mic body an extension of the rigs case greatly decreases their sensitivity to cmc/ humming.
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!
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