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Tons of noise on the floor of an Invader 2000 in SSB

 

This is a question regarding the Johnson transmitters sensitivity to hum, etc on the mic input.

The background:

I've got an excellent Invader 2000 that I've used mostly for CW. It's got a clean (well, for the period) CW note with a couple of close in 60 Hz multple spurs but they are about -40 dBc and there's nothing above beyond 180 Hz.? Otherwise a lovely CW signal.

But with some of the QSO party's ongoing, I hooked up a mic and worked a few guys over the weekend.? And a few of the guys mentioned I had tons of noise on the signal.? In checking it on the audio spectrum analyzer, it looks like it's got 60 Hz multiples all the way across the audio spectrum.

I'm actually not surprised that there would be **SOME** 60 Hz and harmonics present.? In my setup, the D104 has a W9AC JFET buffer driver in the base.? The buffer has an output Z of about 150 ohms.? That output feeds through shielded cable up to a relay box where the mic cable is routed onward (via shielded cable) to one of 4 transmitters.

If I hook a D104 (no buffer) up directly to the Invader, the signal is not too bad with noise down about -40 dB or so.? Not great but acceptable.? And if I turn the mic gain to zero, the noise drops to nothing.? So clearly the rig connected to a stand-alone mic works "OK."

But with the mic fed via the switching box, the noise is only -20 to -30 dB down - really lousy.? And here's the funny part.? The same switching system also drives a Hallicrafters HT-32A and a Drake T4XC.? Both the Hali and the Drake have noise floors (mic keyed, no talking) of about -50 dB to -60 dB down.? Better in fact that the Invader with the mic directly connected.

So this makes me wonder if there's something about the Invader that makes it somewhat more susceptible to these 60 Hz environment noise and in the setup using the switching system, it's somehow generating tons of harmonics.? Clearly given the Hali and Drake love the system, I can't say that there is a fundamental issue with the mic relay setup specifically.

One other bit of info.? If I disconnect the mic cable at the switching box end, the invader shows no noise.? But when I hook it up, and float or ground the mic wire at the switching end, the noise remains.? Literally no change with the mic wire either way.

There seems to be something about this connection to the switching box that drives the Invader nuts specifically.? But does not bother the Hali or the Drake at all.

Appreciate any comments anyone may have.? I'm not a SSB guy so these mic topics are pretty foreign to me and I don't have any practical experience to draw upon here unfortunately.

Thanks!

--
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
 

An old post, but will comment anyway.? I was never able to find an Invader to buy, but did buy some Hallicrafters stuff (HT-44) from the same period and had lots of 60 Hz hum on the transmit audio.? In my case, most of the hum was due to the fact that they chose to bundle wires carrying the line voltage to a front panel switch with everything else including shielded audio lines going to/from the front panel mic gain control.? I ran new wires outside the bundle for the line voltage and installed new shielded cable, also outside the bundle.? Almost all the hum disappeared.??
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While restoring an RME-6900 receiver recently, the first thing I noticed was that there was ZERO background hum.? The RME engineers had followed the advice in the RCA tube manual to a Tee.? Instead of using the chassis ground as the return for the filaments, they ran twisted pair to every tube socket with no ground involved.? In addition, and here's one I've never seen before but is in the RCA tube manual, they applied +80 VDC to the filament string.? According to the manual, this is an effective way to reduce filament hum in receiving tubes.? No reason that wouldn't work I guess with the audio string tubes in a transmitter.??
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73, Floyd - K8AC

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