[TenTec] RE: RFI to power supply speaker

Jim Parsons k5rov@worldnet.att.net
Mon, 13 Oct 1997 12:03:02 -0500

OM, I have the RS DSP filter, and I also have the service manual for it,
with schematic. I had the same troubles you have.

The schematic shows that the input audio is isolated by a transformer.
However, the output audio is directly wired into the internal circuitry
with no isolation at all.

I have used a large capacitor to couple my output audio. Without doing
that, I would get very loud squealing on my audio. It was impossible to
use. The capacitor isolates the DC return system, and just passes the
audio. I am thinking of going inside the filter and putting a transformer
on the output to have isolation on both input and output.

73, Jim, K5ROV

At 04:31 PM 10/13/97 +0000, you wrote:
>I'd like to thank all who replied, and let you know my progress.
>There was something I failed to mention: I'm running a Radio Shack DSP
>unit. This is an UNSHIELDED device in a plastic enclosure. I had
>forgotten about that. That was an ommission on my part.
>I've run into problems in the past when trying to ground this device. It
>seems that it operates with a "floating ground". If you connect the
>"ring" (DC ground) of the power input jack to "shack ground", you get an
>instant blown fuse in the DSP unit!
>So, I went to work from there. I disconnected the external speaker
>connection from the back of the radio, and with the radio internal
>speaker, there's no RFI. Hmmmm. OK, so I connected everything else, one
>item at the time, to see if I could spot where the RFI was coming from.
>I tried a set of speakers directly connected to the external speaker
>jack, and used shielded cable. I got RFI here as well. So, it would
>seem, the cure would be to use the internal speaker. But how about the
>DSP unit? I'd lose it's advantages! So, the problem is definitely a
>external to the radio/power supply. There must be simply too much RF
>floating around in my shack. A cheap field strength meter shows some
>deflection. The peak deflection appears to occur near the RF connector
>at the back of the radio. I thought perhaps a bad solder joint in the
>PL-259, but switching coax jumpers disproved this. Perhaps hard-line
>jumpers might help?
>My antenna (for this radio) is about 50 feet away, and is a delta loop
>that starts about 7 feet above the ground, and the top is probably 40
>feet high, sides about 70 feet apart. It's fed with coax. Just throwing
>that in for informational purposes. There are antennas for other bands
>near it, and it's possible that they could be conducting RF back into
>the shack. But I think the effect would be minimal. I'll look into this
>next. Most of the time, my unused antenna coaxes are left disconnected
>and dangling in the shack.
>I think my next step is going to be to re-work my RF ground, which
>consists of 5 6-foot ground rods connected with #6 solid wire in a
>straight line leading away from my shack. I have that connected to the
>shack with stripped coax braid, which is about 8 feet long. I've check
>all bands, and the RFI seems to be equal across the spectrum, so it's
>not frequency dependent.
>So, that's where I'm headed. Will let all know what I find. Thanks, Web
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James (Jim), Parsons, K5ROV USAF, Ret.
k5rov@worldnet.att.net   QCWA, NWQRP, Fists, ARRL
JOHN 3:16

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