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From: harrism@dg-rtp.dg.com (Mike Harris)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Radio Service Question
Keywords: Funny Popping Noise
Message-ID: <1801@xyzzy.UUCP>
Date: 3 Jan 90 23:04:31 GMT
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Reply-To: harrism@dg-rtp.dg.com
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Organization: Data General Corp., Research Triangle Park, NC
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My ICOM-701 HF transceiver has been giving me a funny popping noise. 
It comes at random yet frequent intervals. This popping noise is barely 
audible in the audio unless the audio is cranked up high. It is accompanied 
by an aparrent de-sense of the radio to incoming signals.

I thought at first that the band (usually 10m) was fading frequently, 
however, I soon noticed it happening on all bands, & then I noticed the 
popping noise.  It is easy to duplicate now as I turn the rf-gain all the 
way down & turn the audio up. There it is. It doesn't appear to be the 
audio stage as the cw sidetone is not affected by the popping.
The S-meter is appears to be uneffected by the popping, however, I'm not
certain of this yet. It appears that the radio remains on frequency, although
it appears to sweep 50-200 hz or so at times. This might rule out the PLL
(or it's voltage supply)

Some history. I recently purchased the radio at a hamfest known DOA. My 
suspicions were the PLL and, with that, I sent it off to ICOM. The problem
turned out to be a DC-DC converter (DC-2?) for the -9V reference supply (for 
the synthesizer op amps, etc). Now, when it went, it also took out a small
resistor. (4.7 ohm, 1/4 watt). We don't acutally know which went first.
I don't know whether it was doing this when it returned as I was happy
with my "new" "bargain" radio & it worked fine esp with strong signals. BTW, 
the signals "appear" to drop out by 3 db. Sometimes more.

My suspicion is that the problem is a capacitor which is charging relatively
slowly (time constant 1 second) until it reaches a breakdown threshhold and
discharges. The discharge (short) interferes with the -9 supply & causes the
signal problems. The cap probably was toasted by over voltage when the 
converter went out. Or, perhaps, a defective cap caused the coverter to go.

Anyway, any ideas? Also, tips on isolating the component would be appreciated.
I can get access to a 30 mhz scope, but probably not much else (computer
service dept at work). I could also use my backup HW-101 as an rf signal
source.

BTW, the noise isn't really a sharp pop - more like a pop followed by a 
brief static rush. It's kind of like listening to frequent lightning strikes 
on the radio. BTW, it isn't that as it does it w/o the antenna & w/o the storm.

regards,
Mike Harris - KM4UL                      harrism@dg-rtp.dg.com
Data General Corporation                 {world}!mcnc!rti!dg-rtp!harrism
Research Triangle Park, NC