harrism@dg-rtp.dg.com (Mike Harris) (01/04/90)
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, NCharrism@dg-rtp.dg.com (01/14/90)
Howdy. I'm working on my ICOM-701 hf transceiver. I've pinned the potential
source of the "funny popping noise" to Q37 on the A board. It is
a pass transistor for the receive mode +9v. It is running *VERY* hot to the
touch - you can't keep your finger on it.
The noise starts up when the transistor gets hot - about 20-30 seconds after
power up. It is a tall plastic transistor. The complementary transistor for
xmit is a power transistor with a 1sq inch heat sink attached.
The "noise" isn't present on the inputs to the transistor.
Soooo, Questions:
1) Has anybody had similar problems with their IC-701 (or any radio?) Is this
a known IC-701 failure mode? BTW, I have found other components in the rig
that are being pushed hard.
2) Is it probable that this transistor is shorted and/or bad?, or,
3) Some other circut is being a pig and overloading this transistor. (All
circuts seem to be functioning properly excepting this "noise") I haven't
had a chance to measure the current this transistor is passing.
4) Anybody have specs on this transistor? It is a 2SD468. How about an xref
for substitution?
Thanks, Mike.
Mike Harris - KM4UL harrism@dg-rtp.dg.com
Data General Corporation {world}!mcnc!rti!dg-rtp!harrism
Research Triangle Park, NC