|[AMRadio] Icom R-75 Receiver Noise Blanker Issue on AM|
jcandela at prodigy.net
Fri Aug 6 07:59:57 EDT 2004
Here is what I did to my R-75 to get the noise blanker (NB) working on the
AM mode. See email trail below with the Icom R75 users group.
Remember that if someone is selling an Icom that is "Icom America certified
OK", you better ask them what's wrong with it...They sent my Icom back to me
saying it was fine, and that the Noise Blanker worked fine.....WRONG! It
does work now though...after I fixed it.
--- In icomr75 at yahoogroups.com, "Jim candela" <jcandela at p...> wrote:
> There is a description on adjusting the NB is on our Web page
> alignment instructions. I don't really have the equipment to do
that nor the
> desire. It only tests the NB on SSB, doesn't mention overload
> characteristics, etc. I am somewhat a new-bee to this Icom, and to
> if the NB used a 6L7 instead of a speck of dust (tiny SM
components) for the
> noise gate I'd be more comfortable.
> I just hooked the scope to the NB test point (CP-731), and
tuned to a
> frequency void of signals except nice impulse noise at a fairly low
> The thought was to not have enough signal to kick in the NB AGC
> With the NB off, I adjusted L731, and 732 for max amplitude on the
> which was about 3 volts peak. Switch the NB on, and the pulses
square off at
> the threshold plus a diode drop (Q735 b-e junction). This was about
> volts (SSB), and 1.8 volt (AM) on my radio. Adjusting the source
> FET Q731 definitely has an affect on the gain, and NB signal
threshold. I am
> still not exactly sure how to set it up, but too much gain (wiper
> leads to large signal NB overload, and too little leads to poor NB
> performance for weaker signals in the midst of proportionately
> I'm just learning here, and I don't plan to go much deeper.
For right now
> my radio is working, and that is the main thing. I'm pretty PO'd
> about Icom service, and the local to me repair center. Sure seemed
> radio was a "hot-potato" and it bounced about place to place with
> no bench verification of the problem. Makes me wonder what level of
> put into the rest of the radio's adjustments...
From: Frank Cathell [mailto:fcathell at msn.com]
Sent: Tuesday, July 20, 2004 3:34 PM
To: icomr75 at yahoogroups.com
Subject: [icomr75] Re: Icom R-75 Noise Blanker Issue
--- In icomr75 at yahoogroups.com, Jim Candela <jcandela at p...> wrote:
> Some progress. Lowering the NB threshold or AM made the NB
work, but it also overloaded very easy. I decided that the signal to
Q735 base may have been low since it was peaking at about 1.5 volts
squared off. For kicks I tuned into 7 Mhz where the impulse noise was
present, but low in amplitude, and adjusted the two NB coils (forgot
the L numbers). To my surprise The amplitude came up, and between the
coils, and the NB gain pot, I could get over 3 volts Peak to the base
of Q735. Now the NB works fine on AM or signals from about S5 to
S9+30. Below S5 the threshold needs lowering, and above 30 over, the
threshold needs to rise as the modulation peaks trigger the blanker.
I guess that is how Icom designed it...a compromise.
> It looks like my Icom is pretty much fixed now....
From: Jim candela [mailto:jcandela at prodigy.net]
Sent: Sunday, July 18, 2004 9:08 PM
To: icomr75 at yahoogroups.com
Subject: RE: [icomr75] Re: Icom R-75 Noise Blanker Issue
My theory about the noise blanker not being enabled digitally was not
correct. I dug into the schematic, got out my big lights, and opti-visor,
and started probing around (kind of risky ya-know). The diodes between L711
and L712 get back biased during the blanking pulse coming from pin 3 of the
6 pin dual transistor array Q737. Pin 3 is normally about 1 volts, and when
the blanker works, it pulses up to 8 volts real fast, and often. This is
true for SSB, and CW/RTTY only. Go to AM, and even in the presence of strong
signals, and mega-impulse noise the blanker never triggers.
While wondering why the NB was dead for AM, I started probing for the
control signals. Signal NBLS to pin 2 of Q737 is normally low unless the NB
button is pressed WHEN the MODE is SSB or CW/RTTY. With the NB enabled this
signal goes to + 4.5 volts DC. OK fair enough. What about AM? The signal NBS
goes high when NB is pressed in AM Mode. That blew my theory since the
digital stuff is working. Now here is the interesting part. These control
signals drive two open collector transistors (Q736 for NBS, and Q737 pins
1,2,3 for NBLS). These transistors have pull=up resistors (NBS=R747-> 1K,
NBLS=R748 -> 470) that tie together at the emitter of Q735. This emitter is
pulled high through a separate 6.8K resistor to 8 volts. For the NB to work,
the emitter of Q735 must be low. Since the two pull-up resistors are
different values, we get two emitter voltages for Q735. The math works out
to be 0.5 volts for SSB and CW/RTTY, and 1.0 volts for AM. Measurements in
my R75 give me 0.5 volts on SSB, and 1.2 volts on AM. After observing this,
I pondered the test, and what that might mean. If the emitter of Q735 never
gets low enough, the signals on the base (as seen at test point CP731) might
never forward bias the transistor. So as a test, I listened to a noisy AM
station at about 3.2 Mhz in the midst of heavy impulse line noise. I then
took a grounded probe, and touched the emitter of Q735. THE NB WORKED, AND
THE NOISE WAS GONE!
So what next? It looks like the divider consisting of R746 and R747 is a
little off (drops 8 volts to 1.2 volts and it should be 1.0 volts). Maybe
one of the resistors are out of tolerance. I suppose Icom had two different
thresholds for the different modes.
Is there any chance somebody will measure there receiver as a comparison?
If so, locate the NB section on your radio, and look for a large
electrolytic capacitor (C741 -> 47 uf 16 volts). The plus terminal meanders
about an inch to the base of Q735 and 3 resistors. This is a pretty easy
place to probe, and I referenced to the receiver case.
It seems pretty Ironic that I have to fix this @#$% radio (isn't fixed yet)
after Icom America said in wasn't broke...
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