[ICOM] IC-751A Oscillator

[EP Swynar]

Hi Jim,

Thanks for your thoughts...

Unfortunately, in my case, the receiver works A-OK on all modes on
receive --- but it will not transmit. The 9-MHz crystal at Q3 is FB when
removed & placed into a test circuit...all the voltages into the stage
appear to be normal...

But the darn thing just won't oscillate.

Oh, but how I do miss the good old tube days, when it was SO easy to just
remove & replace a tube, to see if that was the culprit...! And most of the
time, it was, too...

~73~ Eddy VE3CUI - VE3XZ


****************************************


----- Original Message ----- 
From: "JimR" <kd1yv at arrl.net>
To: "ICOM Reflector" <icom at mailman.qth.net>
Sent: Wednesday, January 23, 2008 5:42 PM
Subject: Re: [ICOM] IC-751A Oscillator


> EP Swynar wrote:
> > Good Day All,
> >
> > Has anyone reading this ever encountered an instance(s) with their 751A
> > transceiver wherein Q3 --- the 9.0175-MHz crystal oscillator stage --- 
on
> > the main circuit board refuses to oscillate...?
> >
> > The crystal checks out A-OK here, as do all of the voltages going into
this
> > stage...
> >
> > The problem manifests itself by virtue of no transmitting capability
with
> > the rig...the receiver works great, though, as always...
> >
> > Any & all thoughts & feedback appreciated!
> >
> > ~73~ Eddy VE3CUI - VE3XZ
> >
> > ----
> > Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > Icom FAQ: http://www.qsl.net/icom/
> >
> >
> >
> Eddy,
>
> Back in 1999, the BFO went in my IC751a.  With help from this list,
> including Gary Fiber, I determined that it was the crystal.  Can't
> recall if it is the same one that you mention, and I don't have my
> schematic handy.  Replacing the crystal fixed the problem, so they
> definitely will die in rare cases.
>
> Following is a description of the problems that I experienced at that
time.
>
> 73 de Jim, KD1YV
>
> It stopped receiving any signals on any of the LSB modes (LSB, CW,
> RTTY).  It works fine on USB modes (USB, AM).  This is true on all
> bands, in both ham and GENE modes.  None of the panel settings seem to
> have any effect on the audio, other than shifting to USB or AM.
>
> When I say "stopped" receiving, it hasn't quite totally stopped, but
> the audio level is so far down that it might as well have stopped.  I
> don't have any facility to measure it, but by the best guess of my
> ears, I'd say -25 to -30 dB.
>
> TX seems to be fine on CW and RTTY, but I just checked and found that
> it is not working on phone LSB.  RTTY also moves the needles (external
> meters, too.)  TX works OK when set to USB.
>
> After this, I was able to do some very minor diagnosis, as follows:
>
> I had a bad  feeling about the BFO, so I opened up its box
> (fortunately, not one of the soldered ones.)
>
> I tapped around with a plastic probe, and amazingly, the LSB and CW RX
> came back.  I flipped back and forth between modes for a minute to be
> certain that I was correct.  Then I tapped around the same spot, and
> LSB and CW went away again.  I was not able to restore it again, even
> after many tapping attempts.  If I remember correctly, the magic spot
> was one of the IF-style can transformers, I believe it was the one
> closest to the front of the rig.
>
> I also found that by connecting my MFJ-259 as a frequency counter to
> the BFO test point at CP1, when I am in USB, I can measure 9.013 MHz
> (approximately -- this frequency counter is not the most accurate).
> When I swtich to modes where the RX is not working, the BFO output
> goes away, or drops so low that the MFJ can not measure it any longer.
> (It later occurred to me that the input to this frequency counter may
> well be 50 ohms, which may significantly load down the oscillator.)
>
>
> ----
> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/