[ICOM] IC-751A Oscillator

[JimR]

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
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>
>
>   
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.)