[Icom] Sick 761

Sat, 19 Apr 2003 08:50:29 -0400

William, well at least you have found the problem but not the cause.  The
low drive level is the reason for  the low power.  Sounds like to me that
when you use the probe you are biasing the diode into conduction and
allowing the drive to get to the final.  The probe or screwdriver is
coupling noise into the circuit and is probably just enough extra bias to
turn the diode on.  Just a few tenths of a volt is enough to change the
diode from open to full conduction.  To verify this you can open the bias
tree with a current limited power source and vary the bias to the diode. 
If you can turn it on and oFf at will, the problem will be in the switching
circuit.  Sounds like something there is not turning on hard enough.  Good
luck!  Dave

> [Original Message]
> From: Tony Hwang <[email protected]>
> To: <[email protected]>
> Date: 4/19/2003 2:47:02 AM
> Subject: Re: [Icom] Sick 761
>
> Hi,
> Is the symptom same on any and all bands?
> Happy Easter,
> Tony, VE6CGX
>
> wjdiamond wrote:
>
> >Hi Folks,
> >
> >Well, I have my new 756 PRO II for the shop now but I keep wondering
about
> >the 761.  What am I missing on it?
> >
> >I spent an hour on the phone with Icom Tech support today and based on
the
> >voltage readings, they said that is should be working but it is not.
> >
> >For those of you that are familiar with the 761, I am finding something
> >really strange. If I touch a metal screwdriver or test lead to either
side
> >of Diode D 24,on the RF board, the radio will put out 135 watts to the
bird
> >and dummy.
> >
> >All voltages at the diodes are within .2 volts of what the manual and
Icom
> >says. When looking at the output of J8 with a scope loaded with 50 ohms,
> >the output is about .2 volts RMS. Icom says that it should be 2 V RMS. 
If
> >I use the test lead it goes to 2.2 V RMS.
> >
> >I have replaced all the diodes and caps  and transistors in the Icom tech
> >info sheets and it still puts out about 4 watts.
> >
> >I am sure it is in the RF board and so is Icom. Short of replacing the RF
> >board, neither Icom or I know what else to do.
> >
> >I am sitting here looking at it now and it is just too nice to use as a
> >parts rig.
> >
> >Does anyone have any suggestions?
> >
> >Thanks
> >
> >
> >        William J. Diamond (Bill)
> >        Rogers, Arkansas USA
> >        [email protected]
> >
> >        Ham Radio Operator WR�T
> >      Flying The Flag Long Before
> >            It Was Popular
> >
> > Please Visit My Icom Radio Page At
> >
> > http://pages.sbcglobal.net/wjdiamond/index.html
> >
> >
> >----
> >Your Moderator: Dick Flanagan W6OLD, [email protected]
> >Icom Users Net: Sundays, 1700Z, 14.315 MHz
> >Icom FAQ: http://www.qsl.net/icom/
> >
> >  
> >
>
>
> ----
> Your Moderator: Dick Flanagan W6OLD, [email protected]
> Icom Users Net: Sundays, 1700Z, 14.315 MHz
> Icom FAQ: http://www.qsl.net/icom/