[App_rpt-users] CM108 modification

[hwstar at rodgers.sdcoxmail.com]

> 
> From: Doug Bade <kd8b at thebades.net>
> Date: 2009/07/08 Wed AM 08:13:10 PDT
> To: app rpt-users <app_rpt-users at qrvc.com>
> Subject: Re: [App_rpt-users] CM108 modification
> 
> That explains it...I did not look there :-)
> 
> I need to see if my new Chinese versions have it ... it looks like 
> lots more parts on the board but very similar otherwise... some other 
> buttons added for mute and volume up down on the fob itself... 
> assuming we do not need core.... I am wondering if I can isolate the 
> 4th pin on the speaker and  mic jack and just push ptt out that jack 
> instead of cutting it all out as seems like the other 3 leads do not 
> need to be changed...
> 
> I am considering replacing the npn transistor with a 2n7000 series 
> fet as a series resistor is not needed on the gate side... as long as 
> all runs at logic levels etc...
> 
> Doug
> 
> 
> 
> At 10:29 AM 7/8/2009, Stephen Rodgers wrote:
> >Doug Bade wrote:
> > > The schematic looks logically correct...
> > >
> > > C2 and C3 are in the usbfob.pdf picture..
> > >
> > > C1 does not appear to be.. It seems C1 is on the schematic but not
> > > the construction picture.... I would think it should be there... as
> > > to avoid DC coupling between the stages...
> > >
> > > I just recently got some CM109 based adapters from China that are
> > > currently available and will be making some notes on them as I
> > > perfect a mod..electrically they are the same.... the chip is 
> > really tiny :-)
> > >
> > > Doug
> > >
> > >
> > > At 08:24 PM 7/7/2009, you wrote:
> > >> Ok, I'm modifying a CM108 USB adapter, and though I've done it
> > >> before I can't remember if whether the picture or the schematics are
> > >> correct in the pdf file.  In the picture, I see that for the mic
> > >> input (RX Audio) has the twisted (or just soldered) end of the 68K
> > >> and 470K attached to the 9 pin connector.  In the schematic, the
> > >> twisted end is attached to a capacitor (which isn't in the picture
> > >> and directions) and then to the mic input on the USB adapter.
> > >>
> > >> Can anybody tell me which way is the correct way to hook this up?
> > >>
> > >> Thanks,
> > >> Jeff
> > >> _______________________________________________
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> > >> App_rpt-users at qrvc.com
> > >> http://qrvc.com/mailman/listinfo/app_rpt-users
> > >
> > > _______________________________________________
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> > >
> >As the previous post states, a coupling capacitor is required, however,
> >it is already included in the SMT parts on the USB fob. The pictures
> >don't show it because it is already mounted on the fob.
> >
> >C1 is a 4.7uF multilayer ceramic chip capacitor which is part of the
> >original circuit as noted at the bottom of the schematic with an *.  An
> >additional 4.7uF capacitor is not required,
> >
> >Steve
> >WA6ZFT
> 
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> 

If the CM109 is a 3.3V chip then the 2N7000 will probably not turn completely on due to a higher Vth requirement. Personally, I'd stick with the 2N4401 and the series resistor. 

"I know engineers, they love to change things"
Dr. McCoy, Star Trek

Steve
WA6ZFT