[App_rpt-users] CM108 modification

[Stephen Rodgers]

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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> 
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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