[FADCA] Re: Help needed from all TNC-2 Experts

[Welcker]

OK....here's the deal...

Bud talked to me yesterday (Saturday) at the hamfest but the light 
didn't come on.

Basically there are two types of radio audio inputs when using 1200b.
Amplified (those with a low level mike input)
Not Amplified (those with a mike preamp or high level input)

Most Ham radios require very low level of audio at the mike input.

Most commercial radios require much higher levels of audio at the mic input.

Lifting one side of the resistor gives you the higher level of audio due 
to the
loss of termination by the 560 ohm resistor in the output network from the
opamp.  Some change in the mark verses space level is suffered but usually
not of any consequence.

On the commercial radios the feed the DC to the mike preamp up the audio
line through usually a 560 resistor.  If you don't want that DC lift the 
resistor in the radio.
That also gives you a little more audio margin from the tnc since that 
audio termination
is now gone.

Hope that helps
Doug



Chuck Hast wrote:

> On 10/15/06, bud Thompson <budt at cfl.rr.com> wrote:
>
>> (How  many are there out there?)
>>
>> We have run across several MFJ TNC-2 clones (i.e. 1270c, etc, etc.) that
>> have zilch TX audio output at the 5-pin din to the radio.  Circuit 
>> tracing
>> indicates plenty of TX audio from the modem clear out to a 0.33 
>> capacitor
>> coupling to Pin 1 of the 5-pin din.
>>
>>  circuit looks like this:
>>
>> TXA generator ---> 0.0047mfd->-7.5k resistor-> 0.33mfd->Pin1 5pin din.
>>                                                                         
>> |
>>                                                                         
>> |
>>                                                                   560 
>> ohm
>>                                                                         
>> |
>>                                                                      GND
>>
>> That 560 ohm (swamping?) resistor to ground is really swamping the 
>> circuit.
>> Lift if off ground and there is plenty of TX audio.
>>
> I wonder if the .0047 is bad and passing less audio, or the load on the
> TNC is higher than expected? Is there any audio with no load on pin1 of
> the din?
>
>