Author Topic: Poor frequency response on FT8 data (lower end of spectrum)  (Read 235 times)

0 Members and 1 Guest are viewing this topic.

Offline wcurlew@juno.comTopic starter

  • Newbie
  • Posts: 3
  • Country: us
I have an FT100D ham radio that I am operating with FT8, and AFSK (data) mode. With FT8 you transmit tones in roughly 50hz bandwidths from about 350hz to 2600hz. I like to hang around the 500-800hz area. Turns out the radio was severely attenuating those tones, anything under about 1k was severely attenuated and very little output power was seen. This is on Upper SideBand (USB).

I have confirmed that the input signal is clean and at about 30mv consistant all along the path to the junction of C1304 and r1335 in the attached diagram. Don't see anything at the other side of R1335, but I believe this is expected as the op amp is current driven and no voltages are present.

I can't reach the op amp or the mux to see the output of the op amp as they are under another circuit board soldered in over them.

Before I start hacking up the circuit board, my question is since the mic input (on the other side of the dual op-amp) is working fine, and the AFSK frequencies above around 1000hz are passed OK, do you concur with me that the most likely culprit is C1455? Possible second culprit might be the FET Q1061, but this is used to control the gain of the AFSK data input, and that function is working OK. I suppose the op amp might be at fault, but since the 1000hz+ tones pass and the gain control function both works I believe it is OK as well.

Love to hear thoughts,

Thanks
Bill
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf