Motorola GM950E, Service Manual

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Receive Audio Circuits
6.3-14 Theory of Operation
The SPK+ and SPK- outputs of the audio PA have a DC bias which varies proportionately with FLT
A+ (U0401-7). FLT A+ of 11V yields DC offset of 5V, and FLT A+ of 17V yields a DC offset of 8.5V. If
either of these lines is shorted to ground, it is possible that the audio PA will be damaged. SPK+ and
SPK- are routed to the accessory connector (J400-16 and 1) and to the control head (connector
J0101-1 and 2).
6.4 Handset Audio
Certain hand held accessories have a speaker within them which require a different voltage level
than that provided by U0401. For those devices HANDSET AUDIO is available at J0101-14.
The received audio from the output of the AFIC´s digital volume attenuator is also routed to U0105-
4 pin 9 where it is amplified 15 dB; this is set by the 10k/68k combination of R0154 and R0155. This
signal is routed from the output of the Op-amp U0105-4 pin 8 to J0101-14. The control head sends
this signal directly out to the microphone jack. The maximum value of this output is 6.6Vp-p.
6.5 Filtered Audio
The AFIC has an audio whose output at U0103-22 has been filtered and de-emphasized, but has
not gone through the digital volume attenuator. From AFIC U0103-22 the signal is routed through
gate U0107-2 and AC coupled to U0106-4. The gate controlled by AFIC port GCB4 (U0103-2)
selects between the filtered audio signal from the AFIC or the unfiltered discriminator signal from the
IF IC U5201.The output at U0106-4 is then routed to J0400-11. Note that any volume adjustment of
the signal on this path must be done by the accessory.
6.6 Discriminator Audio (Unfiltered)
Note that discriminator audio DET AUDIO from the IF IC U5201, in addition to being routed to the
AFIC, is also routed to the option connector J0103-5 (See Secure Rx description blocks for further
information).
6.7 Option Board Audio
Discriminator or filtered audio, enters the option board at connector J0103-5 and J0103-4. On the
option board, the signal may be processed and then fed back through (J0103-2) to AUX RX IN of the
AFIC (U0103-6). From then on it follows a path identical to conventional receive audio, where it is
filtered (0.3 - 3kHz) and de-emphasized.
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