Vega DSP-223, Technical Manual

Page: 23 / 59

Tone Remote 19
5 Theory of Operation
The DSP-223 is a Digital Signal Processor (DSP) based product. Many of the signals that once could be
probed on older products, are handled within the DSP itself. This includes DTMF decoders, notch filters,
tone decoders, and all of the audio summing. Most of the gain controls are also included within the
software of the DSP. Because of this, hardware is simplified at the expense of software creation time and
user serviceability. The next sections discuss the portions of the DSP-223 that can be tested using a
common oscilloscope and voltage meter by a qualified service technician.
5.1 DSP-223 Audio Input Paths
The DSP-223 has only a single stereo Analog to Digital Converter (ADC) U25. The left channel converts
the Line receive audio and the right channel converts the Radio receive audio. A CMOS switch, U2, is
used to break those paths and insert handset audio for transmission down the line or to the radio. Up to
the point of entry into the ADC, the audio is analog and handled by standard op-amps. These paths can
be followed on the schematic and probed with any high-impedance measurement device.
5.2 DSP-223 Audio Output Paths
The DSP-223 has two stereo Digital to Analog Converters (DACs), U3 and U4. U4’s left channel drives
the sidetone for the handset. U4’s right channel is used for the CTCSS tone generation. U3’s left
channel is the line transmit audio source and the right channel of U3 is the radios transmit audio source.
Once the analog signal has been reconstructed on U3 and U4, they can be measured by conventional
techniques.
5.3 Troubleshooting Aid
Problem Possible Solution
What is position “A” on a jumper
plug On early PCB’s the “A” position of the jumper is the double line side of
the silkscreen around the jumper plug.
The DSP223 will not decode Is the power LED on the front panel ON?
Is J11 in the “A” position?
If in 4-wire, make sure the console TX is connected to the DSP223 line
RX.
Ensure alignment procedure Section 3.1 of the manual is correct.
Check telephone line continuity from dispatch console
Verify that proper PTT frequency is selected Section 4.2.2.5 of the
manual (Rev 2.0 operating software or higher)
The DSP223 will not PTT radio Does PTT LED on front panel turn ON?
Check for proper PTT relay configuration/wiring
Check to see if JP2 is soldered closed (if common is grounded)
Verify that proper PTT frequency is selected Section 4.2.2.5 of the
manual (Rev 2.0 operating software or higher)
Verify that function tone is not locked out Section 4.2.2.9.1 of the
manual (Rev 2.0 operating software or higher)
Radio TX audio is low or
distorted Check setup procedure Section 2.3.1 of the manual.
Check alignment procedure Section 3.3 of the manual.
Set J26 to opposite position, recheck.
EE and error beeps occur Overdriving of the Line RX ADC is occurring, adjust R68 down
Turn error beeps OFF, Section 4.2.2.11 of the manual (Rev 2.0
operating software or higher)
Radio RX audio is low or
distorted Check setup procedure Section 2.3.2 of the manual.
Check alignment procedure Section 3.4 of the manual.
Check telephone line continuity to the dispatch console