Motorola ASTRO XTL 5000 Basic Service Manual page 153

Basic Troubleshooting: Audio PA Out Bias
9.4 Audio PA Out Bias
The audio PA is a BTL (bridge-to-load) amplifier. When the radio is in receiver mode, the output of
both speaker leads bias up to half of the supply voltage. Considering that both leads are at the same
potential, the average DC voltage across the load is zero. At maximum volume the signal voltage to
the load is double that of one amplifier peak-to-peak because the amplifiers work together 180
degrees out of phase. Caution must be taken not to short either lead to ground, which might cause
component damage. Even though audio may still be heard from the speaker, the shorted side will go
into current limiting and eventually into thermal shutdown, shutting off the amplifier. These protection
devices are not intended to be tested.
9.5 Replacement Board Procedures
Once a problem has been isolated to a specific board, use one of the following recommended repair
procedures:
• Install a good board from your inventory into the radio.
• Order a replacement board from Radio Products Services Division at 1-800-422-4210. Refer to
"Appendix A Replacement Parts Ordering" on page A-1
• Troubleshoot the defective board using the ASTRO XTL 5000 Digital Mobile Radios Detailed
Service Manual. (Refer to the
specific manual number.)
6871769L01-A
"Related Publications" section of this manual on
for further information.
page xvi
October 30, 2006
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for the