The Motorola XIR M3688 service manual can be obtained from various sources, including:

The radio receives perfectly, but when you press PTT, the red LED comes on, but no one hears you. Manual Solution: The manual directs you to Section 4.2 (Transmitter Alignment). You will learn to measure the PA bias voltage. A common fix is adjusting pot R432 (or replacing Q404, the driver transistor). The manual provides the exact voltage table for each stage.

: Use only genuine Motorola parts for consistent performance and safety.

Additional information often found in service documentation includes: and their technical meanings. Wiring diagrams for the rear accessory connector.