The manual shows that the 220 MHz band uses a dedicated harmonic filter. A burnt ferrite bead (L41) is a common failure. Fix: Locate L41 via the PCB map in the manual; replace with a 0603 ferrite bead (600Ω @ 100MHz).
Some sites offer a that is actually for the older QYT KT-8900 (single-band board) or a generic “AT-778” platform. The KT-7900D is a derivative of the AT-778 motherboard used in many Chinese radios (Retevis RT95, Zastone, etc.). qyt kt-7900d service manual
The standard user manual included in the QYT KT-7900D box covers basic operation: changing frequencies, setting squelch, and programming memory channels. It does not include: The manual shows that the 220 MHz band
An official, detailed service manual for the QYT KT-7900D is nearly impossible to find. qyt kt-7900d service manual