Batboard

I have a UHF HT750 that has no receive audio. No beeps and no signal audio. No audio on internal speaker and no audio on speaker-mic. Transmitt is good and led shows receiver activity. What causes this?

One of the most common causes is that the radio took a bath and then sat upright with the battery attached for some time. The audio IC/circuitry is at the bottom of the PC board and has B+ applied to it all the time if I remember correctly. Usually electrolysis takes its toll on the circuit board traces in a fairly short period of time because the traces are so fine. I assume that you've already checked the flex and connector on the PCB. It's starting to look like you'll need a back cover kit to repair that one.

Circuit board is clean. New flex and speaker did not help. There is no audio on volume control, it looks like it is a DC control type circuit. DC does vary.

If you can remove the shield over the audio amp IC, you might try taking a look at the input and enable pins on the IC. If you have audio on the input pin and valid logic on the enable pin you should have audio across the BTL output of the chip if the IC is OK.