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Posting it here for posterity, and for the possibility of somebody passing-by having some idea.

I’ve confirmed that it wasn’t an issue with power supply. The static noise itself comes from the RF noise in the air, but that’s beside the point, as the radio is supposed to remain silent.

Analysis

With the help of the radio rig’s technical supplement, as well as the datasheets for the involved components, I was able to get the grasp for the radio’s circuit logic.

In short:

• captured signal gets passed thru the mute gate to the volume control circuit
• the volume control circuit passes it to the audio output

What happens in between:

• the volume control circuit depends on the virtual ground set by a CD4094B shift register, marked Q1048, thru a 5 volt signal
• the same shift register also controls the mute gate, also thru a 5 volt signal
• the shift register in question is controlled by the microcontroller (processor of H8S/2268 series) thru clock signal and serial data, which seems to be shared with other components, notably an ADC.

It is noted in the block diagram that the clock signal comes from the microcontroller. The circuit diagram shows it would be the pin 74 of the microcontroller. However, the documentation for the microcontroller, the pin in question is for clock input. There are also pins for the clock output, but not for that pin.

It worth also to note that the H8S microcontroller can have powered-down states. There are multiple different types available, but the only one that applies here would be the software type. Under this state, an interrupt signal can be used to ‘wake up’ the processor back into the normal on mode. In the case of this rig, the power button serves both as a GPIO input to activate the powered down mode in normal mode, and as an interrupt source to wake up the processor in the powered down mode.

Observation

In the normal on state, the shift register marked Q1048 output 5v for both the signal going to the audio control circuit’s virtual gound as well as the one going to the mute gate.

In the standby state, at first this condition persists. After a while, tho, when the loud static appear, the shift register seems to be disabled as both the signals I observed above both become 0v.

Also when the loud static appears, shorting the microcontroller’s RESET pin to the Vcc seems to get rid of it.

Interpretation

When the radio rig get ‘turned off’ to standby mode, most of the radio circuitry seems to still be active. Furthermore, the only thing that are disabled completely are the screen and the input buttons, except for the power button.

I am still investigating the behavior of the microcontroller, the mute gate, as well as the volume control within this state.

So far what I’m seeing is that the clock signal runs normally until it doesn’t, and that is what allows the audio signal to pass with maximum volume.

It is also possible that the signal is supposed to be stopped by the mute gate even before it gets to the audio volume control. This is the avenue I’m currently exploring.

Let me know if you have any thoughts!