Ruark Vita Audio R2i DAB/FM radio seems to work but no sound from any source

[rh100605]

This is the second Ruark R2i that seemed to work according to the display but produced no sound no matter what source was selected.
 
A clue was that the current being drawn was about 200ma at 12 volts, which indicated that the final amplifier was ok

Between the sources and the final amplifier is a Princeton technology PT2314 chip that controls source/tone / volume. I checked the voltages and there was no supply. It should be 9 volts

At the front of the main circuit board above the headphone/aux in sockets is an axial 2 watt 56 ohm resistor (R1) and a surface mount Zener diode (D1) marked AZ.
The Zener, which is a 9V1 400mw device, was short circuit. For this to happen three times probably means that the Zener was dissipating 400mw or more.

I removed the surface mount zener diode and replaced it with an axial 1 watt 9V1 Zener  between the 56 ohm resistor leg next to the zener diode and the star ground point a few millimeters away. Be careful not to power up the radio with the Zener diode removed as you will apply 12 volts to the PT2314.
The radio recovered.

I also checked the ferrites in series with the audio signals between the PT2314 and the TDA7226 as if they have a high impedance (>1 ohm dc) the audio becomes distorted. The same circuit is in the Ruark R1 Mk3 and failing ferrites caused rapid volume changes and distortion

[MathWizard]

So the chip that had no power, was it's 9V Vcc the zener+resistor regulator ? Or is that zener a protection on the aux input channel ? What voltage is on top of the 56R, and what's the voltage across it ?

A lot of chips these days will have some basic protections in them, so that they shut down if some voltage or current is too low or high. There's a lot of computer logic circuits in some of them too, like the one u mentioned, speaks I2C with the microcontrollers/processors
https://datasheet.lcsc.com/lcsc/1809291610_PTC-Princeton-Tech-PT2314E_C90034.pdf

Any PCB pictures ?

[rh100605]

Thanks for the reply, I omitted to mention that I had traced the PT2314E power supply pin to the joint of Zener and 56 ohm resistor.  That was why I checked and removed the Zener.
The input voltage to the 56 ohm resistor is 12 volts. So it got warm when the Zener diode shorted.

As the PT2314E is CMOS it consumes very little current so a good approximation for the dissipation in the Zener is that the current is the 3 volts dropped across the 56 ohm resistor divided by 56 or Zener current  I=3/56 amps (53ma). Multiplying by the 9.1 volts across the Zener gives power or P=9.1 *3/56 =0.4875 watts.

So a 400 mw rating was quite marginal.... 43ma through the Zener would be 400mw and assumes 10ma drawn by the PT2314E.

The useful and rare datasheet attached, which also contains all the PT2314E commands, states that the average current in normal use is 30ma at 9Volts
Giving a zener dissipation 9v x 23ma or  207mw. The current when the device is not initialised or is shut down is not stated.
The zeners do fail though.....

PCB picture attached.