MK190 no sound - stuck in mute?

I purchased a MK190 stereo amplifier kit a short while ago which has now stopped working properly – no sound from speakers (except really faint).

Problem was first noted after the amplifier was mistakenly plugged into a 5 volt regulated supply, but was then unplugged and the usual 12v switched regulated supply used. LED came on when switched on, but I can only get a slight sound output if the input source is turned up to maximum, and the amplifier volume also turned to maximum. The sound is really faint and only heard if you put your ear in the speaker.

It has worked fine for weeks and I have checked all the solder connections and examined the board closely to ensure there are no bridged connections and everything seems to be okay.

I note that the chip used NXP - TDA1517 has a mute/standby facility: 0-2 volts for STANDBY, 3.3 to 6.4v for MUTE and 8.5 to 18v for ON. I was wondering if the 5v supply had done something and it’s stuck in mute!

I had a great time building this and I hope someone has an idea what maybe wrong. :question: