1970s Prince Amplifiers PS-100 'Piggy' 15w Solid-State Combo

Super cool '70s solid-state combo from Japan! Manufactured by Prince Tsushinkogyo Co. Ltd. out of Nagoya, the Piggy amps were offered in a range of versions from very basic models all the way up to the PS-100. Two inputs, a basic EQ, as well as volume and gain controls adorn the front. There is however a final control that's interesting labelled 'A=440hZ'. This when turned on plays the a tone that is A at 440hz for tuning purposes! As a solid-state amp it sounds pretty neutral, great for a very basic clean tone. 

All controls appear to function correctly, with minor crackle on each pot. However there is quite a loud and noticeable hum that is present in varying degrees, which appears to be some kind of malfunction.