I'd really sincerely appreciate help - I've just bought a lovely seldom-seen amp model (Crafter DSP1) - about 10 or 15 years old. I bought it in a noisy environment, and discovered very conspicuous hum once using it at home.

I managed to find a schematic, which is attached here.

I have long experience with passive guitar circuits, and some with simple stomp/active circuits, but very little with mains audio electronics.

The hum is irrespective of the volume control and starts before the output relay switches on. With the amp loud, the hum is drowned out and the amp sounds very good, but quiet playing is spoiled by it. I estimate the hum at 100Hz (comparing it by ear with a tone generator). UK mains is 50Hz.

As a best-guess before looking for help I speculatively replaced the two big 4700Uf power supply caps and the two additional 470uf caps - no improvement, nor from changing the four rectifier diodes :-(

Plugging a jack into the fx return makes no difference to the hum. None of the amp controls makes any difference. The PCB is in good condition, and with a careful look I see no obvious solder bridges, past repairs, scorching or disconnections.

Of course the symptoms lead me to figure the issue is after the preamp stage, and that it is mains-related hum (post-rectifier?) but I`m really not sure how best to further troubleshoot this.

Please does anyone have any other suggestions (or any encouragement!). I have several basic multimeters to hand, could make an audio probe, but have no oscilloscope. I'll really be very very grateful for help... I love this amp, my first one specially for acoustic guitar, but the hum is really disappointing. I can`t easily afford to get this professionally fixed, and also know both how thrilled I`ll be to fix it with my own hands, and how much it would make it `mine` ;-)

Thank you,
Rob