Audio Innovations P2 humming

Help! My P2 has started humming quite noticeably on both channels.

The hum varies with the volume control, and is audible during quieter passages at normal listening levels.

I’ve tried switching valves out one at a time, also removed the 5687 and the 2 OA2s - no change. I’ve also just checked the circuit dc voltages against @murrayjohnson’s chart and all are in the right ball park, but all are about 5-10% low. Also tried adjusting the PSU trim pots and the voltages across the 180R resistors connected to the anodes of the 5687 are both around 3.5V, not quite the 5V I’d like, but should be ok. The P2 hasn’t been monkeyed with as far as I can tell, apart from the heater circuit bridge rectifier that I replaced about 18 months ago.

I’m suspicious of the (original?) PSU capacitors but running out of knowledge and ideas - can anyone offer some advice on where to go next?

But I want less hum, not more…

Pay a bloke to fix it.

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My best guess would be a high-voltage power supply fault. These symptoms would be generated if either one half of the valve rectifier had stopped working or one half of the HT winding on the mains transformer had gone open-circuit. Do you have a spare rectifier you could try ?

As you suspect, a failed power supply cap could also account for it. Is any of them bulging at the end(s) ? Do they move if you wiggle them gently (might indicate a cracked solder joint or the cap lead having come adrift inside) ?


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Thanks Graeme. I had tweaked the PSU caps gently when investigating the hum on Saturday, with no change or suspicious behaviour from the caps - then went back to it yesterday to check the ac voltages at the inputs to the EZ80 rectifier. Thankfully I got the same voltage at both inputs, so the transformer seems to be ok.

The odd thing was that the hum had gone since the P2 was switched off the previous day. I went over the caps and dc voltages again, and nothing seemed to have changed - so I’m going to monitor it from now on and will buy in some capacitors to replace the ones in the HT PSU.

Thanks again for your advice :+1: