The PSU 0v does go to each amp board. So just to be clear, you suggest I remove the remaining green earth wire from the right phono to the chassis and mains earth, then connect the mains earth to the PSU board 0v connection?
Yep each phono input just through the coax to the PA board. The mains to the 0V of the PSU board. The PSU board to both 0V of each PA. The 100k volume pot (if you have one) should have each cold end to the phono input earth only. That way not current from the mains earth or the power supply earth can get into the phono inputs. With the phone inputs shorted but not connected to an external pre-amp, the amp should be quiet.
Thank you. I'll try that tomorrow. I'll let you know how it works out. I've omitted the volume control, by the way. Seemed pointless as the preamp has a volume control already.
It still hums. Infuriating. I was convinced baudouin0's solution would work, but sadly not. With it wired as above, and both inputs grounded with shorted phono plugs, the 50Hz hum remains. Very puzzling.
I am the most dangerous person to suggest anything - an enthusiastic beginner - but could you try plugging it in with the mains ground lifted in some way? Just to evaluate if it is the source of the hum?
You suggest away, my friend. All ideas are welcome. This is driving me nuts now. I could indeed try that. If it works there is a way to replace the connection to earth that lifts it enough to suspend the hum but reinstates it in the event of the chassis going live. It uses a bridge rectifier. Details on page 24 of this article.
http://hifisonix.com/wordpress/wp-content/uploads/2019/02/Ground-Loops.pdf
http://hifisonix.com/wordpress/wp-content/uploads/2019/02/Ground-Loops.pdf
No you can put a 10R 2W resistor between the mains earth/chassis ground and the 0V. Maybe time to try something else. So if you just short the input to ground directly on the PA board you still get hum with nothing connected on the input or its ground. I guess if you remove the EF86 then the hum goes (that will disable the LTP as well). If you have a oscilloscope a waveform across the speaker outputs would be good - failing that a .wav file. It may also be worth running the heaters of the EF86 and the LTP (if that's easier) off a bench supply with DC.
I notice there is no ground return for the heaters of V1 and V2 on the schematic. Suggest you copy the two 100R resistors for V3 and V4 grounding the centre.
I tried putting a screening can over an EF86. No difference. So I pulled one out, which makes the hum on that channel jump to 100Hz.
I have my eye on an eBay oscilloscope that's near me. Hope to have one towards the end of the week. Of course, I need to learn to drive it.
I have my eye on an eBay oscilloscope that's near me. Hope to have one towards the end of the week. Of course, I need to learn to drive it.
How odd maybe that does happen. Problem is the ECC83 grid goes to HT so its not a valid test. Do you have a bench supply to power the heaters off for the ECC83 and EF86 - I see they can be powered separately. I notice the original elector had 50Hz 60dB down on 1W which is audible - so it may not necessarily be a fault of the construction.
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I am just wondering if its not a ground loop - but the heater supply getting into the EF86 input. Not sure how we can check. It does appear to be floating which is bad practice. It should have the same arrangement 2x100R with the centre to 0V as V3 and V4 supply. The only thing I can think of is to put a 10uF 200v cap from the plate of the EF86 to ground just as a check. This would remove all audio too, but if the EF86 stage is a culprit the hum will also go. I would disconnect the NFB if you try this.
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Yes with the NFB disconnected but with no other mod you could check for hum on the plate of the EF86. It will still not tell you the exact cause - but you will have a lot more to go on. Actually even a modern meter on AC should be able to measure the 50Hz component. Often tracking down hum issues is often a process of elimination and it may not be a fault of your build.
Can I just check: you don't think there's any way my addition of a little PSU running from the mains input and powering the relay switching circuit could be causing this problem?
Don't think so. I think you need a scope/supply to go much further. Maybe a circuit of this and what it is doing.
I'll keep you posted on the 'scope. I'll also refer back to the original Claus Byrith schematic and see what he did with the heaters of V1 and 2.
Thanks for your help thus far - it's much appreciated.
Thanks for your help thus far - it's much appreciated.