Quad 405 hum

[chrissyelle]

Forgive my coming late to this discussion.
I had the same problem: bronze/DIN 34 preamp with either 405 or 405-2, hum. (44/same 405, no hum!)

It’s a ground loop because you are mixing grounding systems. The fix is to cut the Pin2-to-chassis link on the 405 chassis DIN connector (inserting a 10R resistor is a good idea as well) and do not carry the shield through on the case of the cable DIN plug at the 405. (Pin 2 needs to be carried through). Yes, it means that the lead is not the same at each end.
The main clue for this is when others have said that a 405 with isolated RCA inputs straight to the PCB does not have the problem.

The 44/405 combination has incoming earth=chassis=audio 0v. Whilst the 34 came out before the 306, I would suggest they were designed as a 34/306 pair, but the 306 audio 0v is NOT chassis; it floats via a 10R resistor.

When I was installing recording consoles, you had to account for these variations, so all AC/Mains Earths connected and resolve any loop issues at the device end, because all shields/audio commons should be connected at the console. Treat the preamp as your console, no AC earth lifts anywhere, but sometimes dropping the shield at the input of the device.

Hi.

Also late to the party... But I'm having this same issue with a quad 34/405 (slight hum, even when nothing is connected to the 34, and not effected by 34's volume knob).

I opened up the 405 and did notice the link between pin 2 of the Din connector and chassis. But @Ampex185, could you explain exact what I should do? Do I solder a 10ohm resistor between pin 2 and chassis? I don't fully understand what you mean when you suggest "do not carry the shield through on the case of the cable DIN plug at the 405".

Sorry, I'm a bit of a newbie here!
Thank you for your time.

 

[martin clark]

Late entry, but the classic cause of hum in the Quad 405 and 405/2 is C5 (as marked on each channel pcb) the 100uf bootstrap capacitor, dying with age.

Easily replaced with any convenient 100uf/50v replacement cap - easy, cheap DIY.

 

[chrissyelle]

Late entry, but the classic cause of hum in the Quad 405 and 405/2 is C5 (as marked on each channel pcb) the 100uf bootstrap capacitor, dying with age.

Easily replaced with any convenient 100uf/50v replacement cap - easy, cheap DIY.

Oh wow. Ok. So in your opinion, no need to cut "pin 2 to chassis link" on the Din input?

 

[chrissyelle]

So I tried replacing C5 and the hum is still there.

@Ampex185 can you clarify what you did to resolve it? I don't understand what you mean when you said "do not carry the shield through on the case of the cable DIN plug at the 405".

 

[chrissyelle]

Hi all
So I inserted a 10R resistor between pin 2 and chassis on the 405 din input socket. The hum has gone away, mostly, in the right channel, however the left is still there. I noticed that the leads to the left channel amp (from the din connector) also have the shield going to chassis. Should I do something with this one?

 

[chrissyelle]

Pardon me, but @Robert : Do you have any advice for me? After inserting a 10R resistor between pin 2 and chassis on the 405 din socket, the hum on the right channel is gone, but still remains in the left.