Quad 405 Hum - one channel only

[Weeman1973]

I’ve got an old Quad 405 I was playing around with an it’s got a good old buzz / hum coming from one channel only.
It’s definitely an output issue as it persists with or without anything attached to the input.
It is the same volume even with music playing so sort of disappears once you get to a volume but when music is paused or in between tracks it’s noticeable.
Any ideas what could be wrong? I’m not a big enthusiast in fixing issues but if it’s a simple fix I’d have a look at doing it.
As the amp doesn’t owe me much I won’t take it to Quad for a recap our whatever as it genuinely isn’t worth it to me but if I can fix it myself I’d like to have a go.
Any suggestions?

[Weeman1973]

I’m no expert but the corrosion here doesn’t look particularly helpful in eliminating hum..

https://uploads.tapatalk-cdn.com/20200607/03b9ad22a810debaca84e303e209f638.jpg

[cjnuk3]

What do the large power supply smoothing capacitors look like? If they're puffed up and/or leaky that would cause the problem.

[Pharos]

The brown resistors look as though they have overheated, and beneath the left lead there is burning on the cct board. I would check the values in the other channel and compare. Either they have changed value, or a semiconductor in the same cct has gone caput at a guess.

[scotty38]

Agreed they’ve burned out so even if not related to the hum it’s had some fault or other.....

[Barry]

It looks as if work has already been done on it at some time in its life: the blue 220uF capacitor is not the original. If the 'brown' resistors have overheated so much as to burn the PCB a centimetre or so below them, then they could well have damaged neighbouring components.

If the Quad 405 is the Mk. I, then the voltage dropping resistors that feed the input IC were under-rated and would run hot (and could eventually fail). Despite the scorch mark, is it possible to identify the resistor comonent number?

[HarryCrumb]

C5 is the usual suspect for hum, though you have obviously got an issue at R39. What PCB number is it? I'm currently refreshing my Quad405. Ivee got noise on both channels but more "hum" on the left. It's common I believe.

Have you researched Bernd Ludwig and Keith Snooks work? They both show fixes for this.

[HarryCrumb]

Although I'm very new to this sort of thing myself, I have been reading nothing but 405 schematics for the last few weeks. I can't see any other instance of having two resistors at R39 like you seem to have in that photo. Possibly thats old damage and a red herring...?

If you're interested you could read Bernd's work here - https://www.desmith.net/NMdS/Electronics/Quad405/405_Qw_14.pdf

Keith goes even further, and he also mentions an important issue with PCB's 9/10 and earlier that have a fault concerning R39, though I don't think it's your issue. In any case, solving the "hum" is one of the easier things to do and seems to involve removing C5 and adding another transistor at Tr1. Though, once inside, you may find yourself drawn in to the mysterious and intriguing world of quad modifications.... good luck!

[Weeman1973]

Thanks all for the replies - really appreciate the input.

As I said at the top of the thread, I’m really not into the ins and outs and how to upgrade, replace everything etc so will probably leave this alone and either throw in the back of the garage or pass onto someone who has the inclination the restore it to it’s former glory.

[reddish75]

New caps and resistors that need replacing for both amp cards will cost under £10, very easy amp to work on schematics freely available