"Kevin Seal"
Phil Allison

The 100uF 6.3V capacitor C2 on the input I.C. is the usual culprit for
hum.


** That ( tantalum) cap has nothing to do with any hum problem.


It is a response setting cap for the DC offset servo.


Apologies, I meant C5. It usually pops it's can and dries out, I've
changed it in about 30 amps so far with great reduction in hum.


** Even with C5 removed the result is a barely audible, 5 mV rms hum.


In every amp I've dealt with , C2 is an NP electrolytic. Never seen one
with Tants in.


** A tantalum is the usual & specified part used.

http://quad405.com/405-1schematic.pdf

See C2 on the parts list and diagram.

It is marked as being 3 volt, 100uF, polarised and having a tolerance of
10%.



His 405s have been modified in a way that is highly prone to creating
hum.

Enlighten me.


** Try reading the heading, the original post and this link I posted two
days ago:

http://www.mhennessy1.f9.co.uk/rogers/ls58.htm

The "mod" consists of a PCB of similar size to a power channel board, fitted
with an input transformer all crammed into one end of the already cramped
case.

BTW

You can cleary SEE a tantalum capacitor ( blue in colour) in the pic of the
405 PCB about half way down the page.




........ Phil