In message , Phil Allison
writes

"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.

I've never measured the hum, I just used my ears.
On all 30 of the foldback amps I've repaired, replacing the duff C5 has
resulted in a total reduction in the hum fault reported.

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%.

Quite right, and on the 405-2 which is the OP's second amplifier, C2 is
an NP electrolytic.


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.

The main problem with hum as described on that web page is to do with
the fact that the amp had been fitted with a toroidal transformer before
it was purchased for the project.
Nothing else is mentioned about the extra mods causing hum.
--
Kevin Seal (at home)
F800ST
{kevin at the hyphen seal hyphen house dot freeserve dot co dot uk}