I've just been lucky enough to have purchased a pair of the stunning
Rogers/BBC LS5/8 speakers, complete the with a pair of the BBC modified
Quad 405 frequency splitting amps (actually, one 405 and one 405-2...
no idea why!)

The main problem is that both amps exhibit a low level but perfectly
audiable mains hum - even with their gains right down and nothing
plugged into the (XLR) inputs. Am I right in thinking it's reasonably
easy to fix by changing some capacitors? Any help greatly appreciated!!

Many thanks.....

spwyatt wrote:

I've just been lucky enough to have purchased a pair of the stunning
Rogers/BBC LS5/8 speakers, complete the with a pair of the BBC modified
Quad 405 frequency splitting amps (actually, one 405 and one 405-2...
no idea why!)

The main problem is that both amps exhibit a low level but perfectly
audiable mains hum - even with their gains right down and nothing
plugged into the (XLR) inputs. Am I right in thinking it's reasonably
easy to fix by changing some capacitors? Any help greatly appreciated!!

Any 405 is pretty ancient by now. You should consider replacing all the
electrolytic caps.

Graham

"spwyatt" wrote in message
oups.com...


** Groper alert !

I've just been lucky enough to have purchased a pair of the stunning
Rogers/BBC LS5/8 speakers, complete the with a pair of the BBC modified
Quad 405 frequency splitting amps (actually, one 405 and one 405-2...
no idea why!)

The main problem is that both amps exhibit a low level but perfectly
audiable mains hum - even with their gains right down and nothing
plugged into the (XLR) inputs. Am I right in thinking it's reasonably
easy to fix by changing some capacitors? Any help greatly appreciated!!



** The hum is almost certain to be a result of the internal modifications -
ie, it has been built in due to earth loops or magnetic field injection from
the mains transformer.

Likely very difficult to fix with the mods still in place.

Find a good service tech and let him have a look.




........ Phil

"spwyatt"
The main problem is that both amps exhibit a low level but perfectly
audiable mains hum - even with their gains right down and nothing
plugged into the (XLR) inputs. Am I right in thinking it's reasonably
easy to fix by changing some capacitors? Any help greatly appreciated!!



** This article is worth reading:

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


Even if only as a warning.




........ Phil

In article .com,
spwyatt wrote:
The main problem is that both amps exhibit a low level but perfectly
audiable mains hum - even with their gains right down and nothing
plugged into the (XLR) inputs. Am I right in thinking it's reasonably
easy to fix by changing some capacitors? Any help greatly appreciated!!

Failed electrolytics are a common fault on these. Remember they could be
near 30 years old. I'm sure if you Google you'll find details on the best
replacement makes, etc.

--
*Most people have more than the average number of legs*

Dave Plowman London SW
To e-mail, change noise into sound.

In message .com,
spwyatt writes
I've just been lucky enough to have purchased a pair of the stunning
Rogers/BBC LS5/8 speakers, complete the with a pair of the BBC modified
Quad 405 frequency splitting amps (actually, one 405 and one 405-2...
no idea why!)

The main problem is that both amps exhibit a low level but perfectly
audiable mains hum - even with their gains right down and nothing
plugged into the (XLR) inputs. Am I right in thinking it's reasonably
easy to fix by changing some capacitors? Any help greatly appreciated!!

Many thanks.....

The 100uF 6.3V capacitor C2 on the input I.C. is the usual culprit for
hum. But as others have said, I would change all the electrolytics on
the PCB.
--
Kevin Seal (at home)
F800ST
{kevin at the hyphen seal hyphen house dot freeserve dot co dot uk}

"Kevin Seal"


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.

http://www.geocities.com/quad_esl63/...c/power405.jpg

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


But as others have said, I would change all the electrolytics on the PCB.


** Which will have no effect on the OP hum issue.

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




........ Phil

In message , Phil Allison
writes

"Kevin Seal"


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.

http://www.geocities.com/quad_esl63/...c/power405.jpg

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.

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


But as others have said, I would change all the electrolytics on the PCB.

Still maintain that all the electros should be replaced in old 405's.

** Which will have no effect on the OP hum issue.

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

Enlighten me.
--
Kevin Seal (at home)
F800ST
{kevin at the hyphen seal hyphen house dot freeserve dot co dot uk}

"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

In article , 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.


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.


"Input" transformer in a power amp eh?.

Not too clever an idea shirly??..
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





--
Tony Sayer