Quad 405 strange noise post upgrade

[lance_the_boil]

Hello All,


I have just upgraded my aging Quad 405 Mk 1.

I have replaced the Opamp's and changed the electrolytic caps.
I used the excellent mods by Bernd Ludwig posted at http://www.audiocircuit.com

One channel works fine, the other has a problem.

It makes an 'orrible wailing screech for about as long as it takes me to rip the power lead out of the amp

The DC voltage on working output is 4mv and 5mv on the wrong 'un.

I have checked all the capacitor polarities (and corrected the one got wrong....oops) and can find no reason for this. My best guess is that one of the 100nF caps connecting pins 7 and 4 of the Opamp to ground (pin3) is dead as they are supposed to prevent oscillation. My only reservation with this is that they are supposed to prevent rf oscillation and to the best of my knowledge I cannot hear rf nor can my test speakers reproduce them.

Any ideas???????

I'm stumped.

Lance.

[lance_the_boil]

I have now solved this.

After several weissbiers and half a bottle of wine I sat down in a badly lit room with the circuit board and stared at it thinking what on earth was wrong.

At this point I spotted the blob of solder that was connecting 2 components that were not supposed to be connected.

D'Oh.


some later and all is fine.

Sounds MUCH better than before the upgrade. Probably mostly due to the fact that the power supply caps were not the 10,000uF that were supposed to be there but some 2,200uF that some clown must have fitted at some point.