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Thomo 20th June 2007 08:06 AM

Rebuilt Quad405 Problems
Hi everyone.

I recently got hold of a Quad 405 amp. It was in very poor condition internally, so I stripped the boards and rebuilt with all new components,

I also performed the mods listed here:

The amp does not have any speaker protection (yet).

The problem is that I am reading -42v at both speaker outputs.

Anyone have any ideas please?

Regards, Lee.

geminni 20th June 2007 11:13 AM

I'm also Quad405 happy owner:)

With voltmeter test dc supply, to be shure that you have + and - rail. Maybe you don't have one rail so thats why you have same volt on spk out for both amps.
If thats ok then check the board connections. If you made your new boards maybe mistake is on them, some off connections, or some shot connections....
Allo check the supply for IC, check the zener diodes, maybe one is soldered backward. Can happen, once i did that :)

You must put some DC protection, for speakers sake! ;)

Thomo 20th June 2007 03:07 PM

Hi. Thanks for your help.

I checked the supplies to IC and there was a break in the positive supply track - on both channels!

I don't really know what the person who owned this previously was doing - but 45 for a Quad 405 in any condition, I consider that a bargain!

Anyway I am now reading only a few millivolts of dc offset now, I've installed velleman speaker protection and am about to connect some old speakers.

What mods have you performed on your 405? or is it just stock?

Kind regards, Lee.

geminni 21st June 2007 07:08 AM

Ah, your welcome;)

Well, i don't know which is my version, because friend gave me schematic so i made it by that, some 10 years ago. My input IC is TL071, also there are 3 small coils which are not present in original schematic.
My quad405 is somewhere between original and last modified version :)
I can say that i am very satisfied with quad405. Testing with some regular hi-fi comercial amplifiers it's like heaven and earth-can not be compared :D

How much millivolts is dc offset? Small dc is not dangerous but some 100mv is much, by my standards :)
Check the volts after zener diodes, that is Ic-s pin 4 for "-" and pin 7 for "+", they must be equal. If one reds 15,3 and other 16,2 (or something like that) i would change zener diodes and check resistors before them, use 1% 2w resistor there. I'm not shure if zeners are 15 volts on every schematic, on my they are.

Thomo 21st June 2007 09:24 AM

Hi. Just letting you know that all is well. The amp is making very sweet music, and with a few hours burn in it should improve further (if you believe in such things).

The dc offset is very low only around 6 millivolt.

Just connected to my Tannoy LGM's and I am very impressed. Much better than the 405 I modded for a friend, and also much better than my Gainclone. I will go dual mono with the power supply as soon as I can afford the caps.

Thanks for your help, kind regards, Lee.

geminni 21st June 2007 12:10 PM

You're welcome!;)

alex mm 25th June 2007 07:58 PM

Quad 405 Reborn
Hi , I start two weaks ago to build again the new version . :) Schematic PCB and final picture other picture ;) all the best Alex mm :cool:

geminni 26th June 2007 06:54 AM

Hm... Nice! ;)
I see that one 6,9uh is missing, with 22 ohm parallel with it. Also there are no some diodes, transistors, 75 ohm resistors around them which were some kind of protection?
How does this version sound?

Symon 15th July 2007 09:42 AM

Hi Alex,
Looks very nice, but I have a concern that the PCB layout doesn't match the schematic.
The problem I noticed is then the schematic shows the boot strap capacitor is a 47uF in parallel
with a 3.3uF. But the PCB connects one of them to the negative rail. Is the PCB layout the
version built?

If this is the version built in the picture, it appears that the 47uF is connected backwards across
the lower 560R, so it won't live very long.

- Symon

alex mm 15th July 2007 07:10 PM

QUAD 405 REBORN PCB corection
Hi , Symon u right I have been corected PCB error , and I hope now its ok
All the best Alex mm :)

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