Quad 405 clone

[HiFiWill]

Hi, I recently bought a 405 amp with 34 pre-amp. One channel of the amp was faulty (both fuses blow at turn on). With the 405 came two Ebay clone boards. They are similar to the ones I've seen on Ebay except that they have a heatsink fitted and the output transistors are of the can type. Anyway... I decided to keep the 405 as quad intended and sent the amp module off for repair, meanwhile whilst I'm waiting the clone boards kept crying out to be tried.. so one +/-50v supply later (and with only one amp connected, just to be on the safe side) I turn on. All seemd fine until I connected an input. Then much distortion and hum and then came the smoke (not a lot and I can't see the damage). I have since tried the other channel and all is well. The first channel however remains loud and distorted but not smoking. I'm certain that all wiring etc was ok, the only two things I can think that I might have done wrong are; 1, overloading the input on the first attempt and 2, not running a separate 0v to the heatsink. Any ideas what I've done to my project??

[Tajzmaj]

Hi.
I have a lot of experience with 405......clones...originals....
Can you post some pictures..? There can a lot of things go wrong. I suspect that you somehow miss the ground conection. Original board have a bolt which connect ground to the heatsink.....If there was no ground on the board it oscillates.....and probably burn the 10E resistor at the output zobel network...
Because I'm in the London theese days maybe we can meet and I can help you....also with repairing originals....
Regards, Taj

[CopperTop]

I, too have had a smoked tweeter as a result of the faithful cloning of the original grounding error on the Quad circuit diagram. As I recall, the input ground is connected to the power ground by a low value resistor, but when fitted in the Quad enclosure, all grounds were connected together. On the diagram, the speaker ground return is shown going back to the input ground, not the power ground, so if you don't connect the input ground to anything, the circuit oscillates and bang goes your tweeter!

[HiFiWill]

Originally Posted by Tajzmaj

Hi.
I have a lot of experience with 405......clones...originals....
Can you post some pictures..? There can a lot of things go wrong. I suspect that you somehow miss the ground conection. Original board have a bolt which connect ground to the heatsink.....If there was no ground on the board it oscillates.....and probably burn the 10E resistor at the output zobel network...
Because I'm in the London theese days maybe we can meet and I can help you....also with repairing originals....
Regards, Taj

Brilliant!! Many many thanks! R10 had indeed gone down to 1K. Smiles abound once more!!

[Tajzmaj]

R10 ?
It is 1k and it is connected to the output of the ic1 .
I meant R39....and sometimes even R2 which connects both grounds...
They should be 10E both...
Regards,Taj

[ljm_ljm]

I made some QUAD405 design. But don't know if I have provided version?

have "LJM" letters ON THE BOARD?

[HiFiWill]

Hi, many thanks for the help so far... Replacing R10 with 1k8 as shown on the board got the channel working but you're right, both r39s have got hot, they still measure 10E though!
I have realised however that each channel works if supplied from a different source, the minute I try the same source, hum & distortion on both channels. The same happens if I link the two input signal earths or try to put the signal earth to the chassis.
Is this because of burnt R10s or does r10 get hot because of the lack of earth?
I seem to be going round in circles, not certain of cause or symptom!

[jaycee]

If you get a humm when you join channels, that is a ground loop.

If the 10R resistor which appears to be part of the Zobel network is getting hot, the amplifier is oscillating.

Signal earth should not be on the chassis. Connect that to safety earth. The input sockets should be isolated from the chassis. Connect both signal grounds at one point only, and then connect this point to 0V star earth at the main supply capacitors, via a 10 ohm resistor. Somewhat like this:

[Tajzmaj]

Hi sorry for late reply...I have a lot of job, I'm trying to rent a flat in London....
Well quad. There you have 2 10E resistors at the each board. One is a part of output zobel filter. The other connects both grounds. The power ground and the input ground. And what I think is.....
You have no power ground at your boards so they oscillate and burns zobel resistors....and because amp wants ,,to ground itself ,, it is happening through those 10E resistors (R2) it is even worse if you have unisolated rca plugs bolt to the chasis ......
So you need to connect the power ground to the both boards. It is for an example the opposite side of zobel output network which isn't connected to the output. You have there 10E resistor in series to 100n cap. I don't know which goes to the output and which to the ground but one side is ground and the other is speaker output.
If you need help feel free to ask....and if you need I can send you 405 service manual via email. Mine is
taj.pecnikar@gmail.com
Best regards, Taj

[bdm47]

read threads on quad 405 clone probems ? i am about to use 405 ljm 2006/v1/2010/v2 . seems to be an issue on gnd of the boards . as both my boards have a connection point marked gnd i am alittle confused . ( nothing new to me ) has anyone not had a problem ? and how do they perform once finished . i have only recently started down diy rd after 40 yrs all tube then . also i want to mount tip42s on single heatsinks ? i thank all who can help bernard