Identifying Quad 306 mods + repair questions

[RaggaMuffin]

I recently acquired a Quad 306 power amp in excellent condition for free that was going to be thrown out, so of course, I had to take it! It powers on fine, and music plays through it, but the sound is quite muddy and distorted. Not massively, but enough to make it not fit for purpose as it is right now.

I opened it up to have a look, and I couldn't see any components that had obviously burnt out or anything. I did though notice that some modifications and some recapping had been done though. It looks like the 4 main (power supply?) capacitors have been replaced. The speaker inputs have been replaced, and looked like they were wired really badly to the PCB, so I desoldered the wires, and replaced them. This unfortunately did not help with the sound. I used 20AWG wire rated for 600V for this, is this wire fit for purpose?

Just a note, I'm not very experienced with electronics, I have some very rudimentary knowledge and basic soldering and multimeter skills, but I am keen and willing to learn.

On the underside of the board, there are 8 100nf capacitors, film capacitors I think. I have done a bit of research, and I'm pretty certain these are to decrease the input sensitivity, as the Quad 306's sensitivity is 375mV out of the factory. I have attached some photos of this.

Could anyone help me work out by how much exactly this is decreasing the input sensitivity? And could this mod be causing the sound issues? Would desoldering them be ok? I will attach a photo of the top side of the PCB, in case anyone can spot any other changes that might have been made that I've missed. Some of the solder joints on the underside are touching each other, could this cause anything?

Also, what should I do in a situation like this where an amp has a sound issue? I have seen some advice elsewhere that involves sending a sine wave through the amplifier, but I don't think I have the equipment to do this.

Thanks in advance for any help!

https://i.imgur.com/mFeIJQU.jpeg

https://i.imgur.com/mCJlulj.jpeg

Here are the photos: https://imgur.com/a/lowlBhW

p.s. The shematic is here https://usermanual.wiki/Document/Quad306Schematic.1625445474 and the manual is here https://www.dadaelectronics.eu/uploads/downloads/02_Quad-Service-Manuals/Quad-306-Service-Manual.pdf.

[RaggaMuffin]

I think I found some information on the mods that were done. It's a kit from here: https://www.dadaelectronics.eu/uploads/downloads/01_Dada-Electronics-Revision-Upgrade-Manuals/Quad-306-Upgrade-Revision-Manual-V1.4.pdf . The resistors at R6 have been changed from 120kohm to 62kohms. I don't think R13 has been changed, so the input sensitivity hasn't been changed I don't think. R9 reads as 27 whereas it should be 33, and it does match the colour for a 27ohm, so this might be incorrectly installed. R11 reads as 120ohm but should be 47ohms, so it looks like this either shouldn't be here as well, or was put in for some reason. R21 reads as 0.01 or 0ohms whereas it should be 22ohm. It looks like the right colour resistor though, as R20 is a 22ohm as well and it's the same colours.

Oh, and I've just realised, if I am correct that the person who did this was following this companies mod, there are only 8 caps fitted on the underside, as opposed to 10 in their manual.

[Bonky]

I have a DADA-modded 306 and I believe you to be correct.. I had mine professionally fitted by RedHill Audio. It sounds excellent ('though not in use any more).

Do you want me to open it up and take some pics?

[RaggaMuffin]

I have a DADA-modded 306 and I believe you to be correct.. I had mine professionally fitted by RedHill Audio. It sounds excellent ('though not in use any more).

Do you want me to open it up and take some pics?

Ah, that would be excellent, thank you! The only images from Dada are a few snapshots of small areas of the board, so it would be great to see it professionally done and more clearly. Only if you have the time and don't mind, though. Also, I got some advice on another forum about one of the readings I took. R21 is in parallel with L2, so that will give it a very low reading. I'm learning how to read schematics as I go along, so at least I know about checking resistors in series/parallel now for future projects.

Out of curiosity, what preamp have you used with your Quad? At the moment I'm only testing with the pre-out on my headphone amp, as I don't have a normal preamp and my integrated doesn't have pre-out either.

[Bonky]

Hi, I used a Schitt Saga +

...good value...but I sold it on.

R

[Bonky]

https://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/P8pWW_EJS7WLhEK-rjMoRQ?ref_=nav_AccountFlyout_clddrvhttps://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/DRGR57upTCuj1R_loEu8DQ?ref_=nav_AccountFlyout_cldd rvhttps://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/5EQvn_vhQKKQc4GZ4S3BwQ?ref_=nav_AccountFlyout_cldd rv
https://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/gNN6b9xERWGKJ-h_YPWU1Q?ref_=nav_AccountFlyout_clddrvhttps://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/XIT8CdtjTyiUCtab6K7RUg?ref_=nav_AccountFlyout_cldd rv


Hope that works!

[RaggaMuffin]

https://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/P8pWW_EJS7WLhEK-rjMoRQ?ref_=nav_AccountFlyout_clddrvhttps://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/DRGR57upTCuj1R_loEu8DQ?ref_=nav_AccountFlyout_cldd rvhttps://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/5EQvn_vhQKKQc4GZ4S3BwQ?ref_=nav_AccountFlyout_cldd rv
https://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/gNN6b9xERWGKJ-h_YPWU1Q?ref_=nav_AccountFlyout_clddrvhttps://www.amazon.co.uk/clouddrive/folder/CMgNi4OMRpChuT40Vw2CSg/XIT8CdtjTyiUCtab6K7RUg?ref_=nav_AccountFlyout_cldd rv


Hope that works!

It’s odd, I can’t see the links until I press “reply with quote”, but when I follow those links, the page says folders are empty.
Did you press Share on the photo, then “Get shareable link”? If it’s too much bother, don’t worry! Thanks.

[Bonky]

It’s odd, I can’t see the links until I press “reply with quote”, but when I follow those links, the page says folders are empty.
Did you press Share on the photo, then “Get shareable link”? If it’s too much bother, don’t worry! Thanks.

Strange. Please pm me and I can send the pics by email if you wish.

[RaggaMuffin]

Strange. Please pm me and I can send the pics by email if you wish.

Thanks, just sent pm.

[Bonky]

Hi,

5 pics sent by email.

R

[RaggaMuffin]

Thank you! Much appreciated.

[Bonky]

Thank you! Much appreciated.

Hi, please let me/us know your thoughts/conclusions..and what alterations -if any- you are going to make!

Best wishes,

[walpurgis]

Others here might appreciate a look at your photos Richard. If you feel like sharing :).

[RaggaMuffin]

Others here might appreciate a look at your photos Richard. If you feel like sharing :).

I could share them here, if that's okay with Richard?

[Bonky]

...of course...please do.

I'm sorry my first attempt at picture hosting didn't work as intended.

(You'll may need to rotate the images 90 degrees to match yours).

No copyright issues!

BW

[RaggaMuffin]

Hi, please let me/us know your thoughts/conclusions..and what alterations -if any- you are going to make!

Best wishes,

I notice on your board, you only have 4 decoupling caps, across D1 and D2 on each side. Was there a reason for RedHill Audio not installing all 10 caps, as per the Dada instructions? Also, what are those bits between the PS caps? They look like little metal plates bridging the leads.

Changes so far I'm going to make is reducing the input sensitivity to 1V by changing R6. However, I think before I do that I want to try and find the source of the distortion. I've also just noticed quite a loud hum when I tested it just now. I'm not sure if that was there before or not. I noticed a user called Light Dependant Resistor has a lot of ideas regarding the 306, so I might go through the posts he's made or might shoot him a message.

[RaggaMuffin]

Pics of Bonky's example:

https://i.imgur.com/wo3toXh.jpeg
https://i.imgur.com/jNEZagy.jpeg
https://i.imgur.com/K3shF5y.jpeg
https://i.imgur.com/c9Z1xJ6.jpeg
https://i.imgur.com/QWoToqV.jpeg

[RaggaMuffin]

Also, when measuring the voltages listed on the schematic, what is an acceptable +/- deviation % before I need to investigate a reading? When I did the Bottlehead Crack kit, it had a wide acceptable variance of +/- 10%.

[walpurgis]

Pics of Bonky's example

Good stuff. I have a couple of these nice amps.

[Bonky]

I notice on your board, you only have 4 decoupling caps, across D1 and D2 on each side. Was there a reason for RedHill Audio not installing all 10 caps, as per the Dada instructions? Also, what are those bits between the PS caps? They look like little metal plates bridging the leads.

Sorry...I don't know/recall. Are the 'bits' between the PS caps just surface-mount resistors or zeners~?

Changes so far I'm going to make is reducing the input sensitivity to 1V by changing R6. However, I think before I do that I want to try and find the source of the distortion. I've also just noticed quite a loud hum when I tested it just now. I'm not sure if that was there before or not. I noticed a user called Light Dependant Resistor has a lot of ideas regarding the 306, so I might go through the posts he's made or might shoot him a message.

Please see above.

[RaggaMuffin]

It seems to be working great now!

I changed R13 to a 27R, so the input sensitivity is now 1V. I would have thought that would only affect the amount of volume I need on the preamp, but it has got rid of the loud hum, speakers banging when I turn it on and gets rid of the distortion that was present at all volume levels.

Thank you to everyone for all of your help! I'm just going to do some more tests before I finally finish with this.