Quad Disaster

[meBob]

Hello All,

Recently I bought a 405-2 from a private AOS seller and the amp was as described by the seller, that is to say recently serviced by Quad, fully working and sounding good. I am happy with the amp and decided to do a few mods, such as lower the sensitivity to 1.5v and add higher capacity psu caps, there were also a few other Bernd Ludwig mods I was concidering.

I took the top cover off and had a look around, the boards are a 12565 ver 6 and everything was pretty much standard except for the new Kemet 10000uf psu caps, having never been inside one of these before I had gentle root around, but perhaps not gentle enough.

With in minutes of switching back on the right channel started to fail, becoming blared and then suddenly dead (very dead!), initially I thought it was the speaker and after testing it found it wasn't. Upon opening up the amp again, I quickly discovered the problem, one of the LED wires had come away from its solder pad, where it had only been just slightly embeded into the solder, all it took was the slightest of nudges to free this wire from the solder.

After testing the good and bad board side by side and comparing values with a multimeter, TR9/TR10, TR8, D13, D5, D10, N1/N2 are all dead, I feel lucky the short only damaged the right hand side. I actually stopped testing and decided to replace all the transistors and Zeners/Diodes, I also don't want to have different TR9/TR10, TR7/TR8 transistors on each board and so both boards will get the same new ones.

I'm really pissed off with the workmanship, I was careful with the wires and it took no pressure at all to dislodge the LED wire.

I am now where I am and it's in for a penny in for a pound, It's become a full project I'll be chucking money and time at it.

N1/N2 are not replaceable and so they'll have to go from both board along with R35/R36 and C18/C19.

I'd like to also replace C15/C16 and C11 but I'm not sure what types they are?

My mindset is to replace anything (or future proof) now while the boards are out. If there's any anything else that might need

attention I'd be grateful for opinions.

Cheers.

[Bonky]

Commiserations!

Two thoughts:
1. a DADA upgrade kit: http://www.dadaelectronics.eu/shop/quad-upgrade-kits

2. Get professional to look at it. I used RedHillAudio (local) to look at my Quad 306 with the DADA mods.

Also I think from EBXX one can get new boards (Chinese clones actually) but look carefully for the version number.

Keep us informed please.

[nutteronthebus]

The problems with the 405-2. R7 and R8, 3.3k between the 50V rails and the 15V zeners D1 and D2 for the 741 op-amp. If/when one goes open circuit it kills lots and lots of silicon.

They dissipate 0.37W and the part is a 0.25W rating, mounted hard to the circuit board. Replace them with metal oxide 1W part, Farnell 1357889. And space them off the board to allow air to circulate.

The other thing that need attention, depending on the issue of the board, is C10. That can be directly in the heat plume from R30 and R31, which cooks the capacitor - but only on one channel! Anyway replace those too, and mount the replacements on the back of the board to prevent them suffering the same fate.

Dave

[meBob]

Hello Bonky,

Already got the dada kit but need to sort out the above mentioned problems first.

I've moded/fixed a few pieces of kit, going to a pro will be when I've run out of ideas, I'll leave it to last.

I'm trying to remain original as possible, It's a snob thing, but I think it's the right thing to do.

Thanks for your reply.

[meBob]

Hello nutteronthebus,

All of the components mentioned are in the dada kit and I'll get around to it when I'm absolutely sure everything else is working. I'm more worried about resistors or anything else that may have gone wrong.

Thanks for your reply.

[Light Dependant Resistor]

Hello All,

Recently I bought a 405-2 from a private AOS seller and the amp was as described by the seller, that is to say recently serviced by Quad, fully working and sounding good. I am happy with the amp and decided to do a few mods, such as lower the sensitivity to 1.5v and add higher capacity psu caps, there were also a few other Bernd Ludwig mods I was concidering.

I took the top cover off and had a look around, the boards are a 12565 ver 6 and everything was pretty much standard except for the new Kemet 10000uf psu caps, having never been inside one of these before I had gentle root around, but perhaps not gentle enough.

With in minutes of switching back on the right channel started to fail, becoming blared and then suddenly dead (very dead!), initially I thought it was the speaker and after testing it found it wasn't. Upon opening up the amp again, I quickly discovered the problem, one of the LED wires had come away from its solder pad, where it had only been just slightly embeded into the solder, all it took was the slightest of nudges to free this wire from the solder.

After testing the good and bad board side by side and comparing values with a multimeter, TR9/TR10, TR8, D13, D5, D10, N1/N2 are all dead, I feel lucky the short only damaged the right hand side. I actually stopped testing and decided to replace all the transistors and Zeners/Diodes, I also don't want to have different TR9/TR10, TR7/TR8 transistors on each board and so both boards will get the same new ones.

I'm really pissed off with the workmanship, I was careful with the wires and it took no pressure at all to dislodge the LED wire.

I am now where I am and it's in for a penny in for a pound, It's become a full project I'll be chucking money and time at it.

N1/N2 are not replaceable and so they'll have to go from both board along with R35/R36 and C18/C19.

I'd like to also replace C15/C16 and C11 but I'm not sure what types they are?

My mindset is to replace anything (or future proof) now while the boards are out. If there's any anything else that might need

attention I'd be grateful for opinions.

Cheers.
Hi Rob
My understanding is you are jumping at conclusions too quickly with this repair.
The first boards with N1 and N2 fitted were 12333 issue 7 according to the service data manual
and D13 was first fitted at 123333 issue 8

Firstly check the fuses, noting Quad fitted the former crowbar circuit designed to blow the fuses
which is based around a diac and triac

Looking at back where the fault arose with the display led is current limited by R40 originally is a 2k7
which has an outrageous 18ma then across the led, replacing it with a 15k is advisable where 3.3ma
then is powering that led. The original resistor has likely been heating so much that it weakened the
solder joint.... but that would not cause a general fault as you are indicating as the circuit will tolerate
18ma being shorted. However I would check with your multimeter R23 that it is 1k2 and R16 180R
and at the same time R39 the 10 ohm zobel resistor. also R2 a 10 ohm if fitted.

In all my time with 405's I have never seen damage to N1 and N2 - replacing them is not a simple matter
as they are integral with the circuit.

As for C15 and C16 they are 100n and need to be 63v preferably 100v types.

Given the extent of damage to one channel, I would simply contact Quad for a replacement board
and do the change of r40 to be 15k. It should then run happily and reliably for many years to come.

[reddish75]

Isn't the led on these directly connected to the large caps? Did the wire come loose and brush the circuit board?

[Light Dependant Resistor]

Isn't the led on these directly connected to the large caps? Did the wire come loose and brush the circuit board?

The led and R40 resistor are on a very small circuit board under and on the right of the front panel. The wiring is standard a green and red twisted pair
so it would require both wires desoldering or cutting to come adrift. The red wire has 50v or just over positive DC on it and the green is the circuits ground.
The resistor to drop current as I pointed out is far too low in resistance value and any servicing would include changing it to be 15k. This will have the
pleasant effect of dimming the led brightness.

The original led used is a Hewlett packard part and has the feature of have a lower forward voltage than any comparable red led available today, as I recall
its forward voltage is 1.4v.

[meBob]

Hello LDR,

Thanks for your reply.

Both board fuses are well and truly blown.

R40 is good at 2.689kohm. I'll change this to 15k as recommended.

R23/R16 are ok, R39/R2 are alive but out of range at 11.5 ohms (they're still in circuit so the readings might not be too accurate).

I've sent an email to Quad enquiring price of a new board, but i'd rather go down the DIY route. If not too expensive I'll probably go down that root to save time.

If the worst came to the worst with N1/N2 I've seen on Dada that desoldering N1/N2, C18/C19 and replacing R35/R36 with wire links would solve my N1/N2 problem?

C11,C15,C16 do you know if these are polypropylene or polystyrene?

[meBob]

Hello reddish75,

The wires are twisted pair as stated by LDR and also not stranded, they're quite stiff and would not have come into contact with the boards.

Thanks for your reply.

[Light Dependant Resistor]

Hello LDR,

Thanks for your reply.

Both board fuses are well and truly blown.

R40 is good at 2.689kohm. I'll change this to 15k as recommended.

R23/R16 are ok, R39/R2 are alive but out of range at 11.5 ohms (they're still in circuit so the readings might not be too accurate).

I've sent an email to Quad enquiring price of a new board, but i'd rather go down the DIY route. If not too expensive I'll probably go down that root to save time.

If the worst came to the worst with N1/N2 I've seen on Dada that desoldering N1/N2, C18/C19 and replacing R35/R36 with wire links would solve my N1/N2 problem?

C11,C15,C16 do you know if these are polypropylene or polystyrene?


Regarding N1 and N2 it is not just a simple matter of desoldering those parts. The parts Quad used in earlier boards at that position tell us that
substituting N1 and N2 requires then placement of earlier board components R26 75R , R24 22 ohms D3 a diode ,R27 a 15k, R28 a 75 ohm R25 a 22
ohm D4 and R29 15k which are all needed for the circuit to function correctly

As for R35 and R36 these are resistances of 0.91 an ohm each. Each part Quad used was arrived at to provide an amplifier with low distortion and ability
to drive loudspeakers.

You would need with any substitution ( other than the led current limiting resistor R40 ) to have ability to measure distortion before and after.
C11 is a 1 nanofarad capacitor , whereas C15 and C16 are 100 nanofarad. My advice is to use these exact values particularly observing voltage
tolerance. The construction of capacitance material is or should be way down your list of priority at the present time. MKP is one type of
capacitor where each value is available

Your best option is to restore the 405 to its original condition by installing a new board from Quad- forgetting things like removing current limiting and doing partial mods you have seen
at Dada. Quad amplifiers have a good reputation for reliability and performance, none of which can be achieved by removing parts.

Alternatively save up for either Net Audio 405 Mk 3 boards or indeed Dada boards, which I know in the case of Net audio boards offer improvement in lowering distortion
compared to a standard 405 board.

[meBob]

Hello LDR,

Any N1/N2 removal would be problematic with all those resistors involved, best to avoid that.

I'm waiting to hear from Quad and hoping their boards are a reasonable price.

The .Net audio mk3 boards to me seem like a set of mods too far, I've read a few reviews on them and I get the impression they sound a bit clinical compared to an original 405-2, very detailed etc, but moving away from the original sound.

My original moding intentions were just to lower the sensitivity to 1.5v, I have a very nice preamp and all the action was condensed before the 9 o'clock position on the volume knob. Also some larger capacitors and maybe a new opamp, but I would have done the other mods first and then listened to see if the opamp was needed. I do like the Quad 405-2 sound and wouldn't want to move too far away from it.

Thanks for the help, I'll post updates as things happen.

[graham67]

Hi bob, a good compromise is the net audio dual mono psu (or similar) into the std quad boards with sensitivty adjusted to 1.5v.
I have a quad serviced 405-2 and full net audio upgraded 405-3 and consider the dmpsu makes the most cost effective upgrade without changing the quad tone. It opens out the soundstage and improves bass without adding any speaker hum which the 405-3 can do. The 3 has much greater grip and transparency and still sounds very Quad- like (not at all clinical) but loses a bit of the warmth of the 405-2.
Graham

[meBob]

Hello graham67,

That dmpsu looks interesting and the lowering the sensitivity down to 1.5v must happen to improve my system. I'm happy with the overall tone of the 405-2 and have been able to listen for long periods without fatigue (which is something I suffer), the 405-3 if sacrificed any of the 405-2's warmth I'd have to avoid it.

I'm now faced with problem of simply getting the thing working again or simply selling it for spares/parts for next to nothing, I'm still waiting for Quad to get back to me with their prices.

Thanks for your reply.

[Barry]

The problems with the 405-2. R7 and R8, 3.3k between the 50V rails and the 15V zeners D1 and D2 for the 741 op-amp. If/when one goes open circuit it kills lots and lots of silicon.

They dissipate 0.37W and the part is a 0.25W rating, mounted hard to the circuit board. Replace them with metal oxide 1W part, Farnell 1357889. And space them off the board to allow air to circulate.


Agree. I had a 405-1 where the zeners were 12V, so the 3.3K resistors dissipated 0.44W. One of them went OC causing the amplifier to go into a paroxysm of oscillation. No actual harm was done to the amp, but I replaced both with 1W rated devices, which being larger were spaced off the board. At the same time I replaced IC1 along with a few other mods.