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Thread: Quad Disaster

  1. #11
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 1,254
    I'm Chris.

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    Quote Originally Posted by meBob View Post
    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.

  2. #12
    Join Date: Aug 2017

    Location: Liverpool, Merseyside, England

    Posts: 141
    I'm Rob.

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    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.

  3. #13
    Join Date: Oct 2014

    Location: Surrey

    Posts: 131
    I'm Graham.

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    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

  4. #14
    Join Date: Aug 2017

    Location: Liverpool, Merseyside, England

    Posts: 141
    I'm Rob.

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    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.

  5. #15
    Join Date: Jan 2009

    Location: Essex

    Posts: 20,122
    I'm openingabottleofwine.

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    Quote Originally Posted by nutteronthebus View Post
    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.
    Barry

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