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Thread: Quad 306 Fixer Upper

  1. #1
    Join Date: Nov 2014

    Location: Leicestershire

    Posts: 153
    I'm Mark.

    Default Quad 306 Fixer Upper

    Just got myself a Quad 306 (M12890-6, latest board), with one working and one dead channel (just hum).
    I haven't powered it up yet, but there are a couple of burned out resistors that are obvious, and some weird looking 'residue' around a couple of the Power Transistors. Ran a few very brief checks, there are no short circuits on any of the transistors, so I'm hoping that they are OK...

  2. #2
    Join Date: Nov 2010

    Location: Sheffield/Peak District. UK

    Posts: 374
    I'm Richard.

    Default

    I got mine mended and 'breathed upon' by RedHill Audio nr Chesterfield.
    Usual disclaimers.

    bW

    Bonky
    My Gear:
    Source component/s:
    Linn Mimik2 Cd player; Inspire Apollo deck + Roksan Nima arm + AT-0C5MC; Pioneer F656 tuner; Cambridge DAB500; Aiwa R650 Cassette Deck. Logitech Squeezebox Touch
    Amplification:
    Sugden A21a; Mini-T (amptasitc). Mission Cyrus 1
    Loudspeakers:
    Living Voice auditorium IBX-R2
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    N/A
    Any other hi-fi bits:
    Beresford Caiman + Gato DAC; Apple's airport express; Apple iMac i5; Senn HD598, 467, 650s. Beresford Bushmaster

  3. #3
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 848
    I'm Chris.

    Default

    Hi Mark

    R31 is a 2k2 resistor placed in the negative rail, as part of the virtual earth circuit
    It is very unusual for it to be a problem - which indicates T10 a RCA 17556 or Motorola MJ15003
    might need replacing, as well as T12, and obviously R32 the 10 ohm ground lift resistor and R31.

    Also the heatsink needs a bit of a cleanup where something has spilled.
    which might indicate the TO3 silicon washer is no longer insulating, if its a burn mark
    that seems to indicate a fair bit of current trying to access the heatsink metal, or something spilled,
    to create a short.

    On the plus side the 306 is a breeze to work on, and one of the best amps Quad made.

    Carefully check , particularly transistors T10 and its driver T8
    and T12 a ZTX 750 and replace the 2 burn't resistors. Fingers crossed should be all good then.
    The circuit breaker will always engage before the fuse to protect the 306

    At a minimum, power off unplugged when working on the 306 , obtain some solderwick for cleaning up the excess solder
    when replacing parts, and the usual repair tools like side cutters.

    Let me know if I can help further.

  4. #4
    Join Date: Nov 2014

    Location: Leicestershire

    Posts: 153
    I'm Mark.

    Default

    Quote Originally Posted by Bonky View Post
    I got mine mended and 'breathed upon' by RedHill Audio nr Chesterfield.
    Usual disclaimers.

    bW

    Bonky
    Cheers Bonky,
    My intention is to repair, then do the recommended upgrades (bypass caps around the Op Amp Zener diodes, halve the input sensitivity, low ESR PSU caps etc etc)...

    Sent from my LG-H850 using Tapatalk

  5. #5
    Join Date: Nov 2014

    Location: Leicestershire

    Posts: 153
    I'm Mark.

    Default

    Thank you for the tips LDR, much appreciated.
    I measured the resistances between E, C, and B of all 4 power transistors - comparing left to right, they were the same values. I'm hoping this means they are OK. But, that is unlikely I guess, or the ground lift resistor wouldn't have burned out?
    I found a schematic with test point voltages for the M12890-3 board, I can't find anything for the -6.
    Cheers, Mark.

    Sent from my LG-H850 using Tapatalk

  6. #6
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 848
    I'm Chris.

    Default

    Quote Originally Posted by M6NTL View Post
    Thank you for the tips LDR, much appreciated.
    I measured the resistances between E, C, and B of all 4 power transistors - comparing left to right, they were the same values. I'm hoping this means they are OK. But, that is unlikely I guess, or the ground lift resistor wouldn't have burned out?
    I found a schematic with test point voltages for the M12890-3 board, I can't find anything for the -6.
    Cheers, Mark.

    Sent from my LG-H850 using Tapatalk
    Hi Mark
    You could try just replacing the resistors. r31 and r32 , they will either remain intact
    or repeat what happened to them Any visual damage is like a map
    explaining where and what happened. If lets say a liquid was spilled on or near TR10
    R31 and R32 would fail. If R32 went first then the transistors might be OK, but as I
    say R31 is a bit strange to be damaged. I noticed my own 306's have 1watt resistors
    for both, whereas your R31 looks to be 1/2 watt. The circuit breaker will instantly
    pop out if there is an ongoing fault.

    The voltage measurements for Version 3 and 6 will be similar, so Version 3 is good as a guide.

    Here are a few suggestions once it is stable. The current dumping bridge is fractionally
    out with regard to exact balance, so can be corrected with making a change to value of resistance
    It is getting a bit late here so will add the detail to this tomorrow

    Peter Walker Quad's founder did not just see the 306 and 606 as another amplifier,
    rather as I see it, he saw the 306 and 606 as a culmination of all of his knowledge
    and Mike Albinsons knowledge into current dumping. Sadly when the numbers
    and accountants became involved there was buying in of parts - some very good
    but others less so. You will notice with the 306 much copying of what was being
    worked out for the 909. The real essence of the 306 606 909 amps is the use of
    course of current dumping, but also fet components in the opamp and current source
    and input transistors replacing a op amp used in the 405 and 405 mk2 .

    Although I have been critical on forums in the past of the TLC271 - It is actually the very
    best part in that location. The 306 becomes a bit unstable with any other op amp, and after
    many listening tests the TLC271 as used by Quad is extremely good. So leave the TLC271
    in place. It has just the right slowness and therefore needed characteristic in that part of the
    circuit to be perfect.

    The current source labeled CR2 j503 I would argue has insufficient current capability for the
    emitter of T1 and as well the base of T3. What I did with mine was to use a 1.3ma
    current reference vs 0.56ma for a J503. I may revisit this, adding a separate current source for each.

    Cheers / Chris
    Last edited by Light Dependant Resistor; 01-05-2017 at 11:06.

  7. #7
    Join Date: Nov 2014

    Location: Leicestershire

    Posts: 153
    I'm Mark.

    Default

    Chris, thank you for the detailed reply - much appreciated.
    I have read about the bridge imbalance, and Dada Electronics sell an upgrade kit including low ESR 6,800μF PSU caps, revised component values for the bridge (1% tolerance resistors etc), and a number of 100nF caps for various locations - it's not an expensive kit, and saves sourcing bits from here and there!
    Your help is greatly appreciated!

    Sent from my LG-H850 using Tapatalk

  8. #8
    Join Date: Nov 2014

    Location: Leicestershire

    Posts: 153
    I'm Mark.

    Default

    Well, it's patently obvious that somebody has been 'at' this Amp already.
    The Toroidal Transformer secondaries have been unsoldered, and it looks like T9 and T10 may have been unsoldered/resoldered too.
    The spill was something very gluey - almost like contact adhesive to (partially) clean off.
    Although the transistors measure almost identical channel to channel, with no shorts at all, I am (assuming I can source them) giving serious thought to just replacing them all...

    Sent from my LG-H850 using Tapatalk

  9. #9
    Join Date: Nov 2014

    Location: Leicestershire

    Posts: 153
    I'm Mark.

    Default

    The Right Channel of the Amp is most definitely poorly-sick.
    The Left Channel output has a totally healthy 0.5mV (half a millivolt) DC offset, but the Right Channel output is more than 17V - enough to destroy any speaker connected to it!
    Furthermore, the PSU symmetrical DC supply on the left channel is 30-0-30V, but the right is 15-0-40.
    I'm going to remove the Power Transistors and see what happens to the DC PSU symmetry...

  10. #10
    Join Date: Nov 2014

    Location: Leicestershire

    Posts: 153
    I'm Mark.

    Default

    Further work on the faulty channel of the Quad 306 Amp.
    Testing diodes and transistors in circuit is very ambiguous, so I'm taking them out to test them.
    All my suspects so far test perfectly OK.
    Dang it!!!

    Sent from my LG-H850 using Tapatalk

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