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Capacitors clicking when discharging?

Discussion in 'd.i.y.' started by loz_the_guru, Sep 14, 2021.

  1. a.palfreyman

    a.palfreyman pfm Member

    Yes. 480V would be 2mA or 1W. From what I read yesterday, the 440k are the high voltage ones as they act as a current source IIRC so indicate this is the +/-450V supply version.
    Just noticed that there are no 1000pF caps visible next to the 440k resistors on the board.
    loz_the_guru likes this.
  2. loz_the_guru

    loz_the_guru pfm Member

    You're not wrong about the missing 1000pF caps. In fact there's a couple of slots on the board without resistors too, in my ignorance I just assumed these were redundant? I have a few other photos of the other boards which for some reason didn't work, I'll try again when I'm home.
  3. davidsrsb

    davidsrsb pfm Member

    Very few resistors, even THT types, are rated for 900V or even 450V, whatever the power dissipation
    A Vishay 2W is only rated at 500V, a bit low in this amplifier. 3W gives you a safer 750V rating, but still not enough. I have used a few HV resistors in series in APD diode power supplies in the past just to get the voltage rating
  4. a.palfreyman

    a.palfreyman pfm Member

    Yes, my Stax valve headamp has large (2W?) resistors stacked in series.
    Be interesting to see what @martin clark says when he's had a look....
    I quite fancied having a go at one of these but the high Vs and cost (circa £1200 for the parts) put me off a bit. There is a guy in China (myheadfi, who I think was mentioned on the thread linked above) building these and selling on ebay for £1200.
  5. martin clark

    martin clark pinko bodger

    Loz & I are in contact and a provisional peek is arranged later this coming weekend.

    Thanks for the developing thoughts and observations from all contributors meanwhile - really helps focus thoughts on approach to diagnosis (as does, obvs, HV measurement safety issues ..)
    loz_the_guru likes this.
  6. 337alant

    337alant Negatively Biased

    PFM to the rescue, priceless :cool:
    loz_the_guru likes this.
  7. loz_the_guru

    loz_the_guru pfm Member

    My sentiments exactly! It's a fantastic community.
  8. martin clark

    martin clark pinko bodger

    Well -while @loz_the_guru can report his own thoughts - I'm glad to say we met earlier today, and spent more time chatting than really worrying too much about the amp!

    The punchline: yes, the ticking reported is thermal in origin, and the three 240K resistors @a.palfreyman first suggested as running warm. They are.

    Basically I sent some minutes going over it first with a bright light, and the boards themselves are clean- put together quite neatly enough and free of flux and obvious things that might cause a problem (dots of solder, debris, dust, not a hint of 'tracking' etc.) All the HV caps etc are reputable quality and appear suitably rated based on build notes in threads like those linked above by others.

    Power -up, quick nominal voltage check: all spot-on, rails bang-on 480v where noted, so initial thoughts are positive; power down. The supply HV rails bleeds-down close to 0v very fast. That's nice behaviour to know...

    Re-apply mains power and let it idle. I went over all the heatsinks, discrete parts and all resistors with my Fluke IR gun. Used close-up this has a fine enough field of view to clearly register the centre of a non-SMT transistor or a 2w resistor on a packed board. No immediate surprises, so left it on. The amp is totally silent mechanically, as is the transformer (which settled 4degC above ambient). Idling with the top off, the heatsinks on the power amp boards all settled quickly to modest and sufficiently- equal values - 38-42degC typ. All insulators on transistors in place, neatly assembled, clearances ok - no issues. Ditto on the supply board, including the 15ohm resistors which run significant current.

    But those three 240K resistors come immediately up to 65-68degc and after a bit of a soak, settle near 90degC - yes, with the lid off. That's rather too much for long-term reliability imo. In chatting about this, Loz commented the reported ticking had first been noticed when he'd had his dac sat on top, but didn't think it had happened since. Aha.

    Looking at the 240K parts more closely then closely - they are mounted right down tight to the pcb, and the leads are cut short and soldered tight -no pre-bend in the lead-outs , no possibility to expand at a different rate to the pcb, and indeed, while monitoring voltage after turn-off - I heard a very slight tick from the central one.

    tl;dr: Proposed remedy:

    The 240K resistors are dissipating up to 1w each, installed on a pcb footprint for 22.5mm lead spacing.

    I'm inclined to replace the three 240K parts with 5w metal-oxide resistors, spaced-off the pcb by 15-18mm. Plenty of space for this...and metal oxide is very, very robust against overload. A quick look at suppliers suggests the 5w size is available readily with a 700v+rating. Standing-off with a careful pre- bend in the legs will make space for the longer 5w resistor body, and allow much better convection around (and flex with any thermal expansion) without getting close to adjacent components.

    Only two other things I've suggested we address:

    1. yes, that indicator LED appears to be connected to the transformer AC input! That has to go; replace with standard 5mm part fed from tapping a 15v rail from the opamp servos would be ideal, and easy to do. We've left is disconnected for now.

    2. Also - there's no secondary insulation over wired connections to the IEC input socket and voltage selector switch. Again - easy to improve-upon. The case is properly bonded to mains earth, btw.

    I suspect, hope, this could lead to another meet, with more music, one evening TBA. Very nice to meet you, Loz! : )
  9. a.palfreyman

    a.palfreyman pfm Member

    Nice job @martin clark. I'm intrigued b'cos I noticed that there is a fourth 240k resistor at the bottom of the last picture Loz posted that doesn't to appear to have got as hot.
  10. loz_the_guru

    loz_the_guru pfm Member

    Sorry for the delayed update to this from my end, a busy day with kids and work. I came to update this but can’t add anything to the extremely thorough documentation above, although having met with Martin I don’t know why I would expect anything else! Thanks so much not only for the verdict, but also in talking all the various checks through with me and general re-assurance. I’m now happily listening to some Pantha Du Prince and feel much safer in the knowledge it’s unlikely to lead to a lethal zap.

    It was also fantastic to see and hear those beautiful Quad electrostats, and I would be very keen to do so again at some point in the future. I’ve already been chewing over a basement conversion to see if I could accommodate some.

    When I get some time at the weekend I will look into sourcing some 240k 5w metal-oxide replacements. I’ve just been doing a bit of reading on the difference between carbon film vs metal film – lots to learn still!
    martin clark likes this.
  11. davidsrsb

    davidsrsb pfm Member

    I have always been tempted by Stax headphones, but I get nervous of those voltages next to my head, especially when I see that the amplifier has not been as well engineered as it could have been for safety
  12. a.palfreyman

    a.palfreyman pfm Member

    This is a DIY job. You can get the boards / main parts from Mjolnir audio who also supply the finished article. Look really good, but to rich for me.
    Keep hankering after buying a set of boards though...
  13. davidsrsb

    davidsrsb pfm Member

    I know its DIY, but I have come across plenty of both DIY and commercial products that are potentially lethal.
    One of the first projects I worked on had a 3000V 1A regulated supply for a RF power valve. The designer of the PSU did a good job. Unfortunately the designer of the dummy load didn't and it exploded rather spectacularly as a long chain of power resistors flashed over to earth. It was like a Hollywood set.
    a.palfreyman likes this.
  14. martin clark

    martin clark pinko bodger

    Ha! I've actually got a pair of the Stax in-earphones... 580v of bias inside the pinnae...
    Last edited: Sep 21, 2021
  15. a.palfreyman

    a.palfreyman pfm Member

    Gets rid of unwanted ear hairs too....
    Yes of course you are right, no excuses for unsafe construction practices.
    martin clark likes this.
  16. loz_the_guru

    loz_the_guru pfm Member

    I think if headphones are your thing you owe it to yourself to give a set of Stax a go at somepoint. I've owned numerous sets of cans, all at a range of pricepoints and to my ears they just offer something different. I've gone through numerous iterations, starting off with an ancient SRM-1 Mk2 amp from 1986, and they've all sounded excellent.
    martin clark likes this.

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