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Tannoy HPD "Rebuild"

Discussion in 'd.i.y.' started by Vinny, May 23, 2020.

  1. Vinny

    Vinny pfm Member

    I have some 12 inch HPDs that will be getting re-coned, but before that I am going to try to fix what seems to be a regular problem with Tannoy drivers - the counter-bores in the mounting holes have largely stripped due to insufficient difference in diameter between bore and counter-bore. Because the metal of the basket is soft, the counter-bore is easily stripped so that the drivers have to be mounted using plug-ugly bolts and/or washers that pull up on the front face of the basket.

    If I fail, nothing is lost and the speakers will be as they are now. If it works, the drivers will be secured in the cab's by flush-mounting M5 SS set screws.

    So, linked to that.
    As part of re-coning, are the formers and voice-coils transferred, or are new ones fitted as part of the new cone assembly?
    As part of re-coning, are the foam strips that fill the galvanised U segments, that in turn secure the foam to the basket, replaced?
     
  2. cooky1257

    cooky1257 pfm Member

    I don't fully understand what the problem or the fix is regarding the baskets..pics?

    Assuming it's a genuine cone kit then everything cone,surround,spider, voice coil are included. I doubt they give you fresh front gasket foams though.
     
  3. Vinny

    Vinny pfm Member

    Thanks, as ever.

    Originally, the unit would have been secured to the cabinet using 4 fasteners (bolts or screws) that sat with heads recessed into 4 lugs that were cast as part of the basket. So two bores each position, one to take the shank of the fastener and then a larger one that took the head of the fastener. The fastener tightened up on the land between the two bores and the head sat recessed into the front of the basket.

    All of the 8 on the two drivers here are badly or totally stripped so that there is effectively just a single bore. To mount them, you have to use a fastener (and washer) with the head proud on the front of the basket. Which I think looks a mess.

    Having done some hunting through thread tables and bolt dimension charts, I can drill the mounting holes on the basket to 8mm (they are basically that already) and, assuming the drill cuts marginally over-size, and as the basket material is soft, CAREFULLY and SLOWLY tap 3/8" UNF for around 5mm from the reverse side of the basket, the magnet side. (Tapping drill for 3/8" UNF is actually Q - about 8.4mm.)

    I have some 3/8" UNF SS studding that I have got drilled 5mm down the centre and will make some 5mm long bushes that I will Loctite into the baskets. (They will be cut comfortably over-length, inserted and then carefully taken back to the reverse side of the basket.) If it works, I will be able to mount the drivers using 5mm SS cap head set-screws that are recessed into the front of the basket. If it fails, everything is back to square one, no harm done, barring any cock-up or mishap from me.

    The idea is just to reinstate the minor bores that have been stripped.
     
  4. Tony L

    Tony L Administrator

    Ouch! Sounds like they’ve been to a ‘Linn-tight’ dealer! My Golds, which are obviously older, don’t even have any scratches in the paintwork around the bolt-locations, let alone gouges. It is unfair to blame the design for end-user abuse.
     
  5. Vinny

    Vinny pfm Member

    Apart from the stripped bores, they are pretty damned good in appearance. One of the paper labels is missing.

    I see what I assume is the same problem on pic's of other drivers, otherwise why use a whacking great bolt, head proud of the front of the basket?

    They won't take much heave-ho as the cast material seems, to me at least, to be quite soft.
     
  6. Tony L

    Tony L Administrator

    You don’t need to bolt a loudspeaker driver beyond finger-tight and maybe a further 8th of a turn maximum. To seriously damage a massive cast aluminium basket would take the sort of idiocy seldom seen outside a 1980s Linn dealer. What you have sounds like the equivalent of an Ittok or Zeta with awful gouges on the bearing shaft caused by a some prat doing up the VTA bolt way, way to tight. I remember taking to Roger at Lockwood before he retired and he implied some people actually bend the baskets by over-tightening them. Have you checked it is still true?
     
  7. Vinny

    Vinny pfm Member

    HOLY CRAP!!

    I haven't, but haven't noticed anything, and dead easy to check once I have dismantled them far enough to do the bores. (One done so far, so will check later.)
     
  8. Vinny

    Vinny pfm Member

    Another question - is it normal to glue the outer part of the foam into the recess of the basket, the part of the foam used to secure the foam to the basket?

    I have disassembled that far and the acres of crap stuck to the basket on that land, under the foam!!!
     
  9. stevec67

    stevec67 pfm Member

    An 8mms drill will make a hole about 8.2 in a hand held drill. Go carefully with your tap, you are not removing much metal but 0.2mm too tight on the tapping drill can make it bloody tight. Last time I did this i was tapping some crappy Mazak casting M8 and thought I would get away with a drill that was"close enough ". The bloody casting split. Steel is more forgiving, but you don't want to snap a tap in there either.
     
  10. Vinny

    Vinny pfm Member

    TBH, I am probably going to splash out on a Q drill, but at least £5-10 for an 8.4mm twist drill just seems a hell of a lot. Compared to what they are worth, and the cost of reconing…...

    The whole lot is going to be done on a pillar drill, including starting the 1st tap - I am going to have to be very close to true to get a good finish.
     
  11. Vinny

    Vinny pfm Member

    Drill ordered, actually around £13!!!

    Snapping a tap is very unlikely - 3/8 in basically aluminium, no real depth, but also not really a worry - that, I could get EDM'd out at work easily enough, but cracking the casting would make the air more than rather blue!
     
  12. stevec67

    stevec67 pfm Member

    That's expensive. Still it's bought now. I would have used 8.5, that's a standard metric size and dirt cheap. Yes, you will have shallower threads and lose strength, but at 3/8 UNF, this being close to M8, you have enormous strength to spare.
     
  13. Vinny

    Vinny pfm Member

    There is only around 5mm of thread, it is shallow (being UNF), it is in aluminium and there ain't much meat left in the boss cast into the basket. I was/am very reluctant to go over size, even by 4 thou.

    I spent AGES working out what was likely to be the best thread, so I am sure that there is no room for using another, unless you go into obscure things like half metric sizes, which would likely be impossible to buy studding for.

    I may actually try to drill SLIGHTLY off current centre, to take most metal off the meatier part of the bosses. I'll get things set-up and see what is possible. Either way a Q drill will do little more than clean-up the existing bores. Maybe start with an 8.0 and then the Q? It depends on what is possible.
     
  14. stevec67

    stevec67 pfm Member

    5mm is OK, it's about the size of a half nut. I'm sure you'll be fine, it's hardly got your granny hanging on it.
     
  15. martin clark

    martin clark pinko bodger

    For such drilling & tapping (or machining) aluminium alloys - white spirit is one lubricant of choice and paraffin (or wd40!) works well too.

    Use liberally, in so far as is reasonable in situ (even a cottonbud dipped in it to dab the cutting tool at small scale) - it will help reduce galling, esp. swarf build up on the cutting edge and so make for a free & clean cut.
     
    Vinny and Mike P like this.
  16. cooky1257

    cooky1257 pfm Member

    The HPD and Golds cones are clamped in position under/ by the u brackets, you can glue them using just tacky pva but they are meant to be clamped( after the basket and the brackets have been cleaned!)
    You can distort the basket by uneven torque but you have to go mental tight to do it.
     
  17. Vinny

    Vinny pfm Member

    First one stripped and cleaned. Basket still true as near as feck it is …………………

    HORRENDOUS amounts of crap removed from the land on the basket where the foams are clamped by the U brackets. SERIOUSLY half-arsed job of refoaming done previously.

    I just hope that the gold paint that I have is a close-ish match, not that more than next to none is visible when the drivers are mounted in cab's. The main thing is to protect the cast aluminium from general corrosion.

    Talking of corrosion... In aluminium, this is generally driven by galvanic action - dissimilar metals in contact, forming a battery which leads to corrosion (oxidation) in one of them. So long as the paint is intact this can't happen with the baskets, but the likely source is the plated steel screws that hold the U brackets coupled with the fact that at some stage the bore(s) that I am trying to repair have been abused and stripped of paint.

    How about using stainless or even nylon screws to secure the U brackets? Anyone any idea of the thread? (Bound to be imperial/BA!!)
     
  18. cooky1257

    cooky1257 pfm Member

    I appreciate the desire to get some nice flush hex socket bolts flush into the frame but if you want original they came with hex bolts/washers that butt up to the lugs cast into the frame(or slots if they are very early HPD's. Fraser of this parish has his mounted like you plan, looks nice n tidy.
    [​IMG]
     
  19. Vinny

    Vinny pfm Member

    That surprises me, mostly because they were held into their cab's with screws and washers with the heads below the front face of the basket. A couple of the bores are parallel, a couple have the best part of two bores left and the rest are in-between.

    If bolts were used originally, heads on the outer face of the basket, someone has already counter-bored them to sink the mounting screws/bolts into the basket.

    Were Golds counter-bored? Because Tony implies that his are.
     
  20. cooky1257

    cooky1257 pfm Member

    As they (Golds) were almost universally rear mounted the bolt heads are sunk into the front side of the baffle so cant see the point tbh.
     

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