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JBL 6290 noisy transformers

Discussion in 'd.i.y.' started by Chops54, May 14, 2020.

  1. Chops54

    Chops54 pfm Member

    I have a JBL Urei power amp and I really like this amp. It's a real powerhouse but the transformers are proper noisy. Is there anything I can dip them in which will silence them when cured? I'm not bothered how long it takes them to dry, I just want something that'll work. Any ideas anyone?
     
  2. cooky1257

    cooky1257 pfm Member

    I've heard power amp tranny hum can be caused by DC in the mains.
     
  3. Chops54

    Chops54 pfm Member

    I don't think it's that Cooky. I've had the amp running off a 230 volt mains conditioner and just sitting there at idle the trannies were rattling for England. They are 220 volt units so I'll admit that I'm over running them but I've other 220 volt kit that's dead quiet. I spoke to one guy who said they do often rattle but sat in a rack in a room at the back of a studio somewhere it doesn't really matter. It's a really good sounding amp and it's been sat on the floor in my workshop for several years so I think it's time I sorted it. I'm going ring round on Monday and see if I can find a motor rewind company with a vacuum laquering facility.
     
    cooky1257 likes this.
  4. Mike P

    Mike P pfm Member

    What is the mains conditioner? Does it actually block a meaningful amount of DC?

    What type of transformers are they (e.g. toroidal etc)?

    Have you measured your mains voltage? Mine is more like 250v.
     
    Last edited: May 15, 2020
  5. martin clark

    martin clark pinko bodger

    Fundamental question given its US kit - are these transformers actually designed for 60Hz..? What does the amp plate state as acceptable input?

    If the transformers are actually for US 60Hz input,albeit with 230v taps or similar - then the cores are undersized, would need to be min 60/50 = 120% the cross sectional area they are at present winding count : else they will be badly saturated, and rattle, and the real fix would be to replace with equiv (or better) VA wound for 240VAC/50Hz.

    (If this is the cause and the amp requires a bunch of customary secondary windings not readily found off-the-shelf, you'll probably best off talking with someone to make what you need - Canterbury Transformers, Primary Windings or sim in the UK. Sorry!)
     
    Mike P likes this.
  6. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    They should be powered from the correct 220V for a start. Yank gear is made for their 60Hz mains and ours being 50Hz means they're under increased stress anyway! All this will tend to make them buzz away...
    Being yank they will likely be non toroidal transformers so check the bolts that usually hold the laminations together. They may be loose and in fact sometimes there can be a sweet spot tightness wise where there is the least noise.

    As Mike hints there are mains conditioners and mains conditioners.. if we are talking a mains regenerator then there should be no DC but some "conditioners" are just RF filters and will let DC straight through! And many places around UK are indeed nearer to 250 then 240V.
     
    Mike P likes this.
  7. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Ah there was a pre echo from Martin whilst I was typing:)
     
    martin clark likes this.
  8. martin clark

    martin clark pinko bodger

    fools seldom differ ;)
     
    Arkless Electronics likes this.
  9. Chops54

    Chops54 pfm Member

    My mains at my home is around 245 volts. I know the amp is American Martin but it came over from Europe though I have no other info. It's dual mono with two large frame transformers. The mains conditioner I borrowed came from a friend I used to work with. It was borrowed from a job we were on and as far as I was aware it had a dc blocking circuit inbuilt. The amp is under a pile of stuff in my office so I'll dig it out later on and see if there's any info on it anywhere. I did consider having another pair of transformers made or having the originals rewound and I did speak to Terry at Canterbury Windings but he doesn't do frame trannies and couldn't help. Tbh I don't think the amp is worth spending that sort of money on. I was going to try a bucking transformer but that conditioner I borrowed put out 230 volts and it didn't consider 10 volts extra excessive.
     
  10. martin clark

    martin clark pinko bodger

    I've found a copy of the manual, but it so US-centric it's not clear if a 50hz variant was made - all reference is to US mains operation.
    Basically the PA section requires about 65-0-65v AC rails for full rated output, and a 12-0-12vac winding for the monitor/relay circuits. it uses a separate transformer, type unspecified, for each channel.

    60-0-60 Vac windings, x 2, would result in negligible drop in output, might be easier to source....

    It's a damn powerful beasty for sure!
     
  11. Chops54

    Chops54 pfm Member

    Martin, there's nothing anywhere to say what frequency it requires. The transformers simply have 220V stamped on them and on the rear panel. There's Instructions in French on the rear panel too but they all seem to have that. I do know they did 240 volt transformers because I've seen one. That one also had rattling transformers.
    I've already had the transformers out and fitted new bolts to tighten everything up but to no avail. They are pretty big! There are two fuses, one for each channel, rated at 6.3 Amps so that should give an indication of the power rating of the transformers? I was convinced it was the laminations rattling hence my thoughts about dipping them. However I realise that would make them difficult to rewind if I really had to. I would consider that if it was cost effective but I've been laid off and can't justify spending big money on it atm. I have a 300va toroid somewhere with a 25V secondary so maybe I could try bucking to get down to 220V?

    Forgot to say, yes it's extremely powerful and sounds really nice with my IMFs on the end of them.
     
  12. martin clark

    martin clark pinko bodger

    OH, if you have that transformer to hand, yes, absolutely it is worth trying to feed it off a supply bucked-down to around 215VAC or so; it'll only cost your time and a little care; the voltage drop will have negligible effect on power delivery for home use, and at 300VA that solution should support <scribbles a bit> way more than the amp could draw in most use domestically. Way more.

    Take care; and utmost sympathies on the lay-off - that's a shite state of affairs and no mistake. Best wishes on that front, too.


    *Afterthought:
    (215/240)^2 = 0.80; might be enough drop in primary voltage, hence max core flux unloaded, to offset the '60hz transformer problem' if that is the cause. It certainly cannot make things worse here. Good luck!
     
  13. Chops54

    Chops54 pfm Member

    I went on Rod Elliots site and he reckons a 200va transformer is good for 2.2kva when used as a bucking transformer so I think I'll try that when I have an evening spare. Won't take me more than an hour or two to make up a decent wood box to put it in.

    I'm not too bothered about being laid off atm Martin. I just need to be a bit careful with the finances until things pick up again. The company I worked for managed to get essential work status but all the bosses buggered off to work from home and left us plebs to it. However as work began to slack a bit due to covid they laid off all non productive contractors so that was that. The covid thing doesn't worry me too much but I do feel safer at home not having to come in contact with other lads. That sounds really bad doesn't it.

    Thanks for kind thoughts.
     
  14. martin clark

    martin clark pinko bodger

    OT but must add:

    'That sounds really bad doesn't it.'

    No, it really does not; esp if one has any kind of underlying health issue, at any age; or even if you live alone - have family / people close to also care-for (inc wish to visit sheltering Rellies ASAP, as I do)/ might expose onward. No-one wants to be a vector; so - to my thought - this is the one time such a directed naked self-interest, is not a bad thing - because it serves the General good at least as well.

    And this too shall pass.

    (As for employment - I suspect we are seeing the same thing from different perspectives on the same sector here (construction) - my view is, there is a lot of hardship yet to come & play-out. Happy to take such thoughts or discussion to PM if you wish;
    And, again - ATB)
     
  15. Chops54

    Chops54 pfm Member

    Thanks Martin :)
     

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