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Amp borked, what to do..

Discussion in 'Sound City' started by benneh, Jun 11, 2011.

  1. benneh

    Sgarrista

    Joined: Apr 9, 2004

    Posts: 9,162

    Location: Nr. brumijum

    I'm just wondering really, my amp blew up earlier. It's just an old 80's Technics amp, but I'm quite fond of it - so I'd like to get it repaired.
    Normally I'd have a go myself, but I've had it apart and I can't find any bulging capacitors or burned resistors etc (basically whenever I turn it on the VU meter for the left channel maxes out and I get an almighty hum/signal).

    Firstly I suppose, is it worth me buying a multimeter and having a poke around?, how easy is it to fault find?. (I've built a couple of cmos amps in the past, but it was basically solder by numbers..).
    Or am I best handing it over the the local tv/stereo repair man?.

    :)

    As an aside. It happened when I switched (amp) inputs between my on-board sound card and my m-audio 1010Lt. And a mixer that the M audio was connected to has recently died a death too.. Could these things be related?. How much power could an AGP card muster through the RCA cables?, and surely there's some type of failsafe employed. Confuzzled I am.. Hmm
     
  2. MikeHunt79

    Capodecina

    Joined: Jan 4, 2004

    Posts: 20,833

    Location: ¯\_(ツ)_/¯

    The first thing I would do is google the model number of your amp, you'd be surprised how many schematics are avaliable now...

    Failing that, I would head over to diyaudio.com and post what you have posted here. I'd buy a multimeter first however, even a £5 one will be ok for now...
     
  3. benneh

    Sgarrista

    Joined: Apr 9, 2004

    Posts: 9,162

    Location: Nr. brumijum

    Cheers chap, also M. Sayyid 4tw.

    M SAYIIIIDD

    I should add, today I purchased a multimeter, found the schematics and signed up to diyaudio :).
     
    Last edited: Jun 12, 2011
  4. MikeHunt79

    Capodecina

    Joined: Jan 4, 2004

    Posts: 20,833

    Location: ¯\_(ツ)_/¯

    No problem, Beans and High Priest are also awesome... ;)

    If you have the schematics then this makes things a million times easier to fix... What do the chips look like which are bolted to the heatsink? If these have overheated then this is quite a common reason why an amp will fail.

    If these look ok then it could be another component, what is the model number of the amp?
     
  5. Greenlizard0

    Man of Honour

    Joined: Mar 15, 2004

    Posts: 28,189

    Location: Liverpool

    I know what you mean about repairing an old amp. They don't make 'em like they used to and provided it fits whatever other hardware you have they often can be picked up for peanuts on eBay.
     
  6. MikeHunt79

    Capodecina

    Joined: Jan 4, 2004

    Posts: 20,833

    Location: ¯\_(ツ)_/¯

    Older amps tend to use thru-hole components, which I find are far far easier to soldering and desolder compared to the surface mount components of more modern stuff.
     
  7. benneh

    Sgarrista

    Joined: Apr 9, 2004

    Posts: 9,162

    Location: Nr. brumijum

    Firstly, for anyone reading this, always discharge your capacitors... Scary.. :mad:

    It's a technics SU-Z1 chap, and I can't visually find a single thing wrong grr. In terms of what's bolted to the heatsink there's just one big thing (see pic, the white stuff atop being thermal paste), which looks to be fine. I have no idea what that is lol.
    The power supply is in a sealed 'unit', Sealed inside a solid metal housing with some type of hard waxy looking substance poured into it.. so I can't get a visual on any of that..

    [​IMG]

    Hmmm, I've done a fair bit of reading but I'm finding it difficult to identify parts. More research needed ;p.
     
    Last edited: Jun 13, 2011
  8. MikeHunt79

    Capodecina

    Joined: Jan 4, 2004

    Posts: 20,833

    Location: ¯\_(ツ)_/¯

    Oops, I forgot to mention the capacitors, you can get a few sparks from them if you accidentally short them out. :eek: :o They won't be at mains voltage however, in fact they should measure around 30-40v DC when the amp is switched on. This is the first thing I would check with the multimeter, and be careful not to short the pins while amp is switched on. You'll probably need to remove the PCB in order this.

    If you measure 30v DC or more across each capacitor then the power supply part of the circuit then the power supply if fine. If this is the case then I would say that it's more likely that the big chip which says STK2028 on it will need replacing. If you're getting a lower voltage then the power suppy has had it, but hopefully this isn't the case....

    For more info on how the Power Supply works have a look here: http://sound.westhost.com/power-supplies.htm#fullwave

    Also, I managed to find this thread which has more info on your amp, it's not the exact same model but it has quite a bit of info about common faults with these amps: http://www.diyaudio.com/forums/solid-state/162811-technics-su-z45.html
     
    Last edited: Jun 13, 2011
  9. benneh

    Sgarrista

    Joined: Apr 9, 2004

    Posts: 9,162

    Location: Nr. brumijum

    Thanks for the heads up dude, poking about atm, 37.5v across each capacitor, so here's hoping!. The investigation is ongoing..