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Cairn 4808a amplifier repair

Discussion in 'd.i.y.' started by glenn jarrett, Jul 24, 2019.

  1. glenn jarrett

    glenn jarrett pfm Member

    Thanks again Arkless
    :)
     
  2. AudioAl

    AudioAl pfm Member

    All I can say re the gaffa tape repair by whoever did it is " Top Bodging "
    You Sir are braver than me as I know nothing re electrickery
    Well done getting it going again :cool:
     
    glenn jarrett likes this.
  3. glenn jarrett

    glenn jarrett pfm Member

    set up 2 meters on mv and measure the voltage across the emitter resistors on both channels
    with amp just powered no speakers or input connected
    the first measurements before adjustment was 7 .0 mV left and 2.2mV right the heatsink luke warm
    after adjusting both pots up to 140mA as soon as you set it it starts to drop counting down as the heatsink temp rises
    after some time it settles to 63mV and 62mV with a heatsink temp of 56 oC
     
  4. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    So nowhere near 10WPC in class A then.... in fact 1.3WPC in class A for 56C heatsink temp with 63mV across the source resistors....
     
    glenn jarrett likes this.
  5. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    Many years ago I worked on an amp where both channels output transistors had blown, I popped in some replacements powered it up it didn't blow so I put my hand on the heatsink and nearly got a griddle pan mark on my hand.
    Turned out some one has tweaked all the controls up to max including the bias.

    I would turn the bias down a bit.

    Pete
     
    glenn jarrett likes this.
  6. glenn jarrett

    glenn jarrett pfm Member

    Big thanks to Arkless and Pete MB&D for the help in getting this up and running
    been running for about 10 hours now
    sounds very good indeed on my stunt speakers
    already blows my NCC220 out of the water :eek:
    I think my next diy build will be a class A
    any recommendations ?
     
    effinity likes this.
  7. IDM

    IDM pfm Member

    Probably depends on how much power you need for your speakers. I love my Nelson Pass F5.

    Cheers
    Ian
     
  8. effinity

    effinity ya memba

    Well done Glenn, you seemed to do that in double quick time although I don't know how many actual hours you spent on it.
    It's great to know it's up and running and sounding good!
     
    glenn jarrett likes this.
  9. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    How much power? Budget? Can you do it from scratch or do you need a kit?
     
  10. glenn jarrett

    glenn jarrett pfm Member

    Having a look around for bare boards loads on ebay but are they any good?
    the diy audio stuff looks very good and plenty of advise on the site but the import duty makes it expensive cost me £35 just for the Mezmerizer pcb :eek:
    i think 25W will be plenty
    i have a modushop dissipante case the 3u one but not sure if it will give enough cooling for 25W
    as for the fets i have 16 x J74 and 6 x 2sk170b Toshiba's so maybe do a few swops on here to get the 2x pairs i will need
     
  11. linnfomaniac83

    linnfomaniac83 I bet you can’t wheelie a unicycle!

    I think it was more stupidity than guts... looks like the soldering was done with a hot screwdriver on a hob and plumbers solder, I taught my teenage daughter to solder and she was (much) better than this on her first go, having the right tools and materials probably helped.
     
  12. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    You will likely need more heatsinking yes.
    If you can find any matched N and P channel pairs from those you could consider the Hiraga Le Monstre but power is only 8WPC in standard form. Although a very simple circuit they are difficult to get right.

    Much easier (although it doesn't look it) would be a Krell KSA50 copy. It could be set to give less then the normal 50WPC class A to save a bit of heatsinking etc.

    Also using the Toshiba fet's in many of the designs are the Pass Labs/First Watt designs...

    The worlds your lobster!
     
    glenn jarrett likes this.
  13. timmitus

    timmitus New Member

    I'm a new member here but want to chime in anyway as I've owned a few Cairn pieces. The Cairn 4808 came in two iterations, the 4808 which is the class A/B 100 watt version, and the 4808A which is a 30 watt class A version. I currently own, and use occasionally, a 4808A. The amp gets extremely toasty if I forget to power it down at night.

    One potential problem with these amps is that xformer on the front-mounted PCb. If that goes out, good luck finding one. Mine went out and after diagnosing it, I contacted Cairn's owner and shipped him the PCb for repair.

    Beautiful little amp.
     

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