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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

    Bought this from the classifieds on here as not working and has been modified .
    Someone has had a go at fixing this but thats the only info i have .
    The mods consist of bigger supply caps changed from 4x 4,700uf to 22,000uf :eek:
    the rectifier package on the pcb has been removed and replaced by a another one attached to the heatsink
    the fuses x4 where moved to the back of the pcb and most of the capacitors have been swapped out for panasonic fc then bypassed with film caps under the pcb .
    the case alone is worth what i payed for it not to mention the Eichmann Cablepods on the back :).
    On powering it up after a few seconds the relay for the speaker output clicked in so i connected my stunt speakers and although the source selector and volume control worked on the lcd display nothing from the speakers .
    Checked the 2x big transformers all ok 2x 20v
    the front display lcd and input selector relays are fed from a small transformer on the front panel pcb .
    So the big transformers just power the output .
    Decided to remove the pcb to investigate further see pics .
    Removed the 4x supply caps and removed the output mosfets on the heatsink which all tested ok .
    removed the fuse holders and put them back on top of the pcb .
    After studying the board i noticed a voltage regulator was missing i traced the tracks to a PGA2311 volume control chip on the back of the pcb after looking at the data sheet of the ic i found out it needs a +/- 5volt supply so i have replaced with new + and - 5v regs .
    I have probed most of the components with my component tester on my Hameg hm605 oscilloscope and none where short all looks good .
    I am hoping its just the supply to the PGA2311 and thats it ;).
    What i would like to know is am i better to replace the main power supply caps with 4,700uf original caps as the 22,000uf wont fit now i have moved the fuse holders back didnt want to leave them under the pcb as it needs to be removed to replace fuses.
    Looking at the build quality of the amp i dont think they used 4,700uf to save money !
    Whats your thoughts on that ?
    here are the pics....

    [​IMG]20190624_160757 by glenn jarrett, on Flickr
    [​IMG]20190722_120259 by glenn jarrett, on Flickr
    [​IMG]20190722_124741 by glenn jarrett, on Flickr

    [​IMG]20190722_124751 by glenn jarrett, on Flickr
    [​IMG]20190724_133039 by glenn jarrett, on Flickr
    [​IMG]20190724_132943 by glenn jarrett, on Flickr
     
    effinity likes this.
  2. glenn jarrett

    glenn jarrett pfm Member

    This is a pic of the same amp off google its just got a different bias setup
    cant find any service info on google

    [​IMG]maxresdefault by glenn jarrett, on Flickr
     
  3. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Just use the biggest caps that will fit. Oh and sort the soldering to that rectifier before you lose a rail and it goes DC on you!
     
    glenn jarrett likes this.
  4. glenn jarrett

    glenn jarrett pfm Member

    Cheers Arkless
     
  5. Pete MB&D

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

    That missing pad for the smoother might cause you problems, the connection is under the cap you won't be able to tell if its soldered.
    I would run a wire underneath the board to reconnect it to the bridge.

    Pete
     
    glenn jarrett likes this.
  6. glenn jarrett

    glenn jarrett pfm Member

    Hi
    Pete
    Yes i lost a couple of pads on the small caps (red wires ) due to the bad soldering of the caps when they where replaced before i got hold of it big blobs on either side of the board !
    i find these helpful when working on delicate boards stainless steel needles few quid on ebay slide the tube on the leg of the component heat the joint and the tube slides through the hole ;)
    https://www.ebay.co.uk/itm/Hollow-N...607748&hash=item521d75a7c0:g:JvIAAOSw~ihck0xl
     
  7. glenn jarrett

    glenn jarrett pfm Member

    Thought while i am waiting for my caps to be delivered i would check the mains wiring :eek:
    really glad i did
    the transformers have been shielded with copper foil tape and are connected to iec earth but i can live with that
    the blue cable supplies mains to the 3 transformers via the connector on the front panel pcb
    which turned out to be 3x 0.5mm silver wire individually insulated
    the spades just pulled of the ends :confused:
    also the transformer mains input wires black & white had gaffa tape wrapped around so i thought it was a cable join cut it off and it was just foil with a fiber backing under the gaffa tape no joins ! o_O
    ( what kind of snake oil is this )
    everything added has now been removed except the copper screening on the transformers
    will re wire with suitable cable .
    [​IMG]20190726_121132 by glenn jarrett, on Flickr
    [​IMG]20190726_122028 by glenn jarrett, on Flickr
    [​IMG]20190726_122128 by glenn jarrett, on Flickr
    [​IMG]20190726_122222 by glenn jarrett, on Flickr
    [​IMG]20190726_122018 by glenn jarrett, on Flickr
    [​IMG]20190726_123257 by glenn jarrett, on Flickr
     
  8. JimmyB

    JimmyB pfm Member

    The foil must have made it sound 'tinny' so hence the gaffer tape to 'man-up' the sound a bit....o_O

    Amazed someone with obviously no knowledge and even worse soldering skills than myself would have the guts to start wiring up mains stuff like this....

    ...and how come the previous tech didn't rip this apart? (Presuming he wasn't the gaffer tape guy).

    Copper round transformers though...now that quite an 'in' look at the moment.
     
    linnfomaniac83 and glenn jarrett like this.
  9. Pete MB&D

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

    I prefer soldered connections for the IEC and one of those insulating boots, neater a better contact and safer.

    Pete
     
  10. glenn jarrett

    glenn jarrett pfm Member

    Ended up changing the rectifier and the output devices just in case of any stress
    as they where very tightly fixed :eek:
    first power on through a 100w light bulb got a qick glow then went out :D
    everything cool even the main heatsink
    now listening to a bit of Dub Syndicate
    to say its got 4x new power supply caps and output transistors it sounds bloody awsome through my stunt speakers
    cant wait for it to bed in .
    [​IMG]20190729_125148 by glenn jarrett, on Flickr

    [​IMG]20190729_135325 by glenn jarrett, on Flickr
    [​IMG]20190729_135336 by glenn jarrett, on Flickr
     
  11. glenn jarrett

    glenn jarrett pfm Member

    I have no idea what the bias should be set at and cant find any information on the web
    is there a trick you can do to find out what it should be ?
     
  12. Pete MB&D

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

    Do you have a signal generator and scope?

    Pete
     
  13. glenn jarrett

    glenn jarrett pfm Member

    Hi Pete
    yes i have Audacity and a dual 60mhz analog scope
    Also got a 4R / 8R dummy load
     
  14. James Evans

    James Evans Bedroom Bodger

    First 10 watts are supposed to be class A, not sure if that gives any clue along the way?
     
  15. Pete MB&D

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

    I was going to say look at a sine wave and tweek it up untill you get no flat bit around 0v, then a smidgen more.

    Pete
     
    glenn jarrett likes this.
  16. glenn jarrett

    glenn jarrett pfm Member

    not sure about the output of Audacity
    its free to download but the output is low through my pc on board sound card
    i think its only 700mv from what i can remember might have to play it through a dac which i have to get the full 2v
    will have a go tomorrow
     
  17. glenn jarrett

    glenn jarrett pfm Member

  18. glenn jarrett

    glenn jarrett pfm Member

    Just found this
    Audacity is a free and open-source digital audio editor and recording application software for pc
    so no good for what i need
    will try to find another one tomorrow
     
  19. glenn jarrett

    glenn jarrett pfm Member

  20. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    For 10W class A you need 0.8A bias which is a measured 176mV across the 0.22R wirewounds.

    Careful though... many companies exaggerate class A power and those heatsinks look a tad on the small side to be in a an amp capable of 10W class A but which then goes on to give 30W in A/B

    If you set it to 10W class A then keep an eye on heatsink temp and on bias drift. It will quite likely need readjusting somewhat as it heats up. If it gets above around 65-70C then back off a bit to get to 60C
     
    glenn jarrett likes this.

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