AKAI M-8 Monoblock Amplifier Recovery - Step-by-Step Guide (maybe)

Discussion in 'Tube Audio' started by SteveAndBell, Jan 24, 2015.

  1. SteveAndBell

    SteveAndBell 1.21 Gigawatts

    Messages:
    383
    Location:
    Brisbane, Australia
    Hi all and Happy New Year!

    I posted a quick vid on YouTube late last year showing my recently acquired AKAI M-8 Monoblocks in running order soon after I pulled them from the main cabinet: https://www.youtube.com/watch?v=U-zcXtBRpGM The rest of the machine was a bit of a basket-case as the alloy cam had crumbled and there were a few other major problems... but the Amps work perfectly :yes:

    I have plans to box them up individually each with their own multi-voltage power transformer (found elsewhere in the Reel-to-Reel cabinet) allowing them to run either on their native 110V or our Aussie 240V and I've gathered the extra transformer & major parts required but haven't yet progressed due to holidays & having too many other things to do. As you can see my YouTube Vid received a comment from a 'Señor Minks' asking how to do this as he wants to do the same to his and as luck would have it I managed to find another M-8 locally with the exact same major problems and at a reasonable price so I nabbed it and thought it appropriate that I do a step-by-step guide on how to pull the Amps from these machines.

    I've linked the YouTube video to this thread so hopefully 'Señor Minks' will join AK and reply with his progress as I go. I don't consider myself a technical whizz but have a basic understanding & respect of audio, electronics and Mains & HV Valve/Tube circuits so grab your popcorn and feel free to add your own comments to this thread along the way because this may help others do the same or similar in the future. Be patient though, for those in the know this obviously isn't a difficult job but this may take a while and drag on a bit.

    OK, first thing's first, do not attempt this is you don't understand the dangers of mains potential voltages plus just as dangerous the stored energy of a HV capacitor. Make sure your home safety switch is fully functional (hit that TEST button!), never use two hands when working on live equipment (even put one behind your back!), take your time and you should be fine... but it's always healthy to be a little paranoid when working with this stuff as cockiness will get you dead real quick :sigh: Also, be aware that old gear like this may not have been powered up in decades so hitting it with full mains voltage can be bad for things like old & dry capacitors. It's always best to slowly power this stuff up with a Variac initially and always have a fire extinguisher at hand in case some goes 'pop' and crucifies itself in the process. Whatever you do, NEVER leave any gear in 'unknown' working condition connected to power when its unattended and in fact it's good practise to always unplug it whenever you don't need to have it powered up, even when you're still working on it! Safety first :thmbsp:

    Righto, the first actual step... we need to test that the Amps work in their original form before we get hacking away at them. As per the pic attached it's best if you grab a couple of el'cheapo 6.5mm to RCA adaptors as this will allow you to connect a signal to the Line input sockets on each Amp much easier. I'm just using an old Smart Phone via RCA Cable for this as my home made test box accidentally got zapped when working on something else and is yet to be repaired :( While you're at it buy a cheap 6.5mm to 6.5mm audio lead (like a guitar lead), cut it in half, strip the insulation back to expose the conductors and use them as speaker leads connected to the Speaker sockets on the front of each Amp. Be sure to connect the Line-In signal and the Speakers before powering on and as per usual make sure the Volume controls are at minumum (all the way anti-clockwise). Also, these Amps wont make a peep unless the machine is in RECORD mode so while pressing the red button in turn the mode lever around to 'REC'.

    OK, we're nearly ready to go for the fire-up. Check that your Line or Headphone Level signal source is actually working and the volume is up at maximum. Double-check your speaker connections making sure you have no short circuits and that the wires are properly connected at the speaker terminals. Check one last time that you have the unit in REC mode and those Volume controls are set to minimum then power it up! You should notice the tubes glow after a few seconds but leave it for another 20-30 seconds to ensure they're properly warmed up then slowly turn the Volume up on each Amp. Hear anything? If so, slowly continue adjusting the volume up and up and you'll hopefully have yourself sweet, sweet music :music:

    If not there are many things that could be preventing this but we wont know how to start troubleshooting until you tell us what the problem is. An example on my original units was that one of the big slider switches that's activated when the REC mode is set was faulty on my Right channel Amp. A few seconds poking around with a small plastic rod found that straight away and pressing on it firmly got it working fine and that was all I needed to prove all was OK at this stage. My 2nd M-8 (pictured) has a different problem, the Right channel Amp isn't Amplifying the input and just makes all sorts of strange humming & pulsing noises when the Volume is turned up past a certain point. Tapping into the 5-Pin DIN connector on the side of the cabinet I managed to get it to amplify the music I was feeding it so again that's all I need to know at this stage but good to keep in mind for later.

    So 'Señor Minks', can you get yours to this state and if so how does it sound?
     

    Attached Files:

  2. BmWr75

    BmWr75 Addicted Member

    Messages:
    5,287
    Location:
    Northeast MS
  3. SteveAndBell

    SteveAndBell 1.21 Gigawatts

    Messages:
    383
    Location:
    Brisbane, Australia
    Cheers for that. I've already been through many of the M-8 threads on AK however I'm starting this particular thread to help someone who needs a hand-hold guide on how to pull these Amps, not for my benefit.

    Not difficult for most of us here but probably a bit daunting for a beginner hence the detail above.
     
  4. Señor Minks

    Señor Minks New Member

    Messages:
    1
    Hi SteveAndBell,
    Thanks for following through on this! Very excited to get started on it next week (after familiarizing myself with "the dangers of mains potential voltages" and "the stored energy of a HV capacitor"). Thanks again and I'll check in soon.
     
  5. SteveAndBell

    SteveAndBell 1.21 Gigawatts

    Messages:
    383
    Location:
    Brisbane, Australia
    Ha, no worries SM. It's sometimes difficult to assist others remotely and even more difficult when you're not sure how competent the other person is but as long as you don't go poking around too much then you should be A-OK.

    Oh, and welcome to AudioKarma too! I'm happy to have invited you here. It's a great community where you should never be afraid to ask questions no matter how silly you think they are because there's so much experience on tap here and other members are always willing to help whenever possible. I'm hoping others will be following this thread and will jump in if they know a better way of doing something because as I've already said, I know enough to be dangerous but I'm far from knowing it all :thmbsp:
     
  6. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,814
    Location:
    Woodinville, WA
    Watching closely as I have a pair of M8 amps sitting next to me. The biggest puzzle is figuring out how to remove those damn sliding switches...and get the circuit down to the bare minimum of parts.

    -D
     
  7. SteveAndBell

    SteveAndBell 1.21 Gigawatts

    Messages:
    383
    Location:
    Brisbane, Australia
    Hmm, you may be disappointed then Derek as I was only going to lock the switches to the REC position as most others have done... but your desire to strip these back to the bare essentials makes a lot of sense, less parts = less faults! Maybe there's someone else on AK who can help with the switch removal?

    To be honest I haven't looked further into that as I haven't progressed on my original two Amps yet. Big switches with lots of wiring mess but they are just switches afterall so if no-one else has done it them maybe I'll (eventually) attack one by pulling the switch and documenting what needs to connect to what. We'll see...
     
  8. BrokenHill

    BrokenHill Well-Known Member

    Messages:
    754
    Location:
    Newhall Pass CA.
    I just bought a second set of Roberts 770X mono amps that I found unexpectedly on Craigslist a couple of weeks ago, the price was about a third of what I see them
    go for on e-bay and they had already been pulled out and separated from the rest of the unit, the seller was very close to my location so I grabbed them since I like
    these amps so much I want a pair in my bedroom and a separate pair in my living room.

    I will be starting all the Tracker X mods soon and will be removing that slider switch because I strip these amps to a minimum, I will post as much as I can on what I
    did to bypass that slider switch and remove it completely out of circuit, hopefully this weekend I will get all the tubes tested and get the amps all cleaned up and see
    if I can get them to pass sound before I start all the mods.
     
  9. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,814
    Location:
    Woodinville, WA
    Any updates? :lurk:

    -D
     
  10. BrokenHill

    BrokenHill Well-Known Member

    Messages:
    754
    Location:
    Newhall Pass CA.
    Just got my pair of 770x amps done last night, one of them started to have a little bit of white noise and I am looking into that now, cleaning the tube
    sockets and just checking over everything for possible mistakes.

    Other then that they sound excellent, superb imaging, I will post some pics for now and later I will try to explain some things about that switch removal.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  11. BrokenHill

    BrokenHill Well-Known Member

    Messages:
    754
    Location:
    Newhall Pass CA.
    Got all the bugs worked out of my amps, I cleaned all the tube sockets and replaced the volume pots, one of the original potentiometers was noisy so I replaced them both with military spec
    Alessandro 500K pots.

    [​IMG]

    Here is a picture of the switch in the on position and I marked the terminals that are connected and active with the blue marker, having the switch out and marked should help for you to see the
    terminals that are active and the ones that are open and out of circuit, they're are two sides of the switch, both sides are the same but separate, the bottom will be harder for you to see when
    the switch is mounted in the unit, the blue markings apply to the top side and bottom side wires when making a complete circuit.

    I basically used the process of elimination to remove the switch, you have two amps start on one first so if you make a mistake you can backtrack what you did to make the amp work again and
    you will have the other one to look at to help reverse what you did wrong.

    First I connected the speaker terminals directly to the output transformer tap and removed all other wires, I left the 100 ohm resistor in place and the small bare wire from the negative terminal
    to chassis ground.

    Second I used one of the existing 1/4 inch phono jacks for input or you can use a RCA jack somewhere for that instead of the 1/4 phono jack, I have done it both ways.
    I used the existing wire wound 1 meg resistor behind the 1/4 inch Line jack with the 10K resistor to ground then wired it straight to pin 2 of the 12AD7 tube, nothing else connected in between
    input and pin 2 of the 12AD7 tube, there is a 500k resistor on the bottom of the 12AD7 tube socket that goes from pin 2 to the center of the socket and that is ground, leave that resistor in place.

    Power cord on both amps can be wired directly to the power transformer, the cover on the main power transformer can be removed to gain access to the taps, I did leave the toggle switch on the
    front face connected to the power cord to turn on or off the power to the main transformer.

    Now you should be able to pass sound through the amps regardless if that slider switch is on or off, when powering up the amp you definitely should use a variac until you get these amps rebuilt
    and all caps checked or replaced also there is always a chance that something got wired wrong.

    With the amp unplugged you can now start snipping out the wires that are not being used in the circuit on that switch, look at the picture of the switch and you can see the ones that are not
    marked with the blue marker that they are doing nothing when that switch is in the on position, they can all be removed first thinning down all the unnecessary wires and making it easier for
    you to see the wires that go to the terminals that are marked blue in the picture.

    I then followed the wires that are attached to the switch terminals that I marked with the blue marker to where they go in the amp, look closely to where they go and what they are connected to,
    some can be simply be removed and some will just need a shorter wire soldered in to bypass the switch to complete the circuit without the switch, if your not sure about some of the wires do it
    in small stages and then bring the amp up on the variac to see if it still works that way if you do something wrong it will be easier to back trace to fix or reverse.

    Remember just do one amp at a time so you will have the other one to look at if you need to reverse anything you might have done wrong, I spent about 6 or 7 hours per amp to do all the
    modifications although I had another pair that I did before so I had something to look at, the first pair took longer because I had to study all that wiring from that switch very closely to figure out
    how to root out all the unnecessary wiring.

    I used the Tracker X mods very closely although I did move some of the electrolytic capacitors to places were I now had more room after removing unnecessary components that were not needed
    for just audio amplification.

    Most people just leave the switch in place and lock it in the on position, that's up to you and if your up to the task of removing all the unnecessary circuits that were for recording purposes and no
    longer needed.
     
  12. SteveAndBell

    SteveAndBell 1.21 Gigawatts

    Messages:
    383
    Location:
    Brisbane, Australia
    Whoa, thanks BrokenHill! Very well done and the effort of posting pics & text here is much appreciated :yes: Once I've finished the new storage cabinets off in my workshop I'll be busting to sink my teeth into this on my first pair of Monoblocks.

    All we need now is for Señor Minks to respond so we can help him doing this with his pair. Señor Minks, are you out there?
     
  13. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,814
    Location:
    Woodinville, WA
    Fantastic! I'll have to block some time on the weekend to do the same work on my amps.

    -D
     
  14. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,814
    Location:
    Woodinville, WA
    There's quite a bit that can come out when it's all said and done. Lots of runs of wire to and from that switch can be pulled out and connected directly to minimize loops of wire inside the amp. I'm still not quite ready to remove the EF86 socket and associated wire/components, but I'm getting close...

    -D
     
  15. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,875
    Location:
    Scottsdale, Arizona
    Last edited: Apr 26, 2015
  16. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,814
    Location:
    Woodinville, WA
    In my haste I ripped out the umbilical and associated wiring for connecting the left and right headphone channels. I'll need to reverse-engineer that bit...

    -D
     

Share This Page