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Akai M7 monoblocks meter problems

Discussion in 'Tube Audio' started by Omegaman, Jun 15, 2014.

  1. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
    Scottsdale, Arizona
    Akai M7 monoblocks

    Just completed this Akai M7 recap and conversion to stand alone monoblocks. This is the second conversion I have done, its easy.

    Tracker-x mods with BmWr75 updates for clarity.

    It went well and both amps sound clean and quiet

    One meter does nothing
    One meter pegs out max when the amp reaches operating temp

    I did not check if these meters worked prior to doing the conversion to monoblocks as the deck was in miserable condition
    I checked all resistors

    I am trying to understand the meters in order to try to get them functional?

    I know they probably can't be perfect but just understanding them might help me out

    [​IMG]

    I think the twisted blue wires are the lamps 6.3v
    There are pots to control the 6.3v and the 5.7v off the transformer


    I tried disconnecting the red wire but the meter still pegged out

    I could bypass the meter white wire that goes to the speaker out plug
    I could also pull both meters and swap them to see if it is the circuit or meter


    Which wire is the supply side of the meter?


    I found this thread:
    This will help

    http://www.audiokarma.org/forums/showthread.php?t=413464&highlight=Akai+Roberts+mono

    I have to get a 25kohm pot
     

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    Last edited: Jul 24, 2014

     

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  2. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

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    I've wondered if that would work, essentially trimming R20.
    Let us know what it does.

    I might have some extra meters if you think yours are broken.

    RWood
     
  3. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
    Scottsdale, Arizona
    Attaching the 10k pot

    I now have a nice precision 10K variable resistor.

    Here is a picture of the red and white twisted pair wires that run from the output transformer to the meter and speaker jack
    You can see the red/white wires attach to the meter, then the white wire goes on to the speaker jack

    The gray wire wound 5k ohm resistor is R25 in the schematic (I think)

    Is that correct? Are we talking about the R25 5k resistor being replaced by the 10k pot

    Is R20 correct or is R25 the correct resistor to replace with the 10k pot?

    The black wire on the other end of the resistor goes to the transformer
     

    Attached Files:

    Last edited: Jun 19, 2014
  4. cademan

    cademan Addicted Member

    Messages:
    9,027
    Since these record/line amplifiers are from reel-to-reel tape decks, shouldn't the meter only work when the record button is pressed meaning that the meter is for monitoring in record mode?

    There was probably a molex type connector that connected the tape deck to the record/line amplifier and the reel-to-reel had the record button located on it.

    I'll look at the schematic but I suspect this would be the non-working meter problem.
     
    Last edited: Jun 20, 2014
  5. zandru

    zandru "Hard At Play"

    Messages:
    1,080
    Greetings Don,

    The old “cart before the horse conundrum” with resistors.

    I most likely would have proved the meters early on.

    My downloaded Akai M7 amp schematic seems to be slightly different.

    http://elektrotanya.com/akai_m7_sm.pdf/download.html


    The Best of skill & luck, Hope you sort it out today…Cheers…
     
  6. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

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    Yes, R25 not R20 (my eyes....). The way I'm reading it is the hot signal to the meter is coming off the OT secondary. Then its' reference to ground is tempered by the 5k resistor. Make that variable, and you adjust the sensitivity of the meter. I could be wrong, but it should be an easy and reversible test.

    Cademan, those meters also work during playback in addition to record. For our purposes the need for accurate record calibration doesn't apply, but it would be cool to have the meters work while we're in permanent-playback mode.
     

     

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  7. cademan

    cademan Addicted Member

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    Yeah, I forgot about playback, but doesn't playback also require some switching from the deck so the meters show in playback mode?

    I just saw a couple of those Akai/Roberts on the bay that I wouldn't mind experimenting with.
     
    Last edited: Jun 20, 2014
  8. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
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    Last edited: Jun 20, 2014
  9. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

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    I got out some M7 amps and a couple loose meters and tried them out with my bench receiver. They worked fine!

    First, the two blue wires at the top of the meter casing are indeed for the lamps, two of which are inside each meter. They run off 6.3v. The 5.9v winding would work, but I believe that winding was intended to be dedicated to the EF86 tube. If you aren't using that tube you could run the VU lamps off that winding.

    Then from the two big screw terminals on the back of the meter, the white wire goes to to the hot lead of the speaker jack and the + side of the OT secondary. This is a busy junction, as the Neg Fdbk is also taken off here, as well as the 8Ω dummy load that kicks in when there is no speaker connected.

    Red goes over to the main switch, SW1, the big spring loaded slide switch that gets engaged when the record lever is pulled. Cademan, this is where the meter either goes through the 5K resistor to ground, or goes straight to ground, depending on how the switch is set. In Playback, it goes through the 5K to ground.

    To test these meters, I got the speaker leads from my bench amp, a homely but sturdy Astro Sonic ss, and crimped ring terminals on each lead. I also included leads in each crimp that I connected to a 10K pot. With the volume turned down, I connected them, then brought the volume up slowly and the meters reacted just as you would expect them to.With volume high, they really jumped. The pot had almost no effect from 10KΩ down to 300Ω. Lower than that, it suppressed the range of the meters.

    Omegaman, perhaps you can try your meters out like this on a low power solid state amp just to see if they still behave as they did before.With luck, they'll work and you can then hook em back into the amps.

    RWood
     
  10. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
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    First off, thanks for jumping in. This is all new to me

    In Playback, it goes through the 5K to ground
    Isn't it locked in record mode not playback?

    Is the 10k pot test still worth pursuing?

    I'll pull the meters and run the test with a SS amp.

    sounds like a job for my lepai amp
     
    Last edited: Jun 20, 2014
  11. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

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    When the switch is to the right (looking straight at it), the way the spring is pulling it, the red lead from the meter goes through the 5K to ground.

    When it's pulled to the left, it goes straight to ground. I assumed this was Record position but, now, ah could be mis-taken :yes:

    I do believe the 10K pot test is superfluous, yes. But, um, you needed a battery anyway, right?

    I am wondering if your one meter was pegging because it had a bad connection to ground. Remember it must go through a switch to reach ground. For a playback-only amp, I would bypass the switch and terminate that 5K resistor directly to ground.
     

     

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  12. Omegaman

    Omegaman Ultra Super Member

    Messages:
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    Maybe hitting all those connections with solder would ensure connection

    Checking it out the 5k is disconnected and my dwindling meter shows connectivity from the red wire to ground as noted

    If the 5k is not engaged in the circuit, then running the amp with the 5k disconnected will result in...nothing, same performance?

    I hooked it up and it does indeed play as before without the 5k connected.

    Amp sounds good.

    same meter pegged out, I'll try an ALT GND

    Time to pull the meter and run test 1A

    maybe we can run it thru the 500k tone pot and use that to trim the meter for show
     
    Last edited: Jun 20, 2014
  13. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

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    Yes,I just read through the Tracker-x mods and since he bases those on the amp running in Record mode, with the switch to the left, then the red lead from the meter would bypass the 5K and run straight to ground.

    However, if my experiment holds true for you, employing that 5K resistor between red and ground should goose the action of the meter just a bit, so you could give it a try and see if it makes it less dwindling.

    On the pegged one I would hit the solder connections as you said, or better yet bypass the switch entirely.
     
  14. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
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    Running the 5k to ground did not change the meter action. One nothing, one pegged
     
  15. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

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    Bummer.

    Did you try the external ss amp test?

    Offer stands on these meters if you want them
     
  16. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
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    Test results in

    I connected the meter in parallel with the speaker out from my little lepai 2020A+

    The meter that did not move, still does not move
    The meter that was pegged max does move in sync with the music

    The 10k pot did nothing until the ohms were turned way down and then it just inhibited the signal

    Swapped meters
    When I connected the max'ed out meter to the dead meter amp the meter worked
    When I connected the dead meter to the maxed out meter amp nothing

    So I got one amp working with one working meter
     
    Last edited: Jun 21, 2014

     

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  17. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

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    Meters Я Us - heading your way!
     
  18. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
    Scottsdale, Arizona
    I had one good meter with one bad light and one bad meter with two good lights

    When I opened the "good" meter I saw that the wire for the lamps had MELTED and shorted like Apollo 13

    The lamp showed continuity so I re-wired the lamp and now both lamps work on the good meter

    Connected to test amp and meter worked as advertised

    Connected to tube amp and meter VIOLENTLY pegged and banged against the stops

    So I disconnect both meters

    At least both meters light work

    I'm concerned about why the signal is so violent in the tube amp meter, although the amp sounds fine
     
  19. Omegaman

    Omegaman Ultra Super Member

    Messages:
    2,894
    Location:
    Scottsdale, Arizona
    Here are the old caps that I removed from both amps

    I don't have sophisticated technical tools so I just shotgun "CHANGE EM ALL"

    [​IMG]

    Here are the monoblocks rockin the house (4 watt style) through a pair of Ampex D19 speakers

    [​IMG]
     

    Attached Files:

    Last edited: Jun 21, 2014
  20. cademan

    cademan Addicted Member

    Messages:
    9,027
    The problem with shotgunning is that sometimes you make things worse.

    I would go through and double check the replacement capacitors for correct polarity.
     

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