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Thread: Altec 9475a preamps

  1. #11
    Join Date
    Sep 2001
    Location
    GOODLETTSVILLE, TN.
    Posts
    137

    Default altec amp

    Q1-4 2N2484 Q5-6 2N2219 diodes 1N4148 or 1N914. strap pins 5 & 6 for +24 out. pin 15 to audio common. pin 13 to frame gnd. replace C 6-7 with 100pf MICA caps. Put 910 ohm res. in series with feedback R/C where the 3.3V is located on schematic. Hope this helps. Let me know if this works.

  2. #12
    Join Date
    Sep 2005
    Posts
    25

    Default

    thanks John!

    Ill give it a go and get back to you much appreciated.

    Tim

  3. #13
    Join Date
    Sep 2005
    Posts
    25

    Default smokin!

    HI John,

    finally replaced the transistors last night (took a while to source them) ,

    Before doing so I followed your hook up points and managed to get a weak siganl from the pre with a fair amount of hiss I had made the mistake of connecting xlr grounds to frame ground and not audio common.

    Unfortunately on replacing the transistors the 22ohm resistor lit up like a candle and started smoking again.

    it also seems that diode 1 (upstream from the +24v) seems affected (goes out of spec and I replace it again!) but I have double checked many a time regarding the power hook up, all the caps are replaced the resistors seem in spec, the transformers seem to test ok for continuity ( I think) Im baffled!

    as yet haven't placed the 910 ohm resistor in but will do so , Im a little confused, do you mean place two 910ohm resistors between the R9 & R5 and R10 & R6? Or place a single 910ohm resistor in the circuit further along perhaps between r6 and r5 ?

    Hopefully that'll make a difference its a bit hard to take measurements at this point because the 22 ohm resistor pops before I get a chance to pick up the multimeter.

    any help would be grand!

    cheers
    Tim

  4. #14
    Join Date
    Sep 2005
    Posts
    25

    Icon7 9475A working thanks for your help!

    Thanks John for your help and anyone who contributed the 9475a is now working! I re-checked and found a couple of resistors and a cap point were not getting a proper connection.. It all came good as a 747 swooped over the house, the first thing I heard and it sounded loud!

    Apart from that Ive only being talking through a 421 into it , sounds great but Im looking forward to giving it a proper go.

    The only prob now is buzz, so Im still working on it its just a bit loud to pass I think so will attempt tackle that , maybe the budget power supply I bought I guess, buzz seems to shift as I wave my finger over the circuit board.

    Still haven't put the 910 ohm resistor in series with the R/C feedback bit yet so will try my interperatation of that.


    thanks again John for your help to a real beginner .


    Tim

  5. #15
    Join Date
    Sep 2001
    Location
    GOODLETTSVILLE, TN.
    Posts
    137

    Default 9475 amp

    Some 2N2484 xsistors were made with a yellow plastic body. They are light sensitive. Paint them with black paint. Try checking noise with lights out or put it in a light proof container.

  6. #16
    Join Date
    Sep 2005
    Posts
    25

    Default Aha!

    Thanks John,

    much appreciated! It worked I switched the lights off and it went as quiet as a mouse, sounded great and this was obviously on full tilt as well!
    Looking forward to the next recording session and trying it out on some bass or drums I was thinking.

    Strange thing was when I turned the light back on it was still quiet! Maybe there was a loop with the light switch or something... anyway very heartening to know.

    Next step is an optional pad at the front and an output attenuator which should be a new challenge. Should be fun.

    Thanks for all your help.

    Tim

  7. #17
    Join Date
    Sep 2001
    Location
    GOODLETTSVILLE, TN.
    Posts
    137

    Default Altes 9475

    I was in error about the 9475. I was thinking of the 9470. It is same circuit as the AM-16. Notice no caps in audio path. The Peerless div. of Altec would not build the xformers. Robot coil built them. The input xfmr was much better than the AM-16. ( and larger ).

  8. #18
    Join Date
    Sep 2005
    Posts
    25

    Default

    Quote Originally Posted by JOHN_HALL1
    I was in error about the 9475. I was thinking of the 9470. It is same circuit as the AM-16. Notice no caps in audio path. The Peerless div. of Altec would not build the xformers. Robot coil built them. The input xfmr was much better than the AM-16. ( and larger ).
    Ah right,

    Ive seen the 9470 on ebay advertised as being the same as the langevin.

    So would I be right in guessing you designed the 9475 as well ?

    If you did, do you know if there was a difference in functionality for the 9475? Im just curious to know how it came about and whether there would be much of a difference from the 9470 in terms of who or what it might be designed for and the sound etc. Would the 9475a have the robot coil xformers? There is no label on them that I can see.

    Cheers

    Tim

  9. #19
    Join Date
    Sep 2001
    Location
    GOODLETTSVILLE, TN.
    Posts
    137

    Default Altec Amps

    I checked and I had left Altec before the 9475 was released. I must have meen thinking about the 9470 and replied in kind. Sorry for the goof. Blame it on old age. The info about updating the xsistors is still a good thing to do. Glad it helped.
    John

  10. #20
    Join Date
    Sep 2005
    Posts
    25

    Default

    thanks John,
    all your help and expertise
    is much appreciated Im really happy with how it turned out.


    Tim

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