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Thread: Acoustic B120 schematic please

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    Acoustic B120 schematic please

    Anyone have Acoustic B120 schematic by chance? It is from the 80s.

    this one
    Click image for larger version. 

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    One more try for the hail mary.

    If no one has the schematic maybe someone can help me figure out what these burnt to a crisp resistors are, but I could figure it out myself with a schematic

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    Old Timer J M Fahey's Avatar
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    A picture would certainly help

    That said, trace where from-to are they connected.

    Typical woud be across power transistor BE , if so 100 ohm is a cassic value; if in series with transistor base common values are 5 or 10 ohm, etc.

    Those always burn because when transiustors short or open they are subject to 10/100 times the expected current.

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    Juan Manuel Fahey

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    I will send a pic in a bit. It is true that one of the burnt resistors is 100 ohm. the other one though, I have no idea what the value would be

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    Supporting Member TomCarlos's Avatar
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    This might be a long shot.... but if push comes to shove....

    It looks like you can purchase a schematic from MusicParts.com - http://www.musicparts.com/products.asp?Company=Acoustic

    The other thing? Didn't the ACC amps of the 70's use similar power boards, eq boards, preamp boards? So would we find similarities between the B120 and the regular ole 124/124 guitar amps?

    Please give us the numbers from your PC boards - maybe we can track it down that way.

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    Click image for larger version. 

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    So looks like we got a burnt 100 ohm resistor that is associated with the output transistor like JMF said, also there is a bulgey 10 ohm resistor, and then right next to it is an unknown burnt resistor.

    This is an 80s Acoustic

    I'll get back with more

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    Last edited by nsubulysses; 03-14-2019 at 10:25 PM.

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    Burnt resistor I was assuming was 100 ohms and bulging 10 ohm resistor go to emitter of power transistor (B- it says printed on the PCB) NEC 2SD555.

    other burnt resistor goes to collector of the RCA 480059 (RCA 40408)

    Amp powers on but no sound. Which makes sense because this amp has a speaker fuse which is blown. I would guess power transisors are shorted and maybe more. i haven't delved much into i yet though since I was trying to find the shcematic first. But maybe that's not possible.

    Board says 170144. It also say "120W PWR AMP/SUPPLY PCB"

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    Last edited by nsubulysses; 03-14-2019 at 10:59 PM.

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    Old Timer J M Fahey's Avatar
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    70s Acoustic amplifiers were all single supply, then in the last year or two (79 to 81 or so?) they released a last batch of split supply amps, designed by Stewart Rabe, who after Acoustic failed started AMP (Amplified Music products) which later became SWR
    My point being I suggest you search for any late model (I mean from way back then, not the current Guitar Center reissues) schematic, it should be close enough to help.
    I think vintagekiki posted a ton of links to Acoustic amps, search them.
    Not too sure about the NEC transistor, an amp old enough to use RCA404** ones would have used *all* American transistors, and all with some Acoustic house code; a Japanese one does not fit there and looks like a much later replacement.

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    Juan Manuel Fahey

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    Acoustic Schematic

    Although the power module you have is slightly newer, the attached picture and schematics should help you out.
    Note the power amp schematic (page 3) of the 904/906 mixer. It will be almost identical to what you have.

    I have repaired at least 20 of those modules over the years, so I can tell you that in addition to replacing the burned resistors, there are going to be many shorted and/or opened semiconductors you will need to replace as well. Take your time and check everything. It will fail again quickly if you don't get everything.

    Steve

    Acoustic 904, 906 Powered Mixer Schematics.pdf
    Acoustic 100 Watt Power Amp Module Picture.pdf

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    Supporting Member TomCarlos's Avatar
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    Steve.... I cannot open the 904/906 pdf file.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by TomCarlos View Post
    Steve.... I cannot open the 904/906 pdf file.
    http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/Acoustic/Acoustic_904,_906_Powered_Mixer_Schematics.pdf

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    Senior Member vintagekiki's Avatar
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    There is no rescue from horror
    You can fix it if you try to use comparable similar schematics of power amps.


    http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/Acoustic/Acoustic_B220_(170149)_Power_Amp_Schematic.pdf
    Acoustic B220

    http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/Acoustic/Acoustic_122_Power_Amp_Schematic.pdf
    Acoustic model 122

    http://www.ampix.org/albums/userpics/10134/Acoustic_215B-120.pdf
    Acoustic 215B-120
    corrective
    Both output transistors are NPN, MJ15024, MJ15003 or similar.


    http://music-electronics-forum.com/attachments/34426d1434490511-acoustic-g120-112.pdf
    Acoustic G120-112

    When comparing the layouts of the PCB, there is a similarity.

    https://music-electronics-forum.com/attachment.php?attachmentid=52938
    Acoustic B120

    https://www.tapatalk.com/groups/unofficialacousticcontrolcorpmessageboard/imageproxy.php?url=http://i421.photobucket.com/albums/pp300/bassmonkey_album/DSC02958.jpg
    Acoustic 212B-120

    https://www.tapatalk.com/groups/unofficialacousticcontrolcorpmessageboard/212b-120-schematics-t2027.html

    Reminder
    When power amp defect, necessarily must check the output transistors, the driver transistors, the transistor for the quiet current, and emitter resistors in output and drive transistors .
    Output and drive transistors should be matched at +/- 10%
    Do not forget to set quiet current output transistors.

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    Last edited by vintagekiki; 03-15-2019 at 04:54 PM.

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