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Thread: Help with 70's Yamaha guitar amp

  1. #1
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    Help with 70's Yamaha guitar amp

    Hello , I have an old 70's Yamaha guitar amp. It is a "Hundred B212" . I am getting a fuzzy distortion on all notes. Have tried different guitar , different cables , different speakers , and I've even taken it to another house and tried thru different outlets. Can't get rid of the distortion. I tried to check the idle current , which is supposed to be set at 5mv. I am reading .5mv and adjusting it doesn't change it. Below is the closest schematic I could find. Any help would be greatly appreciated.
    Attached Thumbnails Attached Thumbnails yamaha_g100_series_i_service_manual_-05-1-.jpg  

  2. #2
    Supporting Member loudthud's Avatar
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    Welcome to the place. It looks like pin 6 on the output module should be about +90V, and pin 5 should be about half that.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  3. #3
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    Are you certain that you are adjusting the correct control? Are you reading the voltage at the correct test point?

  4. #4
    Old Timer J M Fahey's Avatar
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    Start by testing the amplifier with other , known good spekers.

    Scratching voice coils sound almost the same as crossover distortion.

    As a counter test, check those spekers with another ampliier.

    Just play a chord and let it die.

    IF it´s the amp, we can help you properly bias it, but first discard speakers.
    Juan Manuel Fahey

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    Already tried different speakers. I have been testing at the point marked "TP" or test point on the schematic. Tried adjusting at the 220 ohm trimpot.

  6. #6
    Supporting Member The Dude's Avatar
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    A few thoughts and/or questions:

    1) You say, "Below is the closest schematic I could find." How close is it? Do the numbers on the output board at least match? I'd hate to have all of us driving in circles looking at the wrong print.
    2) Assuming it is "close", I'd start by checking the voltages on TR22 & TR23.
    3) This is a single supply amp with 80-90V supplied to the collector of TR22, so TR23 should have roughly half that on it's collector. Because of this, the output is capacitor coupled to keep DC from the speaker. Notice that 2200UF cap in line with speaker positive? That cap could also cause problems if it's bad/dryed up/arcing/leaking.
    4) I'm not commenting on the bias issue until we find out how "close" the schematic is to the amp we're working on.

    Edit: Oh, and I forgot,......
    5) Welcome to the place!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  7. #7
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    Here's the page from the service manual I've been going by. the schematic is good. I just have the oldest version of the G100 amp. The first ones just say "Hundred B212 " instead of G100 212. I tested tr22 and tr23. tr22 is 96vdc and tr23 is 11.5vdc.
    Attached Thumbnails Attached Thumbnails yamaha_g100_series_i_service_manual_-03-1-.jpg  
    Last edited by zintere; 03-10-2017 at 03:13 PM.

  8. #8
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    well it gets worse. I replaced the 3 transistors on the MA board , 2 2sc783's and 1 2sa483. Replaced all 3 with new ones. Now every time I turn it on it pops the 3amp fuse on the back.

  9. #9
    Supporting Member loudthud's Avatar
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    Please refer to transistors by their TRxx name as on the schematic. The schematic does not show the 2scxxx number so we have no idea what you replaced.

    You should build and use a "Light Bulb Limiter" (Google it) so that you don't blow a fuse every time you turn the amp on. Also, disconnect the speaker until the power amp is functioning and the proper Voltage on the collector of TR23 can be read.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Sorry for the long delay. I have hit a point that's very frustrating. After further research I have discovered that none of the service manuals I can find match this amp. From what I have found this is a 70's version of the amp and no one seems to have the correct manual or schematic. This one just says " Hundred B212 " on the front. the manual I was using does not match so it's impossible to give you the right numbers on parts. Gonna call Yamaha next and see what they can find.

  11. #11
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    don't forget the joker g1's Avatar
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    Does it not have a model # plate and serial number on the back?
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  12. #12
    Supporting Member Jazz P Bass's Avatar
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    From a 2012 post somewhere on the net: "thanks to all! this amp is prior to the "G" series. it was made in 74 or 75. I was able to get the question thru to Yamaha and they responded that they don't have the schematic nor owner's manual archived."

  13. #13
    Old Timer oc disorder's Avatar
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    Hi Zintere,
    Firstly I thought we better make sure of the model number.

    It seems Yamaha call it HUNDRED B212 on the face plate , G100B-212 on
    the back plate (also s/n) and G100-B212 on the advertising !!
    g100-b212-g100b-212.jpg

    The G100-B212 has two channels , reverb and distortion but no tremolo.

    Is this the one you have?
    yamaha-g100-b212.jpg

    I haven't been able to download the schematic but have found 2 that are similar to the page from the service manual you posted.
    One SM is G100-115, 410, 412 Service Manual.pdf which I found here as Yamaha-G100-115-service-manual.pdf

    http://schems.com/bmampscom/yamaha/Y...ice-manual.pdf

    The other SM is Yamaha G100 Service Manual.pdf or G100_SM_E.pdf which can be found here in two parts.

    You will need Winrar (Rarlab) and win zip.

    Yamaha G100, G112, G212 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics
    Yamaha G100, G112, G212 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

    There are 4 separate files in here .. the one I was looking at is G100_SM_E.pdf which is a 100w head.

    The preamps are not the correct ones in all of these.

    Here is the "matching" page to yours from both SM's ... they mention 10mv at the test points.
    page-8-yamaha-g100-sm-page-3-g100-115.jpg

    Analog Alley claim to have a Yamaha G100B-212 Service Manual for $13.95
    https://www.analogalley.com/osccart/...roducts_id=659

    I also found a post at DIYaudio by Amptech who offers the schematic if a member pm's him.
    This was in Feb. 2014.
    Yamaha 100-b212 - diyAudio

    That's about all I can offer in the way of help.

    Hope it's of some use.

    O.C.
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  14. #14
    Supporting Member Jazz P Bass's Avatar
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  15. #15
    Senior Member Enzo's Avatar
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    Try this:
    Attached Files Attached Files
    oc disorder and g1 like this.
    Education is what you're left with after you have forgotten what you have learned.

  16. #16
    Supporting Member Jazz P Bass's Avatar
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    WooHoo!

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    Thanks very much everyone.my board is numbered with all the ID numbers , but they are still quite different from any of these schematics. What I can tell you buy comparing them diagram wise , compared to the one Enzo sent is: I've replaced tr20,tr21,and tr22. I replaced them with new old stock and they all still test good. . The mains in mine are different. Mine uses 2 - 2SD425's. I pulled the mains and checked them and one of them is bad. So i guess I got a long wait , cause to put identical NOS replacements in , I have to order them from China. I will try to keep you all posted. Thanks again.

  18. #18
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    Quote Originally Posted by zintere View Post
    Thanks very much everyone.my board is numbered with all the ID numbers , but they are still quite different from any of these schematics. What I can tell you buy comparing them diagram wise , compared to the one Enzo sent is: I've replaced tr20,tr21,and tr22. I replaced them with new old stock and they all still test good. . The mains in mine are different. Mine uses 2 - 2SD425's. I pulled the mains and checked them and one of them is bad. So i guess I got a long wait , cause to put identical NOS replacements in , I have to order them from China. I will try to keep you all posted. Thanks again.
    Why do you feel the need for NOS replacements? Anything that you order from China will likely not be true NOS transistors.

    The 2SD425 transistors may not be original to the amp and could be replaced with any number of other transistors with no problems.

  19. #19
    Old Timer oc disorder's Avatar
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    As long as they handle 100v @ 12A Bill !
    Silicon NPN Power Transistor
    2SD425 VCBO 140v IC 12A
    2SA1090 VCBO 100V IC 12A (shown in schematic)

    Just got lost and found this .. well might be useful one day


    EchoWars has done some magnificient research re Replacing The STV-3H and STV-4H Diodes over at AudioKarma
    (The Yamaha has one)
    Replacing The STV-3H and -4H Diodes | Audiokarma Home Audio Stereo Discussion Forums

    Conclusion..." So, thus far, the only devices that I have found that mimic the originals reasonably well is the UF1004 (three UF1004's for an STV-3H and four UF1004's for a STV-4H), which is a bit of a bummer, as the device size and the lead thickness make it an unwieldy replacement. What I did do, to minimize the lead diameter issue, is to clip the original leads and use small leads cut from 1/4W resistors to join adjacent diodes. That at least bypassed the lead size issue...if only the body of the UF1004 was a bit smaller."
    52 Bill likes this.

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