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Thread: Gibson G70

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    Gibson G70

    Hey Folks ! Anyone know where I can find a Gibson G70 schematic/layout ?

    Thanks !

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    Old Timer J M Fahey's Avatar
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    Here's the annotated power amp.

    Can search for the preamp if you need it.


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    Juan Manuel Fahey

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    Lifetime Member Enzo's Avatar
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    Um... shouldn't Q6 be an NPN?

    And are they not darlingtons?

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    Education is what you're left with after you have forgotten what you have learned.

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    Thanks Juan ! I'll take all I can find. I'm coming up empty.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Enzo View Post
    Um... shouldn't Q6 be an NPN?

    And are they not darlingtons?
    Hey !!! this is the official GIBSON schematic!!!!!!
    Don't tell me you know more than them !!!!
    Just answer: did Eric Clapton and Jimi Page use a Gibson guitar or an Enzo guitar , huh ?????? <--typical TGP/MyLP/Harmony Central/etc. argument

    Q3 , Q4 , and Q6 are NPN .
    MJE1102/1092 are Darlingtons (notice the 1000/1200mV BE bias I expect to find there) and to boot are way underpowered for a 70W amp : 5A max current , "70W" , in that tiny smaller than TO220 case, like typical Peavey drivers .
    They were notoriously unreliable (I wonder why ) and upgraded 3 times.
    MJE1102 pdf, MJE1102 description, MJE1102 datasheets, MJE1102 view ::: ALLDATASHEET :::
    I suggest TIP142/147 there, but pinout will no match, check both datasheets.

    I posted what I found first, there's an old thread here, also search for other Gibson amps, there's one called something like G50/60/80 somewhere in MEF, all preamps were basically the same.

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Hey !!! this is the official GIBSON schematic!!!!!!
    Don't tell me you know more than them !!!!
    Just answer: did Eric Clapton and Jimi Page use a Gibson guitar or an Enzo guitar , huh ?????? <--typical TGP/MyLP/Harmony Central/etc. argument

    Q3 , Q4 , and Q6 are NPN .
    MJE1102/1092 are Darlingtons (notice the 1000/1200mV BE bias I expect to find there) and to boot are way underpowered for a 70W amp : 5A max current , "70W" , in that tiny smaller than TO220 case, like typical Peavey drivers .
    They were notoriously unreliable (I wonder why ) and upgraded 3 times.
    MJE1102 pdf, MJE1102 description, MJE1102 datasheets, MJE1102 view ::: ALLDATASHEET :::
    I suggest TIP142/147 there, but pinout will no match, check both datasheets.

    I posted what I found first, there's an old thread here, also search for other Gibson amps, there's one called something like G50/60/80 somewhere in MEF, all preamps were basically the same.
    Thanks for the info. I'm trying to find out what the values of the two big can caps are ?

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    Old Timer J M Fahey's Avatar
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    Here's a Gibson G50/60/70/80 schematic, obviously a very different version under the same name.
    It's in SSGuitar
    Login
    Gibson G-80
    As said before, they kept replacing them under warranty, this one is a little stronger, used TIP33 powerhouse transistors .
    Don't know which you actually have, a Darlington or a Discrete one?
    Post a couple pictures.
    For good measure, I'm adding G10/20/30 too so you have all the family.
    By the way, what are you trying to fix?


    PD: that said, yours might be *completely* unrelated

    EDIT: that's the easiest part: 2200x50V or 4700x50V .

    Tne earliest ones used large chassis clamp mounted electrolytics, modern ones will be radials 1/2 (or less) the size, solder their legs where approppriate.

    Unless you want to keep the Mojo and insist on $65 a piece NOS Electrolytics.
    If so, I also have a couple bridges you might interested in, you give me U$5000 in cash, in small denomination unmarked bills and then you can charge toll to cars using it, how's that?
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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Here's a Gibson G50/60/70/80 schematic, obviously a very different version under the same name.
    It's in SSGuitar
    Login
    Gibson G-80
    As said before, they kept replacing them under warranty, this one is a little stronger, used TIP33 powerhouse transistors .
    Don't know which you actually have, a Darlington or a Discrete one?
    Post a couple pictures.
    For good measure, I'm adding G10/20/30 too so you have all the family.
    By the way, what are you trying to fix?


    PD: that said, yours might be *completely* unrelated

    EDIT: that's the easiest part: 2200x50V or 4700x50V .

    Tne earliest ones used large chassis clamp mounted electrolytics, modern ones will be radials 1/2 (or less) the size, solder their legs where approppriate.

    Unless you want to keep the Mojo and insist on $65 a piece NOS Electrolytics.
    If so, I also have a couple bridges you might interested in, you give me U$5000 in cash, in small denomination unmarked bills and then you can charge toll to cars using it, how's that?
    Thanks Juan ,I'm good on bridges lol
    I'll get some pictures asap.

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    Here are a couple of chassis shots.
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    Old Timer J M Fahey's Avatar
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    Ok, then you seem to have the same power amp as the original "orphan/annotated" schematic, because on ther right edge of the first picture I see 2 x TO92 transistors (Q3/Q4?) , a TO220 one (Q5?) , 2 puny small plastic transistors (TO126 case? ... ridiculous) 2 bias diodes clamped to chassis/heatsink and the very old style bias trimmer.
    I'd replace the power transistors with TIP142/147 without even checking them, go figure.
    Pay attention to pinout which does NOT match.

    The preamp might match though:
    2 DIP ICs
    2 trimmer pots (reverb & tremolo)
    5 TO92 transistors
    1 heatshrinked *lamp*/LDR tremolo roach.

    Oh well.

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Ok, then you seem to have the same power amp as the original "orphan/annotated" schematic, because on ther right edge of the first picture I see 2 x TO92 transistors (Q3/Q4?) , a TO220 one (Q5?) , 2 puny small plastic transistors (TO126 case? ... ridiculous) 2 bias diodes clamped to chassis/heatsink and the very old style bias trimmer.
    I'd replace the power transistors with TIP142/147 without even checking them, go figure.
    Pay attention to pinout which does NOT match.

    The preamp might match though:
    2 DIP ICs
    2 trimmer pots (reverb & tremolo)
    5 TO92 transistors
    1 heatshrinked *lamp*/LDR tremolo roach.

    Oh well.
    Thanks Juan ! I don't have this amp in hand yet. It belongs to my cousin ,and I'm waiting for him to bring it to me. We'll get a parts list together also.

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    Quote Originally Posted by J M Fahey View Post
    Ok, then you seem to have the same power amp as the original "orphan/annotated" schematic, because on ther right edge of the first picture I see 2 x TO92 transistors (Q3/Q4?) , a TO220 one (Q5?) , 2 puny small plastic transistors (TO126 case? ... ridiculous) 2 bias diodes clamped to chassis/heatsink and the very old style bias trimmer.
    I'd replace the power transistors with TIP142/147 without even checking them, go figure.
    Pay attention to pinout which does NOT match.

    The preamp might match though:
    2 DIP ICs
    2 trimmer pots (reverb & tremolo)
    5 TO92 transistors
    1 heatshrinked *lamp*/LDR tremolo roach.

    Oh well.
    I finally have this amp in hand. Looks like someone tried to mod? ,or repair at one time. Added caps ? Some places you can see were disconnected from the board ,and hardwired. May have to strip all the mods? ,and start over.
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    Could Admin please move this ?

    Could this thread be moved to repair ,and troubleshooting , or should I start another for repair help ?

    Thanks !

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by gtrplayr1976 View Post
    Could this thread be moved to repair ,and troubleshooting , or should I start another for repair help ?

    Thanks !
    No admin here but you could start a new thread at the appropriate spot R&T, & put a link back to this thread for e-z reference. Costs nothin' but a minute of your time.

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    Quote Originally Posted by Leo_Gnardo View Post
    No admin here but you could start a new thread at the appropriate spot R&T, & put a link back to this thread for e-z reference. Costs nothin' but a minute of your time.
    Thanks Leo_Gnardo lol

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  16. #16
    Supporting Member Jazz P Bass's Avatar
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    Thank you for the schematic J M Fayhey!!!
    I could not find this oddball version anywhere.
    Kudos to MEF.

    I got one of these jewels in with the MJE1102/ 1092 transistors installed.
    Of coarse they where blown.

    I have reattached the schematic: Click image for larger version. 

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  17. #17
    Supporting Member Jazz P Bass's Avatar
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    Here is the schematic with the correct orientation of Q6. (it is a NPN transistor)

    Edit: The LTP (Q3 & Q4) where drawn as PNPs when in fact they are NPN transistors.

    G70 Power Amp.pdf

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    Last edited by Jazz P Bass; 11-13-2019 at 05:57 PM.

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