Calling all techs....(this is a longshot)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mark norwine, Sep 23, 2008.

  1. mark norwine

    mark norwine Member

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    Retired though I might try to be, I was asked to take a look at a friend's neighbor's son's bandmate's bass amp.

    (visions of Pacino in G-III: "They keep pulling me back in")

    Anyway....It's an Acoustic 370. An earthmover, if ever there was one! You all old enough to remember these battle tanks?

    Back to the amp: this one has a PT with the pri winding shorted to ground.

    (visions of Dr. "Bones" McCoy to Capt James Tiberius Kirk: "It's dead, Jim")

    The schematic shows this as Acoustic part # T-401. It's massive.

    Since Acoustic Control Corporation is deader than this kid's amp, getting a replacement is not going to happen via the "usual suspects" (Mojo, New Sensor, etc.).

    Now, I have no real issue in declaring this amp to be "dumpster food", but I told my friend I'd "reach out".

    Anyone, by chance, have an Acoustic 370 "organ donor" with a good PT?

    (or will this thread sink without a trace by tomorrow?)

    Anyway....thanks for reading!
     
    Last edited: Sep 23, 2008
  2. donnyjaguar

    donnyjaguar Member

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    One of the many output transistors is probably shorted causing the amp to draw too much current. The resultant heat likely caused the failure. So I'm saying its probably more than a simple swap to make it right. Acoustic made the most tubey sounding solid state amp's IMO. They are worth keeping.
     
  3. mark norwine

    mark norwine Member

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    Not knowing the exaxct specs of the existing PT, it's tough to say exactly what I need.

    Also....this is a favor to a friend's neighbor's son's bandmate. I want to invest as little time & money as possible, thus my search for a direct organ donor.
     
    Last edited: Sep 23, 2008
  4. UR12

    UR12 Member

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    You can get in touch with Phil at Heyboer transformers and they can make you a duplicate if you want to go that route.
     
  5. Rosewood

    Rosewood Member

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    If you knew the specs you might win the lottery by using something like a peavey power amp trans, ( that's reaching ain't it ) otherwise rewinding or donor looks like it. You just know someone has a 370 in their basement with a good trans, I'd bet on it.
     
  6. mark norwine

    mark norwine Member

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    This is....easily....a $200-250 PT. I'm certain the owner doesn't want to invest that.

    These amp were real good at frying their output transistors (and taking out the PCB at the same time), but leaving the PT in useable shape. I need to find one such amp, presently in use as a footrest or doorstop.

    My hope, entirely!!
     
  7. Rosewood

    Rosewood Member

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  8. OlAndrew

    OlAndrew Member

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    Might wanna try over here:

    http://music-electronics-forum.com/forumdisplay.php?f=10

    There's guys that know somewhat about Acoustic stuff.

    Big transformers don't usually blow unless they've got a good reason, though...have to thoroughly troubleshoot the board before lighting it up, even if you do find a replacement.
     
  9. booj

    booj Member

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    Looks like a 70 volt tranny. Measure the footprint made by the mounting holes. Match that to a new tranny along with the rest of the measurements (height, width,depth). Get one that's the same size. viola
     
  10. mark norwine

    mark norwine Member

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    How do I know? C'mon... All leads disconnected from everything, there's 0 ohms from one pri lead to the case, & 0.2 ohms from the other pri lead.

    Guys, I appreciate all of the input.....really, I do. Trust me, I know how to design amps & I know I can back-engineer in some Peavey PA transformer, etc.

    I was looking for a 370 carcas....nothing more.

    Anyone have one?
     
  11. PRNDL

    PRNDL Member

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    I just finished repairing an Acoustic 370 that did just that ... it started smoking and burned the ground trace through the preamp. By the time it was done, over 20 parts had to be replaced! I used three different vendors, since none had everything.

    You're a little out of date with that request ... I've noticed the used market has dried up over the last few years.

    And ...

    These Acoustic 370's have a HUGE sound. It's smooth like a tube amp but it seems to have a flat EQ up to extremely high frequency, which is most likely from the DC coupling. $470 is about right for one in good condition. These are no longer unknown jems, and they don't make them like this anymore. It's worthwhile to fix a "carcas" instead of using it for parts.

    Also, at a minimum I'd suggest replacing all 8 output transistors, since something blew that PT.
    Trust me when I say it's no fun seeing an amp smoke like that.
    I'd check every transistor on the power amp board to avoid that, since one bad one will start a chain reaction.
     
  12. mark norwine

    mark norwine Member

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    I gave the amp back to my friend this morning.

    Afterall, I'm retired & never said I'd fix it. I said I'd "check it out" and, if I found something simple, I'd then quote a price.

    From the start, the owner didn't want to put anything into it, so unless I found a PT in someone's junk pile (which I didn't), that was to have been the end of the game. And that's how it played out.

    Thanks for the suggestions, though....

    mn
     
  13. PRNDL

    PRNDL Member

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    The owner of the amp I fixed is balking at the final price - $200, which includes $50 in parts (mostly shipping) and $20 for a decent schematic (the free one wasn't readable), even though I got an OK to continue before ordering parts.

    He was mostly concerned about my guarantee since the last tech charged a lot and the amp didn't work at all (I had to fix some wiring mistakes), and when asked the tech said he'd have to charge even more to fix it.

    Fortunately I told him to bring his bass and maybe a speaker to try it. The last tech wouldn't let him play it in the shop.

    It's been a week and I haven't heard back, but the owner is out of town on business a lot.

    Has anyone had similar issues this week?

    I've had a bunch --

    On Thursday I paid $150 for lawnmower repair only to have it not start. I fixed it myself (it was a bad ground wire -- real simple, but not easy to figure out).

    I also had one of my repairs (a JMP1 preamp) not work. I asked the owner to return it so I could work on it some more (at no charge, except parts), but I think he's sending it to Marshall, which might be better since the issue was weird/complex. I offered to mail a refund, but he didn't give an address. He appreciated my candor.
     

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