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Replace pots on Acoustic 370

Discussion in 'Amps [BG]' started by shadewind, Dec 22, 2009.


  1. shadewind

    shadewind

    Joined:
    Jun 25, 2008
    Location:
    Sweden, Örebro län
    Hello!

    I have an Acoustic 370 head and the rotary pots on the front have gone bad. So I decided I'll replace them. Unfortunately, there is no model number on them and I doubt I can get ahold of the same ones anyway so I decided to check the schematic:

    http://acc.homeunix.net/images/schematics/370a1.jpg
    http://acc.homeunix.net/images/schematics/370b.jpg
    http://acc.homeunix.net/images/schematics/370c.jpg

    According to the schematic, all of the pots are 50k. But when measuring with a multi meter, I get this:

    Vol: 42 k
    Treble: 20 k
    Mid: 22 k
    Bass: 25 k

    So I'm wondering, which is right? 42 k is within normal tolerance of 50 k pots I suppose and the 20 and 22 ones is probably 20 k pots and the 25 k... well... 25 k maybe?

    But why do my measurements differ from the schematic? Different revisions? Or are the pots on my amp just bad and need to be replaced?

    So... should I go for schematic or measurement?


    Another question, I assume the volume pot should logarithmic, but what about the tone pots? Lin or log?

    //Emil Eriksson
     
  2. lfh

    lfh

    Joined:
    Dec 18, 2003
    Location:
    Stockholm, Sweden
    A quick look at your numbers suggests that you measured the pots in-place, right? Remember that there's a DC path through all the pots within the tone control network, so the reading in that case is the total resistance of three branches in parallel. Go with the published values. As for taper, typically lin taper for active tone controls, lin or log for volume controls, depending on load impedance (with the proper loading, a log taper can be approximated using a lin pot). This you can double check after desoldering the pots.
     
  3. shadewind

    shadewind

    Joined:
    Jun 25, 2008
    Location:
    Sweden, Örebro län
    Now I feel kind of stupid, obviously the resistance will be less than the specified value if I measure in place :)

    Anyway, since I'm going to replace them, I may as well desolder them and measure them before ordering. According to the schematics, all resistors should be specced to 0.5W unless noted otherwise so I'll make sure of that.

    One thing I'm wondering... in the schematic, it says that you should set R304 for symmetrical clipping... I assume this is the bias setting... is this something I would need to set if replacing anything? Is it something one ever needs to set after manufacturing in a solid state amp?
     
  4. Jaco who?

    Jaco who?

    Joined:
    May 20, 2008
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  6. lfh

    lfh

    Joined:
    Dec 18, 2003
    Location:
    Stockholm, Sweden
    Don't worry about R304. AFAICT it controls the drive symmetry between the upper and lower output transistor banks. Unless you're replacing components in the power amp or suspect that the amp isn't up to specs power-wise, the factory setting should still be OK. (Of course, the amp is 30 years old or more, so component values may have changed a little over the years. If you do have an oscilloscope, it never hurts to check and possibly fine tune while the amp is on the workbench.)
     
  7. KramerBassFan

    KramerBassFan

    Joined:
    Jan 3, 2009
    +1 to Jaco Who.

    I find it interesting that ALL of your pots would go "bad". (not that im questioning your opinion here)

    Dirty more likely.
    that said, I'd still recommend desoldering and removing them from the chassis.

    Prevents electronics cleaner from getting everywhere. ;)
     
  8. shadewind

    shadewind

    Joined:
    Jun 25, 2008
    Location:
    Sweden, Örebro län
    Maybe I really should try that. I've just heard mixed opinions about cleaning pots. I've heard some people say that it's a lost cause to clean pots, it's better to replace them.

    But hmm... where should I spray the cleaner?
     
  9. KramerBassFan

    KramerBassFan

    Joined:
    Jan 3, 2009
    In the small opening.
    Right next to where the tabs are mounted.

    Don't try spraying it down the shaft- THAT is a lost cause. ;)

    Cleaning them works just fine.
    Now, it if TRULY is broken, then Yes. Cleaning it would be a waste of time.
    But i suspect the pots being mentioned are dirty.
     

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