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Amp problem, can I fix it?

Discussion in 'Pro Audio Equipment' started by filete, Jan 30, 2006.

  1. filete

    filete Guest

    Hi there!

    I am about to move my studio to another place, and I found
    an old power amp (Alesis R-100) that it has this problem,
    I wonder if I can fix it:

    The right volume knob, when you turn all the way down, it continues
    to let sound out, if I turn to other position he gets a lot of variations
    on the volume level.
    As you can see, this makes impossible to stay on a specific volume
    level exactly as the left channel.

    Is this a easy thing to solve?
    And can I fix it or should I get technician help?

    This amp has been for several years (2-3), in the original
    cardbox, but with no use.
    If this problem becomes expensive to fix it, I just might give it
    away and get another one!

    Any help or repair link would be great!

    Thanks!
     
  2. HansAm

    HansAm Active Member

    Without thinking it trough..
    You need to replace the potentiometer. the volum'knob.
     
  3. filete

    filete Guest

    But from the inside amp or the plastic knob itself?
    :?
     
  4. RemyRAD

    RemyRAD Well-Known Member

    It sounds to me as though the ground wire has come off from one side of the volume control, potentiometer? It will turn down but not all the way down (incoming and outgoing are connected but no ground reference).

    You don't need to replace the whole volume control unit just take out your soldering iron and make sure that the ground wire is securely soldered to either side of the potentiometer that is missing its wire. Please make sure to disconnect the amplifier from the outlet first. Problem solved!

    Securely grounded
    Ms. Remy Ann David
     
  5. moonbaby

    moonbaby Mmmmmm Well-Known Member

    And while you are inside the amp...you might spray a little contact cleaner into the potentiometer ("pot"...I won't go there). Anyone with a recording rig should have a can of DeOxit (by Caig Laboratories) you can get from Parts Express. This stuff is great on all sorts of dirty pots, switches, and connectors.Since you stated that the amp had been sitting around in the box for a couple of years, this would be a good time to clean it out a bit. And a can of that compressed air you get for PC keyboards,etc., would also be a good idea. Open up the amp and spray out all the cobwebs and dust with the air, then do what RemyRad said to do with the soldering iron, then spray the cleaner into the rear of the pots and on the contacts on all the connectors. You should be good to go!

    I'm a pot cleaner from days of yore!
     
  6. filete

    filete Guest

    I took the amp to a technician, and they told me that the potentiometers are too old, and it needs to be replace, but they can´t do it, because they don´t find from their suppliers potentiometers that big!

    Is it really hard to find potentiometers for that specific amp?
    Does the new RA150 use the same volume knob (potentiometer) size?

    I would be great if someone could tell me the reference of the potentiometer
    and also info where I could order and instructions how to change them yourself.

    Thanks
     
  7. dementedchord

    dementedchord Well-Known Member

    hate to say it dude but having been a tech it sounds to me like your gettin hosed.... 75-80% of all repairs are physical problems not electrical... and frankly remy's analysisis exactly what i would have expected to find... which granted doesn't account for the other 20-25%... but then anyone having trouble sourcing a volume pot should not be in the business....
     
  8. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    I agree with the others, but since nobody here really knows definitively, heed their advice, and here's some other possibilities. Of course, resolder the contacts and clean the pots. When you say "a pot that big", we're not sure what you mean. You seem to have already confused the knob with the pot that it's attached to, so while the knob might be large, the pot may be a normal size.
    Even if it's a board-mounted pot, and you cannot find an exact replacement, it may be possible to figure out how to chassis-mount it, and then wire it up.
    Board-mounted will have stiff L-shaped contacts that solder directly to a board. They may or may not have nuts that hold them tight against the chassis...behind the knob. Because the are mounted directly to the board with stiff connectors, they are susceptible to breaking connection because of physical movement. The worst is board-mounted with no chassis-mount nut to help secure it in place. Chassis-mounted will be held in place by nuts to the chassis, and wires will connect from the pot contacts to the board. These are the least susceptible because there is no direct rigid contact with the board. It's likely Remy is correct, especially if it's a board-mounted pot.

    If you cannot locate the exact pot, find out what value the original ones are, and see if you can find something that is the same value, with the same knob-shaft diameter, and see if it can be chassis-mounted. Also, keep in mind that there are "audio taper" and "linear taper" pots, and they are different. I assume that would be an audio taper pot. If all you can find are pots that you know are the right value and right diameter shaft, and you know you can mount it and connect it...but the shaft is too long...take a hacksaw to the shaft. Just try to not get one with the shaft too short.

    If resolder and cleaning doesn't work, find out exactly what type of pot it is, first. What is the value? Does it mount directly to the board, or are there wires? How big in diameter is the pot, and the shaft? (The diameter of the pot may make a difference on how a replacement can fit in. If the original pot is quite large, you MAY be able to install a smaller diameter pot, if it has the same diameter mount and shaft. But, if the pot is quite small, there may not be enough room to stuff a larger one in. Also, you may need to be careful of current ratings).
    Contact Alesis first. They may have some lying around. Search on-line
    surplus electronics stores. Places like MCM Electronics have a lot of stuff.
    Try, perhaps, Digi-Key, Mouser...all those places. If you DO find something that will work, might as well order two and replace them both. That way they more closely match, and it may save you from having to go through the whole thing again when the other one goes out.

    Where there's a will, there's a way.

    Kapt.Krunch
     
  9. filete

    filete Guest

    Hi!

    Thanks for your reply,

    I also think the techinician does not know how to fix it!

    so is going to return the amp today here at the studio.
    What really makes me mad, is that, they pickup the amp one month
    ago and took them all this time to tell what they told me!!

    Their company deals with Television and VCR repairs,
    so its seems to me now (and after your replies),
    they don´t know anything about Pro-Audio gear.
    But they don´t admit!!

    Around my area, there is no store/company that work with Studio Pro,
    so I took a shot, I see i bad one now!!

    Anyway, I want to give a try, and be able to look exactly for the problem,
    and be able to fix it myself here at the studio.

    So, dementedchord, since you are/been a tech, maybe you can help me!
    What should I looking for after open the amp?
    Should I follow RemyRAD advise and search for cables that are unsolder on
    the potentiometer pots?

    Can you post any graphic or photo example, so I know exactly what
    I should be looking for?
    As soon as I get the amp back, I will open, and will take some pictures
    so you guys have a idea what kind of potentiometer and knobs its made of!

    Thanks again

    P.S - Kapt.Krunch , I already contact Alesis, but there´s no reply yet!
     
  10. RemyRAD

    RemyRAD Well-Known Member

    filete, if in fact the front panel volume control has gone bad, the other TV technician may have only dealt with Radio Shaft and other smaller Taiwanese volume controls which typically have been a slightly smaller form factor/package than some of the other really good longer-lasting volume controls made in America by companies like Alan Bradley, Clarostat and others, which in the past were slightly larger. While some volume controls are mounted directly to circuit boards and have to be of a certain type, terminals and size, others are connected via wires, which makes for more versatile mounting. When re-soldering the existing connections, use a pencil type soldering iron from 25 to 40 Watts, not a gun. Heat the connections first before adding just a little extra rosin core solder of 60% lead and 40% tin, not that awful "Green" aluminum based crap, unless you live in a Green country that does not allow lead based products such as New Zealand. The solder should flow smoothly before you remove the soldering iron but try not to overheat. It should only take a few seconds. The junk left behind is the "flux" that resides in the center core of the solder, that makes the solder flow which you should not worry about and there is no need to clean it off even though it looks like sh*beep.

    By the way, I think that is a marvelous picture of you! No mental illness there.
    Ms. Remy Ann David
     
  11. stickers

    stickers Active Member

    remy why arent you on aim!
     
  12. JoeH

    JoeH Well-Known Member

    If you can't get an exact replacement from Alesis, and if you can't repair or swap it with something similar, why not wire them out of the circuit and be done with it? Even use clip leads (for now) if you have to, and get back to biz.

    I had a couple of old Yamaha 2100's that had problems with the input pots (which were delivishly difficult to repair at the time). On one of them, I simply jumpered the leads used for attenuation, and the amp worked perfectly fine thereafter - but with no input level control - which I didn't really need anyway for the way I was using the amp (in a monitor rack).

    Assuming it's just input level trim, you don't necessarily need it anyway, you have a number of other options: use your mixer's outputs to control the speaker level (which you'll be doing anyway). If you have something in the path ahead of the amp, (EQ, crossover, line amp, etc.) use THAT device's outputs as trim controls. I'm not a big fan of extra physical components in the signal path anyway - esp those that can cause scratchy sounds and level mismatch - all caused by bad/cheap pots.

    Sure, it's nice to have precision control over an amp's performance, but I don't think we're talking about that kind of thing here, and considering the situation - inability to get parts (For now) or find a good tech (for now) I say: Jumper the sucker and get on with the business of making music.

    As always, YMMV. :wink:
     
  13. dementedchord

    dementedchord Well-Known Member

    i agree with joe with one slight exception... you might want to run the output of your console/soundcard hotter for gain staging issuses.. so jumper it out test it and if you need to pad it just add a couple of resistors to the thing in place of the pot... take two back to back one end to input one to ground and tap the center to go to the next stage... will take alittle experimentation to get the right values... and use 1% tolerance parts so the channels will match...
     
  14. RemyRAD

    RemyRAD Well-Known Member

    That doesn't sound very demented to me??

    Demonic engineer
    Ms. Remy Ann David
     

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