LA2A is shocking ME!!

Discussion in 'Electronic Repair Modifications DIY' started by sprouseod, Feb 6, 2002.

  1. sprouseod

    sprouseod Guest

    I recently aquired and older LA2A. Here is my problem after connecting to my patchbay (1/4"TRS)I get a pretty good jolt when handling the conections in the back of the patchbay. This occurs when one hand touches connections for both the LA2A and any other plug on the back of the patchbay. Thse unit is wired --input (1)hot (5)cold with the shield jumped to terminal 7 (chassis), --output (8)hot (10)cold shield is 7 It seems to pass audio OK. although going past 30 or 40 on the input gain seems to start distorting the signal. Any insight would be helpful

    Richard :confused:
  2. Kev

    Kev Well-Known Member

    Nov 13, 2001
    Bad news.

    This is a valve device and so has high voltage inside. Please have respect for the voltages contained within. Here is a problem of that voltage getting out.

    First suggestion is to take it to a Professional who is familiar with valves .... or tubes if you prefer.

    If you want to have a go, I warn you that I'm not an expert but am endeavouring to gain more knowledge on the subject. I have a number of valve projects on the go at the moment , including an LA2 but so far I'm not happy with them.

    So here is my second suggetion. Go to o/Schematics/Schematics.html and find the Urie LA2 schematic and download and print.

    Check inside the box to see if it looks like the circuit and all bit are there like Transformers etc.

    As a first guess check C5 and perhaps replace it.

    Let me know how things go.
  3. punk

    punk Guest

    be careful to discharge the electrolyte caps before poking around inside. The input gain is actually output, there is no input control. So the distortion is probably not from the LA2 but whatever follows it being overloaded. You should be able to find a schematic on line at - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!

    A bad cap is a good first check as Kev suggests.

    good luck
  4. Kev

    Kev Well-Known Member

    Nov 13, 2001
    Hi punk and great to have you here. wavey.gif
    His view and perspective on valve related gear will be very welcomed.
    Thanks punk.... as I said his perspective is excellent. I should have pointed this out to a new comer to valve gear. I did say high voltage inside but didn't suggest the discharge bit. I tend to assume too much prior knowledge.

    The point about the output is very important as the LA2 has much gain and level available at the output.... sometimes too much.
  5. punk

    punk Guest

    Thanks Kev!

    Also check out this site for a book on how to build an LA2. This should be useful for fixing an original.
  6. rmburrow

    rmburrow Active Member

    Dec 26, 2012
    western PA
    I just looked at a LA-2A on the bench. Does your LA-2A have a two prong power cord? The chassis of the LA-2A is NOT grounded with respect to the AC supply (like most modern equipment with the 3 conductor plug). The AC path is simple, hot side of AC through fuse to hot side of power transformer primary, low side of power transformer primary through on-off (panel mounted) toggle switch to cold side of AC. The AC plug is non polarized.

    Look at the wiring from the AC cord. The white (cold) side goes through wiring which folds when the front panel is opened or closed. Is any of the primary wiring damaged (worn or brittle insulation)? If so, repair. Try reversing the AC plug in the outlet or power strip (in other words reverse the prongs and plug it back in). Any changes or still getting a shock? Try a ground from the LA-2A chassis to other equipment. Any changes or still getting a shock? If you try any of these, feel the power transformer (black transformer to the left, viewed from the BACK of the LA-2A for excessive heating. It is rare but primary AC or secondary power transformer core leakage could put the chassis at AC potential.

    (The non-polarized plug, effective grounding, and high voltages associated with tube equipment caused numerous accidents before AC powering and grounding of equipment became standardized.)
  7. DonnyThompson

    DonnyThompson Distinguished Member

    Nov 25, 2012
    Akron/Cleveland, OH
    Home Page:
    I'm sorry, I can't help but chuckle at this... and I know I shouldn't... I just....I can't help it. I just get this vision of you patching in the LA2 and your hair standing on end ..."sonova...motherfu...!"

    Ahh...the prices we pay and the pain we endure to stay sonically pure at heart.

    Nothing like using gear that can also double as a heating or cooking source. ;)
  8. Kurt Foster

    Kurt Foster Distinguished Member

    Jul 2, 2002
    77 Sunset Lane.
    as Henny Youngman said, "well don't do that". simple fix, put some heat shrink or electrical tape on the LA2 connectors to insulate them ...

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