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    Newbie needs more advice

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    grizzinca

    Posts : 40
    Join date : 2012-01-01

    Newbie needs more advice

    Post by grizzinca on Sat Feb 18, 2012 5:23 pm

    Got the proper resistors, put them in place. now the right channel doesn't play as loud as the left. Rechecked my work and all seems correct. I should mention I also replaced the old speaker terminals with dual binding post. I switched all the inputs around to eliminate a bad speaker or cable. I also ran the amp (st 70) in mono and came back to the right channel not as loud as the left. Where should I look next? I capped the wires from the 4 and 16 ohm leads. Seems maybe something in my soldering work? Bad resistor? Would appreciate some of ya'lls expertise.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Newbie needs more advice

    Post by Bob Latino on Sat Feb 18, 2012 6:27 pm

    Check that you reconnected the feedback line to the driver board on both channels properly. The two lines go from the 16 ohm tap on each output transformer to eyelets 12 (left channel) and 13 (right channel). If they are not connected or one is shorting to the chassis then the two channels will have different amounts of gain.

    Bob

    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: Newbie needs more advice

    Post by stewdan on Sat Feb 18, 2012 6:33 pm

    Hi--

    I was writing this when Bob responed about the NFB wires from the 16 Ohm speaker wires to eyelets #12 and #13 of the circuit board.

    I think some additional info is required in order to help.

    What exactly are you running? A ST70 with the stock driver card plus your resistor changes or something else??

    Did the unit exhibit a low right channel before you made the changes?

    You say you "capped off" the 16 Ohm speaker wires, but it is the 16 Ohm wires that provide the Non-Linear feedback to the Print Circuit board!!

    You might try simple right to left tube swaps -- Turn the power off for the tube swaps below and wait 3 to 4 minutes before powering on"

    Does moving a tube from the right channel to the left channel change the symptom or do you still have the same low right channel?

    With power off, first try swapping 7199 tubes on the circuit board. Try one tube per side at a time and shut down for 3 to 4 minutes between tube swaps.
    (The tube swaps will tell whether there is a tube problem in the right channel or a circuit problem)

    With the power off, swap one EL34 tube from the right to the left channel and see if the problem moves to the left channel. Power off, then swap the other pair, etc.

    If you have already tried tube swaps and still have a low right channel let us know.

    Actually let us know in all cases.

    Stew

    grizzinca

    Posts : 40
    Join date : 2012-01-01

    my st 70 upgrade woes

    Post by grizzinca on Sun Feb 19, 2012 12:56 am

    Thanks everyone for your quick replies. it's like having a tech do the work for me. I think my first fix will be the feed back lines to the driver board. Seems to be the obvious error. No. The amp showed no signs of weakness in either channel prior to me upgrading. If reconnecting the feedback lines doesn't do the trick, I'll try swapping tubes around. Again, thanks to all

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