HUM problem again (A15Mk1)

    Hi,
    I have a bad hum problem with my a15mk1. The layout of my amplifier looks like the one on the schematic, the wiring seems ok (double checked) but lot of noise comes out from the speaker (zzzzzzzz hhsssss ) even from the clean channel of course with the pot all at noon position.
    Now what I want to do is desoldering the second channel and insulate it and try to debug the first channel in order to understand where the hum comes from and then debug just the second channel with the first completely insulated (not just turning off the channel select switch)
    I want to pinpoint out the problem and try to figure out if there is some wiring problem I haven't found yet! so I just want one part of the circuit board to work at time!
    So now comes the questions:
    1) Which wires I have to desolder from a complete circuit in order to have just the first channel working? and nothing more?
    2) Which wires I have to desolder from a complete circuit in order to have just the second channel working? (no boost mode) and nothing more?
    3) which wires I have to desolder from a complete circuit in order to have just the second channel boosted working?
    4) I bought a deluxe version of the amp, so another question is about the paint (that insulate the chassis)..
    Have I to remove this paint under the socket tube to better ground them that could be a problem I have noticed that if I press with my thumb the chassis between socket 2 and 3 with a lot of strength as i would like to bend the chassis (not just putting finger on it) the noise disappear!

    any other help is really apreciated!

    Re: HUM problem again (A15Mk1)

    Hi Krusty

    What a lot of problems! Oh dear!

    You told at Point4, that you can, by remove the colour, by the Tubesocket, your Problem is away !!!

    then do this! The next step is to put the metalcover on the tubes, this helps a lot to kill some problems.
    It looks better without this, but it sounds better with the metalcover over the tubes!!

    I wouldn´t disconect the boost channel! Please first check, preamp and Poweramp lonely, by using the Insert: send /return

    I can´t tell you directly, wich cables are to disconnect the Boostchannel; my Amp and my schematics are in the Studio and I don´t get there this month!

    My A15 makes also a lot of noise in the boost ++, but when playing; you can´t hear nothing else but me and my guitar! ;D

    In the clean Position, there´s no terrible noise!

    Good luck, Hatti

    Jeder ist etwas Besonderes! Auch DU! Lebe entsprechend; individuell!

    Re: HUM problem again (A15Mk1)

    I tried the power amp stage only using return input...
    The power amp is very silent so the noise in my amp comes out from the pre amp stage...

    Regarding the point 4 (removing paint) I found that there was a bad soldering on resistor R33 on V3. Now better situation ma always noise comes out...
    I was just asking about paint and insulation on the chassis...
    My tube has shield on it...and the situation doesn't change if i put the shield or if I remove it...

    The strange thing is that noise comes out even from the clean channel (al pot at maximum of course)...of course the noise is less on the clean channel and on channel 2 (no boost)

    Another strange thing that happens (I started another topic to explain it because I'm not the only one that has this prob) is that if i have all pot in chan 2 turned to off and the gain to the max some sound comes out from the speaker...so there must be a signal path from gain pot bypassing the other chan 2 pot and this is wrong I think...I used alpha pot and the volume pot on chan 2 seems to work correcly (checked resistance on it when all closed)...


    Thanks again
    Krusty

    Re: HUM problem again (A15Mk1)

    Hi Krusty,
    arer you sure, that channel 2 is turned off, when using channel 1? Please check (with amp turned off an power cord removed) that pin 7 of V3 is at ground level, when the channel switch is in "channel 1" position.
    Regards,
    Martin

    Re: HUM problem again (A15Mk1)

    Yes It is...0.02 Ohm when switch is in channel 1 position and 255K Ohm when in channel 2 position (no boost) and 280K Ohm when boost++!
    so the wiring seems ok...

    One solution is to post some MP3 with the "normal noise" a well build A15MK1 does when all the pot are to the maximum...Just to know if I have some mistake or that is the "normal noise level"

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