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    vta st70 low volume right side

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    Its a toaster

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    Join date : 2016-09-30

    vta st70 low volume right side

    Post by Its a toaster on Wed Oct 05, 2016 12:02 pm

    Ok this is driving me nuts. I installed an Octal board in my st70 and it sounded great for about 7hrs of use then then volume on the right side went to almost nothing. I have verified the Inputs, tubes, solder connections, resistors at the sockets, voltages are all good. Where do I look next? Whats the best way to verify that the cap on that side is good? need help please.

    tubes4hifi
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    Re: vta st70 low volume right side

    Post by tubes4hifi on Wed Oct 05, 2016 12:19 pm

    more likely a flakey solder connection than a bad capacitor, but you could easily swap the two output capacitors on side side to the other side and see if the problem follows.

    Its a toaster

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    Join date : 2016-09-30

    Re: vta st70 low volume right side

    Post by Its a toaster on Wed Oct 05, 2016 1:24 pm

    I went back and re soldered the whole right side. no change. tested resisters on the board and they measure correct. rechecked all grounds. going to try cap swap after I get everything discharged. any other ideas?

    Peter W.

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    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: vta st70 low volume right side

    Post by Peter W. on Wed Oct 05, 2016 4:09 pm

    Its a toaster wrote:I went back and re soldered the whole right side. no change. tested resisters on the board and they measure correct. rechecked all grounds. going to try cap swap after I get everything discharged. any other ideas?

    Did you swap the output tubes?
    Check the drivers? If ones side of a common dual-triode is weak - there you are.

    Do you have the capacity to check tubes?

    peterh

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    Location : gothenburg, sweden

    Re: vta st70 low volume right side

    Post by peterh on Thu Oct 06, 2016 1:57 am

    Swapping the outputs as suggested would tell more. It's a matter of rerouting the cap from the
    board AND the feedback from the output tranny.
    If the problem changes side the fault is at the board, if the problem stays it's the output tranny or
    the tubes that is the cause.

    Peter W.

    Posts : 152
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: vta st70 low volume right side

    Post by Peter W. on Thu Oct 06, 2016 7:18 am

    peterh wrote:Swapping the outputs as suggested would tell more. It's a matter of rerouting the cap from the
    board AND the feedback from the output tranny.
    If the problem changes side the fault is at the board, if the problem stays it's the output tranny or
    the tubes that is the cause.

    Hmmmmmm....

    I would think that if the problem follows the tubes, then the tubes might be at fault....

    Or am I missing something?

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: vta st70 low volume right side

    Post by peterh on Thu Oct 06, 2016 7:58 am

    Peter W. wrote:
    peterh wrote:Swapping the outputs as suggested would tell more. It's a matter of rerouting the cap from the
    board AND the feedback from the output tranny.
    If the problem changes side the fault is at the board, if the problem stays it's the output tranny or
    the tubes that is the cause.

    Hmmmmmm....

    I would think that if the problem follows the tubes, then the tubes might be at fault....

    Or am I missing something?
    No, you are not missing anything.
    But moving two connections ( swapping side) will test powertubes AND OT without needing to
    pull the tubes from the sockets. And forget tubetesters, they won't help here.

    Peter W.

    Posts : 152
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    Location : Melrose Park, PA

    Re: vta st70 low volume right side

    Post by Peter W. on Thu Oct 06, 2016 10:25 am

    >>And forget tubetesters, they won't help here.<<

    On the tube-testers: If one of the driver tubes is faulty, specifically a dual-triode, as in the Whoville Christmas tree that does not 'light on one side', it could explain low volume. Similarly, a tube with a shorted element.

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: vta st70 low volume right side

    Post by peterh on Thu Oct 06, 2016 11:06 am

    Peter W. wrote:>>And forget tubetesters, they won't help here.<<

    On the tube-testers: If one of the driver tubes is faulty, specifically a dual-triode, as in the Whoville Christmas tree that does not 'light on one side', it could explain low volume. Similarly, a tube with a shorted element.

    Yeh Yeh,
    testing with another "known good" tube takes seconds and don't involve a tester.

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