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    VTA upgrade board with SDS cap board bias problem.

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    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Wed Feb 04, 2015 5:38 pm

    [size=31] PLEASE HELP!
    I am at my wits end.  I jeep trying to bias this amp and to no avail.  First I did the 1N4007 on the 5ar4 mod.  This stopped the fuse from blowing but now the tubes glow orange and then the fuse blows after about a minute.  When it blows the voltages measures 4v on both left tubes.  I only have in the 5ar4 and the 2 left channel  EL34's. I switched out the 22k resistor on the SDS cap board for a 2.2k resistor for R9 and bridged R10. [/size]

    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Wed Feb 04, 2015 8:05 pm

    https://servimg.com/view/19134842/1


    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Wed Feb 04, 2015 8:09 pm



    Last edited by tharhai on Wed Feb 04, 2015 8:17 pm; edited 1 time in total (Reason for editing : error in image)

    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Wed Feb 04, 2015 8:14 pm


    audiobill

    Posts : 270
    Join date : 2014-03-13
    Location : Philadelphia

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by audiobill on Thu Feb 05, 2015 5:03 am

    Do all the tubes red plate, or just one?

    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Thu Feb 05, 2015 9:59 am

    They all red plate after about a minute. I tried different tubes that were also brand new on both the left and right channel. They all exihibit the same behavior of overheating after they are fully warmed up. Voltage climbs fast up to 4v then stops and fuse blows about 5-10 seconds later. I measured the pins #5 and #6 of each output tube socket last night. Whereas they should be between -45 to -65 (I believe), they all measured closed to zero. (For example 0.03v) This seems to be related to my problem but I don't know where to proceed from here. I also checked all solder joints again and reflowed a couple that looked like they might be cold but no change.

    Bob Latino
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    Re: VTA upgrade board with SDS cap board bias problem.

    Post by Bob Latino on Thu Feb 05, 2015 12:10 pm

    tharhai wrote:They all red plate after about a minute.  I tried different tubes that were also brand new on both the left and right channel.  They all exihibit the same behavior of overheating after they are fully warmed up.  Voltage climbs fast up to 4v then stops and fuse blows about 5-10 seconds later.  I measured the pins #5 and #6 of each output tube socket last night.  Whereas they should be between -45 to -65 (I believe), they all measured closed to zero.  (For example 0.03v) This seems to be related to my problem but I don't know where to proceed from here. I also checked all solder joints again and reflowed a couple that looked like they might be cold but no change.

    If you get no negative DC voltage on pins 5 and 6 on the output tubes then the bias system is not working. Put up a photo of the complete back side of the driver board so that we can look at the board. One of us may be able to find something in the photo that would cause a loss of bias voltage.

    You should be getting -30 to about -45 VDC on pins 5 and 6 with an VTA boarded ST-70 with the 4 bias pots set near the center of their rotation.

    Bob

    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Thu Feb 05, 2015 1:31 pm

    I just resoldered the diode on the driver board. I now get -55v with no tubes and was able to bias the left channel at .4V each tube. Is the -55v from pin#5 & #6 to ground an okay measurement (without the tubes).

    audiobill

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    Location : Philadelphia

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by audiobill on Thu Feb 05, 2015 1:35 pm

    Tharhai, that sounds more like it!

    I note you're in Downingtown; I'm in Chester Springs.

    Bob Latino
    Admin

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    Join date : 2008-11-26
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    Re: VTA upgrade board with SDS cap board bias problem.

    Post by Bob Latino on Thu Feb 05, 2015 1:35 pm

    tharhai wrote:I just resoldered the diode on the driver board.  I now get -55v with no tubes and was able to bias the left channel at .4V each tube.  Is the -55v from pin#5 & #6 to ground an okay measurement (without the tubes).

    Yes - that -55 VDC that you get will vary with the setting of the bias pot .. Try biasing the amp again now that you have bias voltage ..

    Bob

    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Thu Feb 05, 2015 2:41 pm

    Thanks for the help Bob and everyone else that chimed in to help.
    audiobill: I used to live in Chester Springs.  I lived on Dartmouth Road off of Station Boulevard.

    P.S. A picture of the offending broken solder point.  "A small leak will sink a great ship." - Benjamin Franklin

    audiobill

    Posts : 270
    Join date : 2014-03-13
    Location : Philadelphia

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by audiobill on Thu Feb 05, 2015 3:27 pm

    Cool - I'm just off 100 near Eagle.

    Congrats on the fix!

    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by colin86325 on Thu Feb 05, 2015 5:19 pm

    Another local guy here--I'm in Glen Mills. Smile

    audiobill

    Posts : 270
    Join date : 2014-03-13
    Location : Philadelphia

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by audiobill on Thu Feb 05, 2015 7:03 pm

    Hey, Colin!

    tharhai

    Posts : 8
    Join date : 2015-02-04
    Location : Downingtown, PA

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by tharhai on Thu Feb 05, 2015 10:13 pm

    Finished product:

    MontanaWay

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    Age : 58
    Location : Brookings, Oregon

    Re: VTA upgrade board with SDS cap board bias problem.

    Post by MontanaWay on Thu Feb 05, 2015 10:35 pm

    tharhai wrote:Finished product:

    a nice clean looking amp, like the red...now just gotta loose those labels! Smile

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