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    MK3 VTA Board Wiring Question

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    hansfob

    Posts : 18
    Join date : 2010-10-24
    Age : 52
    Location : Rickmansworth, England

    MK3 VTA Board Wiring Question

    Post by hansfob on Mon Apr 16, 2012 7:34 am

    Hi,

    I am currently fitting some VTA driver boards (low gain) into my MK3s to replace the stock boards and have a couple of questions that I am hoping the forum can help with.

    On the stock wiring there is a wire from pad 4 of the driver board to pin 4 of one of the output tubes. Where does this go with respect to the VTA board?

    I am also confused about the green/yellow wire from the power transformer as I saw a photograph (www.tubes4hifi.com/Dyna_MK3-DanF4.jpg) that shows this connected to the VTA board and yet there is no instruction to move it from the current location on the tag strip where it is connected to the .02 cap. Can anybody can guide with that as well please?

    Thanks for any help you may provide


    danf

    Posts : 58
    Join date : 2009-01-19

    VTA MKIII

    Post by danf on Mon Apr 16, 2012 7:53 am

    >On the stock wiring there is a wire from pad 4 of the driver board to pin 4 of one of >the output tubes. Where does this go with respect to the VTA board?

    This connection is not used on the VTA board (part of a different feedback scheme). Disconnect this wire.

    >I am also confused about the green/yellow wire from the power transformer as I saw a >photograph (www.tubes4hifi.com/Dyna_MK3-DanF4.jpg) that shows this connected to the >VTA board and yet there is no instruction to move it from the current location on the >tag strip where it is connected to the .02 cap. Can anybody can guide with that as >well please?

    This wire is the center tap of the 6.3 V filament supply. It should be grounded directly or through a capacitor. I grounded it at the VTA board to minimize hum. The stock wiring grounds this through s capacitor at the terminal strip between the fuse and the power transformer. This will work. If you ground this wire at the VTA board, remove the filament connection to the preamp power supply socket.

    stewdan

    Posts : 160
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: MK3 VTA Board Wiring Question

    Post by stewdan on Mon Apr 16, 2012 8:16 am

    Hi --- There is topic that was posted here on the forum called "octal option??", which is on Page #2 of the Forum Topics.

    Take a look at the 2nd page of that topic, specifically POST #24 where Roy posted a wiring diagram and instructions for the "Octal Version" of the VTA boards, WHICH is still valid for the Low Gain and High Gain versions of the board. (The only difference being were the Tube Filament Leads get soldered on the board.)

    Also, there should have been a couple of wiring diagrams in Roy's Documentation Package with the MK3/Vta Board.

    In answer to your specific question about where to put the wire from the old Dynaco PCB Pad #4 that went to Output Tube Socket #4 --- #4 is where an Output Transformer wire gets attached??? Do you mean Tube Socket #6?

    Let us know




    hansfob

    Posts : 18
    Join date : 2010-10-24
    Age : 52
    Location : Rickmansworth, England

    VTA MKIII

    Post by hansfob on Mon Apr 16, 2012 8:45 am

    Many thanks for your replies Danf and Stewdan.

    I can reward you by saying that I had actually connected the wire from pin4 of the output tube to the negative feedback connection on the VTA board. When I switched on, with just the rectifier tube in, there was a puff of smoke at which point I immediately switched off. The smoke was resistor R8 in the NFB circuit becoming toast and not sure about any damage to C12 yet.

    Although feeling rather embarrassed I would like to know if in your opinion my incorrect connection is almost certainly the cause of R8 smoking and if any other damage is likely. As I say I did not have the output tubes in so not quite sure how this happened.

    In hindsight I wish I had checked on the forum before making the connection but at the time I was sure it was correct.

    stewdan - I did mean Output Tube Socket #4 where there is the green wire from the output transformer, from Danf
    I think this was part of the stock NFB circuit. Thanks for the heads up on the posts, I'll check them out.

    Many thanks again.

    danf

    Posts : 58
    Join date : 2009-01-19

    Toast from pin 4

    Post by danf on Mon Apr 16, 2012 9:51 pm

    Probably the damage is limited to the feedback resistor. The feedback capacitor should be fine. The VTA feedback is supposed to come from the secondary of the OPT, not the primary. The primary has about 450 volts DC on it, and running through the 10K feedback resistor to ground, this would push about 20 W, so the resistor would fry. It is possible that your input tube was harmed, but if you are lucky it's just the resistor. On the original Dynakit board, this connection went through a capacitor, blocking that 450V DC.

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