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    Dynaco ST-70 original driver boards - early and later versions - photos ...

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    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Dynaco ST-70 original driver boards - early and later versions - photos ...

    Post by Bob Latino on Fri Dec 19, 2008 12:19 pm

    Dynaco did make an effort to upgrade the cheap brown phenolic board on the ST-70 in the early 1970's by replacing the "brown board" with a much more durable and heat resistant pale yellow fiberglass board. It still had open solder traces but for it's time it was much more durable and a step up from the original brown board. Photos below.

    Bob

    Earlier phenolic board top



    Earlier phenolic board bottom


    Later fiberglass board top


    Later fiberglass board bottom
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    fortherecord

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    Join date : 2009-05-26
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    Location : Rochester, NY

    Re: Dynaco ST-70 original driver boards - early and later versions - photos ...

    Post by fortherecord on Tue May 26, 2009 10:40 pm

    From my experience, the traces on the later fiberglass boards were much more susceptible to lifting from the board while soldering and working on them. The traces are not easy to fix or replace once broken either. However, the fiberglass boards did not tend to develop that waxy or gooey film on them, which the older boards seemed to do. I'm not sure what caused that.
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    PeterCapo

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    Join date : 2008-12-05

    Re: Dynaco ST-70 original driver boards - early and later versions - photos ...

    Post by PeterCapo on Sat Sep 23, 2017 12:18 am

    fortherecord wrote:...the fiberglass boards did not tend to develop that waxy or gooey film on them, which the older boards seemed to do. I'm not sure what caused that.

    It's a coating of wax purposely applied to the foil side of the phenolic boards.  AFAIK, the foil side, after being etched, was susceptible to moisture ingress that is not good for the phenolic substrate, hence the wax coating.  When working on an original phenolic board, the wax coating should be left as intact as possible and no chemicals should be used on the board, IMO.

    PC boards were innovative for this kind of equipment, when the original Dynacos were designed.  PC boards, at least for non-critical commercial applications (vs. military applications), needed time to develop into something more robust, or so I would think.  The limitations of the Dynaco PC boards should be no surprise - it was probably the best with practical availability at the time, maybe the only practical choice.  If left undisturbed, it is remarkable that the original phenolic PC boards are still intact and working, half a century later.

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    Re: Dynaco ST-70 original driver boards - early and later versions - photos ...

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