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    Rehabing an old ST-70 - IDing the tranny wires

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    dmtparker

    Posts : 12
    Join date : 2014-09-25
    Location : Bocas del Toro, Panama

    Rehabing an old ST-70 - IDing the tranny wires

    Post by dmtparker on Mon Dec 08, 2014 12:49 pm

    I searched on this topic and didn't find anything although it may have been covered elsewhere.
    Old Dynaco's don't die, they are just forgotten.
    I am in the process of rehabing a very old ST-70 that dates from my college days (1960's - and I bought it used from a pawn shop back then!). Somewhere along the line I had decide to 'resotre' it and removed all the tubes, sockets and wiring. Then it got put on a shelf. Now I am trying to 'rehab' it. I found a great resource at Triode Electronics: http://site.triodestore.com/NEWST70Instructions.pdf. This is better than the original instructions! (which I also have). As it turns out, I had bought their driver board 'back when' but never installed it. I also built my own version of the Triode (akaSTS) cap board with slightly different values, so the instructions are almost perfect for me. Even if you are doing a straight rehab or using other boards, it is still a big help as the pics are very clear.
    Now for the problems:
    I have the original and coveted transformers with cloth covered wires, BUT the COLOR of the cloth covering is anybody's guess at this point. I can sort of tell two color from solid color, but that is about it. What to do? Dig out your DVDM amd start sorting out wires. Looking at the Triode manual and the original Dyncao manual, you come up with the following info regarding the Output tranny:
    - The blue & green wires are longer than the Bl/Wh & Gr/Wh wires.
    - The omhage between each pair should be the same
    - the ohmage between the Bl & Bl/Wh should be the highest of all possible pairs
    - Bl to Red should be (approx) equal to Bl/Wh to Red and ~1/2 Bl to Bl/Wh
    - Red to either Gr or Gr/Wh should be the lowest value and about half of the Gr to Gr/Wh value
    - That should be enough info to ID all of the wires on that side of the tranny
    The other side is easy:
    - Com to 16 is highest
    - Com to 8 or 16 to 8 should be about equal (~1/2 Com to 16)
    - Com to 4 should be ~1/2 Com to 8 and much lower than 16 to 4
    - That should ID all 4

    Power tranny:
    - The Blk wires were still black and actually still attached to the AC. If that is not the case:
         - Red to Red should be the highest
         - Black to Black should be next highest and ~1/3 of R-R
         - All others are much lower
         - That should give you the Blk-Blk
    - As said, Red-Red is highest ohmage
    - Red/Yellow - Red is ~1/2 R-R (and ~same to each R)
    - R/Blk - R is much lower than R/Y - R for one R and much higher to other R
    - Wh pair is not connected to any other wires and will have a very low ohmage (i.e. testing Wh to any other wire except white will give infinite ohms)
    - Br pair and Gr pair are identical (and interchangeable) They are also very low ohmage
    - Br-Br/Y is ~1/2 Br-Br but this may be hard to measure depending on your meter. Here it is helpful if you can at least tell which wires are solid color and which at two color. I could.
    - Gr-Gr/Y same as above (but Gr-Br/Y is infinite)
    - This should ID all 13 wires on the power tranny. You will not know Gr from Br, but that doesn't matter as long as you have the correct CT (Br/Y or Gr/Y)
    If you are still in doubt, you can VERY CAREFULLY attach the black wires to the AC line and then attach all 11 other wires to a bread board with test points being sure none of the wires is connected to anything else. Plug it in and measure the voltages across various pairs that you are still in doubt about.
    ONLY DO THIS IF YOU KNOW WHAT YOU ARE DOING.
    YOU ARE DEALING WITH LETHAL VOLTAGES HERE!!
    NOTE I DO NOT RECOMMEND THIS. Testing the resistance should tell you all you need to know.

    There you have it. Even if you pick up an isolated tranny from a junk pile, you should be able to ID all the wires (as long as you know WHICH tranny you have).
    Most of the wires are dried out and brittle. After IDing the twisted pairs, I checked for any breaks (fortunately found none) and then put heat shrink over the twisted pair to preserve it.
    Some wires will probably need to be extended, especially if you use a quad cap replacement (which I recommend!). I have found the best way to do this is as follows:
    - strip ~1" off the old wire and 1" off new 20g extension
    - hold wires so they cross at mid-point of bared ends
    - holding the cross, wrap one terminal end around the standing part of the other (back toward the insulation)
    - repeat for the other end
    - this is called a 'telephone splice' and was reportedly used by linemen when they needed a strong and electrically secure connection
    - I go one step further and solder the splice 'just to be sure'
    - then put a heat shrink over the whole deal
    - if the lengthened wire is to be part of a twisted pair, it helps to twist it while the heat shrink is still hot. Even so, it won't twist as well as regular wire. I would twist it as well as possible and then put another heat shrink over the pair to hold the twist.
    Note: I do not own stock in heat shrink! I buy mine from Harbor Freight. Get the 'marine' as it has adhesive inside.
    So, wire it up and enjoy your 'new' old tranny!
    I hope this helps someone.

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