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    Mk4 output transformer measurements....

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    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Mk4 output transformer measurements....

    Post by Pooch on Fri Feb 18, 2011 10:07 pm

    Hi all.
    When I meter the G/W to B/W resistance on one of my A470, I get a reading of 149ohms. When I measure the G to B on the same transformer I get 60 ohms. The other transformer has 69 ohms on the G/W to B/W and 60 ohms on the G to B. Does this indicate a bad transformer or is this normal?

    Bob Latino
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    Location : Massachusetts

    Re: Mk4 output transformer measurements....

    Post by Bob Latino on Sat Feb 19, 2011 8:44 am

    When you measure those resistances on the primary side of an A-470 You have to make sure that none of the other wires on the primary side are touching each other. If the metal on the wire ends are touching you won't get a proper reading. I am assuming that you made sure that the ends of the other primary wires were not touching. That said, your other A-470 is probably OK. A bigger problem would be if you got NO resistance from one wire to the other. This would indicate an open in one of the windings.

    Bob

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Mk4 output transformer measurements....

    Post by Pooch on Sat Feb 19, 2011 5:28 pm

    Thanks, Bob. I peeled back the wax paper to see if the solder joints were weak, where the B/W, G/W connect to the windings. They seemed OK but I re-flowed them and added a little solder, just to be sure.I hot glued the wires back down and the wax paper on top of the wires. This worked remarkably well, if anyone wants to try the hot glue idea. I'll try and use the transformer, see what happens.Still about 145 ohms on the meter, though, so the connection wasn't at fault.
    Here's my tale of woe Smile I re-did these Mk4s using VTA boards, new PS Multi section CE cap (80uF variety), new wiring, tube socket,jacks,binding posts, power cord. The only re-used stuff is the choke, transformers and chassis. Everything worked fine for a while, then a power transformer went out. I bought new PS transformers and after installing them and bringing the amps up on a variac, I noticed one was having a soft intermittent popping coming out of the speaker I had hooked up. After reading one of your sticky posts, it seems the PS cap may be at fault? So I disconnected the wires to it and measured. The square section (40mfd) was shot(couldn't get a reading, just flashing).I had this bridged over to the triangle (30mfd) section and then wired to the B+.Is this a wrong thing to do? Should I just stick with the 30mfd section, or is this just a bad cap? Come to think of it maybe this cap took out my original PS. Would I be better off to go with a PS board instead of the multi section?
    Thanks again,Bob, for all the help you provide on this forum.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Mk4 output transformer measurements....

    Post by Bob Latino on Sat Feb 19, 2011 6:11 pm

    The quad cap may or may not be at fault. What you should do is a full voltage check on BOTH amps. When you get a reading that is noticeably different from the good amp to the amp with problems, you can usually figure out where the problem is. The voltages that you should get are listed in the Dynaco Mark IV manual that is at the link below. The voltages are for a stock Mark IV. Since you have the VTA driver board, the voltages listed on the 7199 tube will not apply.

    Tubes4hifi Dynaco Mark IV manual

    Bob

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Mk4 output transformer measurements....

    Post by Pooch on Sat Feb 19, 2011 8:01 pm

    OK I'll do that Bob. I'll put it back together and run it via variac at a lower voltage. Last time I got that popping at reg intervals, it seemed to hold the fuse when down on the variac. If I cranked it to higher volts it blew the fuse.

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