The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products

    Dynakit Mark3 Transformer Testing

    Share

    Mark Korda

    Posts: 19
    Join date: 2009-12-30

    Dynakit Mark3 Transformer Testing

    Post by Mark Korda on Wed Dec 30, 2009 6:15 pm

    Hi Bob Latino,this forum is great!I've read almost all the posts.You had a great article on how to test a ST-70 transformer or transformers.I was wondering if you have ever posted anything about a Mark 3 transformers?I got 4 out of the chassis and every thing else to start building,but I'm worried about the trannies before I start....thanks Mark Korda.

    Bob Latino
    Admin

    Posts: 1933
    Join date: 2008-11-27
    Location: Massachusetts

    Re: Dynakit Mark3 Transformer Testing

    Post by Bob Latino on Wed Dec 30, 2009 7:18 pm

    Mark Korda wrote:Hi Bob Latino,this forum is great!I've read almost all the posts.You had a great article on how to test a ST-70 transformer or transformers.I was wondering if you have ever posted anything about a Mark 3 transformers?I got 4 out of the chassis and every thing else to start building,but I'm worried about the trannies before I start....thanks Mark Korda.

    Hi Mark,

    About the only thing you can check with a multimeter on an output transformer is resistance and continuity as outlined in the sticky post above on the Dynaco A-470 transformers. The readings for a Mark III transformer will be pretty much the same as the A-470's on an ST-70.

    1. There should be no continuity between any PRIMARY side wire (BLUE, GREEN, RED, BLUE/WHITE and GREEN/WHITE and any SECONDARY side wire YELLOW (16 ohm), ORANGE (8 ohm), BROWN (4 ohm), BLACK (ground)

    2. There should be continuity between the RED primary wire and all the other primary wires. You should get maybe 10 or 15 ohms to maybe 200 ohms.

    3. On the secondary side check the resistance from the BLACK secondary wire to each of the other three secondary wires. You should get a very low resistance maybe .4 ohm (4/10ths) to maybe 2 ohms.

    If your Mark III transformers measure OK as outlined above I would use them without hesitation.

    Bob

    Mark Korda

    Posts: 19
    Join date: 2009-12-30

    Re: Dynakit Mark3 Transformer Testing

    Post by Mark Korda on Thu Dec 31, 2009 3:44 am

    Many thanks Bob...your something else!...Mark K.,But Bob,does this apply to both power and output transformers,I might of missed a coma?..thanks,the pictures of your new amps were so good,I could not think of anyone buying a Primaluna?...

    Bob Latino
    Admin

    Posts: 1933
    Join date: 2008-11-27
    Location: Massachusetts

    Re: Dynakit Mark3 Transformer Testing

    Post by Bob Latino on Thu Dec 31, 2009 4:26 am

    Mark Korda wrote:Many thanks Bob...your something else!...Mark K.,But Bob,does this apply to both power and output transformers,I might of missed a coma?..thanks,the pictures of your new amps were so good,I could not think of anyone buying a Primaluna?...

    Hi Mark,

    What I said applies to just the OUTPUT transformers.

    If you want to test the POWER transformer then ....

    1. Turn your Mark III power transformer upside down and make sure that all the wires on your transformer are separated from one another. Be careful that your hand does not touch any wire or wires. Lethal voltages from the power transformer!!!

    2. Apply your 120 volt AC house current to the two BLACK wires

    3. Check to see that you get approximately ...

    A. 820 to 870 volts AC ACROSS the two RED wires
    B. 5 to 6 volts AC ACROSS the two YELLOW wires
    C. 6 to 7 volts AC ACROSS the two GREEN wires
    D. Now put the RED probe of your multimeter on the RED/BLACK wire and your BLACK multimeter probe on the RED/YELLOW wire and check to see if you get about 55 volts AC

    If you get AC voltages in the approximate range mentioned then your Mark III power transformer should be OK to use.

    Bob


    Last edited by Bob Latino on Tue Mar 22, 2011 10:52 am; edited 1 time in total

    danf

    Posts: 58
    Join date: 2009-01-19

    Mark III Power Transformers

    Post by danf on Thu Dec 31, 2009 9:01 am

    Hi Mark and Bob,

    I've rebuilt four Mark III's. All of the power transformers would have passed the test above, but I ended up replacing two with new transformers. I remove the end bells of a completely disconnected power transformer and look for a roasted core or cracked output leads. A transformer with stiff and overheated cloth leads is a possible warning sign of trouble within. I am careful to replace the insulation sheets over the leads when I reassemble. You can repaint the transformers when they are apart.

    Regards,
    Dan

    Bob Latino
    Admin

    Posts: 1933
    Join date: 2008-11-27
    Location: Massachusetts

    Re: Dynakit Mark3 Transformer Testing

    Post by Bob Latino on Thu Dec 31, 2009 9:52 am

    Dan,

    You are correct here .. With the transformers out of an amp it is easy to take the bells off and check for cracked insulation, black marks on the core etc... I once had an ST-35 power transformer that passed all the tests. I put it in an amp and a few months later one of the two 6.3 volt AC filaments shorted. The insulation was cracked inside and it just shorted out. I should have taken the bells off and checked before the installation of this transformer but I didn't. The wiring "looked fine" on the outside but there was a small stress crack from a bend inside and a black mark where it shorted. Again, you are quite right when you say to take the bells off and inspect that transformer's interior. Some of those Dynaco power transformers are now over 50 years old. Time and high voltage can take their toll ...

    Bob

    Mark Korda

    Posts: 19
    Join date: 2009-12-30

    Mark 3 transformer Testing

    Post by Mark Korda on Thu Dec 31, 2009 3:11 pm

    Dear Bob and Dan,because of you both,I have cracked the obsticle that kept me from building the Mark 3's.I thank you both so much.Sincerely,Mark Korda

      Current date/time is Thu Sep 18, 2014 10:39 am