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    Recommended replacement PS transformer for Mk4/

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    Pooch

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

    Recommended replacement PS transformer for Mk4/

    Post by Pooch on Thu Mar 11, 2010 5:39 pm

    I think the PS trans on one of my VTA Mk4 has bit the dust. Any suggestions much appreciated.
    While I was replacing the coupling caps, I noticed I hadn't put a jumper wire from lug 2 to lug 3 (grnd) of the 5 lug strip. This takes the gr-yel wire from the PS transformer to grnd. I guess it had been working without the jumper all along. The other amp has the jumper and it works a-ok, too. After I hooked up the jumper I started blowing fuses. So I disconnected the jumper but it continued to blow fuses. To the point of running out of fuses. Tried a different 5ar4, same result, blown fuse. Switched 12at7s, same result. So I did the Cardinal sin and put in a larger fuse. This resulted in a little smoke from the ps transformer. Shut it down imediately. I suspect this transformer is toast, if not before I got creative with it, certainly after. Is there any way I can test it to be sure it's the problem, or only problem? All was working well before I started this. Thanks, Gordy

    Pooch

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

    Re: Recommended replacement PS transformer for Mk4/

    Post by Pooch on Thu Mar 11, 2010 6:43 pm

    Well after looking at the transformer schematic, I pulled all the tubes and measured the resistance on both transformers for comparison. There's no leakage from primary to secondary. They measure identical on everything but the red to red, red to r/y. Here's what I got on these...

    Good transformer.... red to red-181.4 ohms
    pin 6 red to red/yellow-87.5 ohms
    pin 4 red to red/yellow-94 ohms

    Bad transformer..... red to red-26 ohms
    pin 6 red to red/yellow-50.9 ohms
    pin 4 red to red/yellow-51.8 ohms

    Does this confirm a bad transformer?

    Bob Latino
    Admin

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

    Re: Recommended replacement PS transformer for Mk4/

    Post by Bob Latino on Thu Mar 11, 2010 7:18 pm

    Pooch wrote:Well after looking at the transformer schematic, I pulled all the tubes and measured the resistance on both transformers for comparison. There's no leakage from primary to secondary. They measure identical on everything but the red to red, red to r/y. Here's what I got on these...

    Good transformer.... red to red-181.4 ohms
    pin 6 red to red/yellow-87.5 ohms
    pin 4 red to red/yellow-94 ohms

    Bad transformer..... red to red-26 ohms
    pin 6 red to red/yellow-50.9 ohms
    pin 4 red to red/yellow-51.8 ohms

    Does this confirm a bad transformer?

    Hi Gord,

    I don't have a Mark IV power transformer here so I can't do any comparable resistance measurements here for you. Get in touch with Kevin at Dynakitparts and run the problem by him. He sells high quality clones of the original Dynaco Mark IV transformers.

    Bob

    DynakitParts
    Admin

    Posts : 183
    Join date : 2008-11-30

    PA-135 Power Transformer

    Post by DynakitParts on Thu Mar 11, 2010 8:23 pm

    [b]Hi Gord,
    Appears your power transformer's high voltage secondary
    winding failed. Measuring the resistance here on one of our new
    Dynakit PA-135 replacments, I get 176.2 Ohms. I expected this to
    be slightly lower than the original since we adjusted our design
    for 120 vac at the Primary as opposed to 117 vac with the original
    Dynaco.

    To be absolutly sure it is the power transformer, which is likely,
    after removing this...tape off exposed leads except for the (2)
    Black primary's and the (2) Red secondary's. Apply 120 volts to the
    Primary leads and measure the voltage at the Secondary Red leads.
    This should be around 365 to 380 vac. A safer method would to use a
    variac working with a lower voltage at the primary side. The turn
    ratio is roughly 3:1 which with 25 vac at the primary will yield about
    75 volts at the secondary. Just enough volts to get your attention if
    you should slip...Let's hope not.

    Hope this helps you out...If needed, I have stock on the PA-135's

    Regards, Kevin @ Dynakit

    Pooch

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

    Re: Recommended replacement PS transformer for Mk4/

    Post by Pooch on Thu Mar 11, 2010 11:35 pm

    Thanks Bob and Kevin. I'll run that check you suggested, Kevin. Once I sell a Sherwood, I'll call you for the replacement tranny.
    Gordy

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