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    ST70 transformer dead?

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    stellan

    Posts: 3
    Join date: 2010-03-05

    ST70 transformer dead?

    Post by stellan on Fri Mar 05, 2010 4:13 pm

    Is there a simple way to check in circuit for a dead ST70 transformer PA521 (240V)? With no tubes installed the fuse blows immediately when switched on. I'm using a cap board instead of the quad cap and it has worked fine for 5 years until there was a loud flash over in the GZ34 and the fuse blew.
    /Stellan

    Bob Latino
    Admin

    Posts: 1957
    Join date: 2008-11-26
    Location: Massachusetts

    Re: ST70 transformer dead?

    Post by Bob Latino on Sat Mar 06, 2010 4:20 pm

    Hi Stellan,

    What I would do is with the amp OFF and all the tubes out is see if there is a short from either of the primary windings (which on a PA-521 set up for 240 volts would be the BLACK and the VIOLET wires) to ANY of the secondary windings . One primary wire (BLACK or VIOLET)will be connected to the fuse post and the other will be connected to to the on/off switch. Measure from the the BLACK primary wire to ALL the other SECONDARY wires on the power transformer. Also measure from the VIOLET wire to all the wires listed below. In all cases you should get no continuity.

    1. The two GREEN wires
    2. The two RED wires
    3. The two WHITE wires
    4. The two BROWN wires
    5. The RED/BLACK wire
    6. The RED/YELLOW wire (chassis ground)
    7. The GREEN/YELLOW wire
    8. The BROWN/YELLOW wire

    If there is any continuity between the BLACK or the VIOLET wire and any of those listed above the power transformer has a short and must be replaced. That said, it is also possible for the power transformer to have a short that does not show up on this continuity test for shorts. Sometimes a short will also occur only when voltage is applied and a spark will jump from one winding to another which will immediately blow the fuse.

    Let us know what you find ?

    Bob

    GP49

    Posts: 499
    Join date: 2009-04-30

    Re: ST70 transformer dead?

    Post by GP49 on Sat Mar 06, 2010 10:26 pm

    I've often encountered Stereo 70 power transformers with a shorted B+ secondary. If by following Bob's instructions above the results are inconclusive, disconnect the red/blue wire from the transformer to the bias circuit (to eliminate a shorted bias rectifier as a cause), leave all the tubes out and install a new fuse...you can use a lower-amperage one such as a one-amp, as there should be no current flowing in the secondary under this condition. If the fuse blows, replace the transformer.

    Bob Latino
    Admin

    Posts: 1957
    Join date: 2008-11-26
    Location: Massachusetts

    Re: ST70 transformer dead?

    Post by Bob Latino on Sun Mar 07, 2010 7:18 am

    GP49 has got a point here ... Even with all the tubes out of there the RED/BLACK bias supply wire will still be connected to the selenium rectifier (or diode replacement). A short in the selenium rectifier (diode) or some of the components on lugs 1 through 4 of the seven lug terminal strip could cause the fuse to blow.

    Bob

    stellan

    Posts: 3
    Join date: 2010-03-05

    Re: ST70 transformer dead?

    Post by stellan on Mon Mar 08, 2010 12:03 pm

    Hi guys,
    There was no continuity so I went on and checked the bias diode on the quad cap/power supply module and it showed the same resistance either way so I replaced it and put the tubes in and powered up on my variac and it worked just fine. Many thanks to you Bob and GP49 for all help.
    Stellan

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