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    ST 70 power trans problem

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    xdodge

    Posts : 7
    Join date : 2012-11-01

    ST 70 power trans problem

    Post by xdodge on Sat Feb 07, 2015 2:16 pm

    So this started out as a Regulator tube problem( fireworks in the tube and then blow fuse). put in new GZ34 and same thing. Then found I had a bad Ruby EL34, replaced that and all seemed to be ok, at least I wasn't blowing fuses any more.

    But I seen to have very high voltages and think my Power Xformer may be bad.

    I believe it is the stock one,  PA 060 Custom Coil, with all tubes out and all measurements taken with a good meter I get

    Line in  123VAC

    Two GREEN wires -7.1 VAC as measured across the two leads
    Two BROWN wires - 7.1 VAC as measured across the two leads
    Two WHITE wires - 5.92 VAC as measured across the two leads
    Two RED wires - 830 VAC as measured across the two leads
    GREEN/YELLOW to RED/YELLOW 180mV VAC
    BROWN/YELLOW to RED/YELLOW .6 VAC
    RED/BLACK to RED/YELLOW 61.4 VAC

    With the Reg. tube in ( with diode mod)

    GZ34 – Pin 2 to ground – 568 volts DC
    Pin 8 to ground – 568 volts DC
    Pin 4 to ground – 206 volts AC
    Pin 6 to ground – 201 volts AC

    What do you think ????

    Thanks

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST 70 power trans problem

    Post by GP49 on Sat Feb 07, 2015 3:52 pm

    1. What is your line voltage?

    2. UNLOADED AC voltages on power transformers are not a reliable indication of anything.

    xdodge

    Posts : 7
    Join date : 2012-11-01

    Re: ST 70 power trans problem

    Post by xdodge on Sat Feb 07, 2015 6:35 pm

    Line in is 123

    so should i put all tube in and measure again ??

    Thanks

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST 70 power trans problem

    Post by GP49 on Sat Feb 07, 2015 8:04 pm

    Yes. You need to check with the tubes in, and drawing normal current. Expect several percent high, based on the 123VAC line voltage. Also, if you have increased the power supply reservoir capacitance, that will increase DC voltages.

    xdodge

    Posts : 7
    Join date : 2012-11-01

    Re: ST 70 power trans problem

    Post by xdodge on Sat Feb 07, 2015 9:58 pm

    So......looks like I have been chasing my tail around Smile
    Put all the tubes in and did some testing and drinking......all is better now !!!!
    I did find a big difference in the new JJ Regulator and an Old RCA that I had. The JJ was about 10% higher in almost all the readings. I am using the RCA now and will start looking for a few old school Regulator tubes. Also I made a home made variac out of a dimmer switch, when I bring down the voltage to 115vac it really makes a difference in the reading. I don't think it is good enough to to run all the time so will find a permanent solution. Laughing


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