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    ST-70 Rectifier Voltage Readings

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    Bugs

    Posts : 91
    Join date : 2009-04-17

    ST-70 Rectifier Voltage Readings

    Post by Bugs on Mon Jan 17, 2011 7:43 pm

    One of my ST-70s developed a bad buzz and just all around poor sound quality compared to my other ST-70.

    I decided to check my voltage readings to see if anything was unusual. The rectifier pin 2 and 8 readings to ground are about 325 V which I understand to be ~ 100 V too low. The voltage readings on pins 4 and 6 to ground are 0.1 mV which I'm assuming this is how my meter reads 0 V.

    The voltages on the quad cap and EL 34s are correspondingly low. For the EL 34s, the readings across pins 2 and 7 is 0.2 mV and pin 3 to ground is 0 V.

    The quad cap readings range from 276 V to 325 V.

    I'm assuming that my power tranformer is toast?? Should I be checking anything else?

    Thanks

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 Rectifier Voltage Readings

    Post by Bob Latino on Mon Jan 17, 2011 8:11 pm

    Bugs,

    On pins 4 and 6 check the AC voltage (not DC) to chassis ground. You must have had your scale on DC to measure only .1 volt on pins 4 and 6 of the rectifier socket. You could NOT be getting .10 VAC on pins 4 and 6 and then get 325 DC on the quad cap. You should get about 360 AC from both pins 4 and 6 to chassis ground.

    The most common cause of low B+ DC voltage is either a bad rectifier or a bad quad cap. If you get about 350 to 370 VAC from pins 4 and 6 of the rectifier tube then the power transformer is OK. If get low voltage here then the power transformer is bad.

    Bob

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: ST-70 Rectifier Voltage Readings

    Post by Bugs on Tue Jan 18, 2011 12:15 am

    Thanks Bob, you were right I was measureing DC. Pins 4 and 6 measure ~383 VAC.

    I switched rectifier tubes and ruled that out as a source of the problem.

    These are the out of spec readings.

    Output tubes:

    Pins 3 and 4 ~ 319 VDC - all tubes.
    Across pins 2 and 7 , no measurable reading - all tubes

    Quad Cap:

    Lug to B+ voltage ~ 284 VDC
    All other lugs ~ 322 VDC

    Does this sound like the problem is the quad cap?

    Thanks

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 Rectifier Voltage Readings

    Post by Bob Latino on Tue Jan 18, 2011 6:10 am

    Bugs wrote:Thanks Bob, you were right I was measureing DC. Pins 4 and 6 measure ~383 VAC.

    I switched rectifier tubes and ruled that out as a source of the problem.

    These are the out of spec readings.

    Output tubes:

    Pins 3 and 4 ~ 319 VDC - all tubes.
    Across pins 2 and 7 , no measurable reading - all tubes

    Quad Cap:

    Lug to B+ voltage ~ 284 VDC
    All other lugs ~ 322 VDC

    Does this sound like the problem is the quad cap?

    Thanks

    Bugs,

    Across pins 2 and 7 is an AC voltage. You should get 6.0 to 6.5 VAC.

    If you replaced the rectifier tube, about the only thing that could cause the low voltage is a leaking quad cap - replace it ...

    Bob

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: ST-70 Rectifier Voltage Readings

    Post by Bugs on Wed Jan 19, 2011 5:12 pm

    Thanks Bob.

    There is a reason I don't do this sort of thing for a living. Quad cap is on the way.

    Best

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