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    Setting Bias on ST70: issues I can't find elsewhere on board.

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    DC92

    Posts : 6
    Join date : 2015-05-28

    Setting Bias on ST70: issues I can't find elsewhere on board.

    Post by DC92 on Fri Jun 05, 2015 5:26 pm

    I'm trying to test the bias. I've got a stock driver board and EL34 tubes that came with the machine when I bought it.

    I have a Radio Shack tester. When I put it on a battery, it reads it correctly.

    When I connect it to the right bias et, I get a reading of .338

    When I connect it to the right bias et, I get a reading of .048

    I know it still has the selenium rectifier . . .

    When I connect it to music, it plays, though I do have a nice hum in both speakers.

    When I try to adjust each bias screw, nothing really changes . . .

    When I flip the tubes, the readings follow . . .

    The meter is set at 2V

    Thoughts?

    Thanks!

    Bob Latino
    Admin

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

    Re: Setting Bias on ST70: issues I can't find elsewhere on board.

    Post by Bob Latino on Fri Jun 05, 2015 6:24 pm

    Open the amp up, turn it upside down on a towel on a table, turn the amp on. Set you meter to the highest DC scale. Measure the DC voltage from pin 8 of the rectifier tube to chassis ground. You should get 430 to 450 volts DC. If you get below 400 volts, then either the rectifier is bad OR (more likely) the quad cap is bad. The fact that you mentioned that you get a constant hum makes it more likely that the quad cap is bad. Unhitch all the wires/resistors from the quad cap and mark where each wire/resistor goes. Get a new 80, 40, 30, 20 uF quad cap from Dynakitparts.com Install the new cap with the 80 section facing the rear of the amp. Connect all your wires and resistors back up. You should now be able to bias the amp properly and the hum should have gone away.

    Bob

    DC92

    Posts : 6
    Join date : 2015-05-28

    Re: Setting Bias on ST70: issues I can't find elsewhere on board.

    Post by DC92 on Fri Jun 05, 2015 6:44 pm

    Bob Latino wrote:Open the amp up, turn it upside down on a towel on a table, turn the amp on. Set you meter to the highest DC scale. Measure the DC voltage from pin 8 of the rectifier tube to chassis ground. You should get 430 to 450 volts DC. If you get below 400 volts, then either the rectifier is bad OR (more likely) the quad cap is bad. The fact that you mentioned that you get a constant hum makes it more likely that the quad cap is bad. Unhitch all the wires/resistors from the quad cap and mark where each wire/resistor goes. Get a new 80, 40, 30, 20 uF quad cap from Dynakitparts.com Install the new cap with the 80 section facing the rear of the amp. Connect all your wires and resistors back up. You should now be able to bias the amp properly and the hum should have gone away.

    Bob

    Thanks, Bob. I'll try that out. Is it concerning that turning the bias screws don't change anything at this point?


    Bob Latino
    Admin

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

    Re: Setting Bias on ST70: issues I can't find elsewhere on board.

    Post by Bob Latino on Fri Jun 05, 2015 6:58 pm

    Two other things you should probably do is ...

    (1) Replace the selenium rectifier with a diode.

    (2) Replace the two 50 uF capacitors on the 7 lug terminal strip. After 40 to 50 years of use sometimes these won't hold a charge anymore.

    When you measure that DC voltage off pin 8, you will probably find low voltages. This does relate directly with the inability of the bias pots to properly bias your output tubes.

    Bob

    DC92

    Posts : 6
    Join date : 2015-05-28

    Re: Setting Bias on ST70: issues I can't find elsewhere on board.

    Post by DC92 on Sat Jun 06, 2015 2:42 pm

    So I tested the 8th pin on the rectifier, and I actually got 480-90 volts. I haven't been able to change the selenium rectifier because my hardware store doesn't carry diodes, so I need to order them.

    So, in sum, it looks like it's running hot.

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