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    run away bias and hum

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    wedg714

    Posts : 74
    Join date : 2011-11-23

    run away bias and hum

    Post by wedg714 on Thu Nov 24, 2011 4:55 pm

    hello. i just finished installing the new vta board in my mark III. when i turn the amp on with the rectifier and outputs in the amp, the bias goes off the charts and i get a steady hum. i shut the amp down after the bias reaches .90 so i don't blow any more fuses. it's steadily climbing when i shut it off. i tried a 5y3 rectifier and i can get the bias to stop at about .85 with the bias controls turned as far down as they will go. it did this with the original pcb in it. that's why i thought i'd try the vta board. thought it might be the original board. i've tried rectifier tubes from another working mark III and it does the same thing so i don't think it's the rectifier. i've changed quad caps and it does the same thing. could it be the choke? any help would be appreciated

    wedg714

    Posts : 74
    Join date : 2011-11-23

    Re: run away bias and hum

    Post by wedg714 on Thu Nov 24, 2011 7:49 pm

    after looking at some other posts, i measured the resistance of the 11.2 bias resistor and found it to be 3.6, also with the amp off the reading at the bias check points is also 3.6 for both tubes. i,m going to change the 11.2 resistor and see what happens. any help is appreciated

    Bob Latino
    Admin

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

    Re: run away bias and hum

    Post by Bob Latino on Thu Nov 24, 2011 10:09 pm

    You are supposed to use TWO 10 ohm resistors with the VTA Mark III driver board -NOT ONE 11.2 ohm resistor. Use one 10 ohm resistor from pins 1/8 on each output tube to chassis ground. Also - you have to remember to cut the wire on the original amp that went from one output tube - pins 1 and 8 to the other output tube - pins 1 and 8 - Again you neeed TWO 10 ohm resistors and not one 11.2 ohm resistor.

    Bob

    wedg714

    Posts : 74
    Join date : 2011-11-23

    Re: run away bias and hum

    Post by wedg714 on Fri Nov 25, 2011 7:51 am

    even with the 2 10 ohm resistors and the wire cut between the outputs, i get the same thing. runaway voltage and hum. i've even tried a solid state rectifier and the same thing

    wedg714

    Posts : 74
    Join date : 2011-11-23

    Re: run away bias and hum

    Post by wedg714 on Fri Nov 25, 2011 7:53 am

    at least with the amp off and the tubes out the readings at the bias points is 10 ohms

    wedg714

    Posts : 74
    Join date : 2011-11-23

    Re: run away bias and hum

    Post by wedg714 on Fri Nov 25, 2011 9:08 am

    i just did the following check: with the tubes out and the amp on i checked the ac voltage at pin 4 and 6 of the rectifier and they were both at about 430. i then checked the dc voltage at pin 2 and 8 and the both started out at about 325 but were decreasing. they both went down to about 225 in less than a minute. could the power tranny be bad? i can't test this with the rectifier in since it will run away and blow the fuse.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: run away bias and hum

    Post by anbitet66 on Fri Nov 25, 2011 10:58 am

    If you tried a solid state rectifier with the same results, check your output tubes for shorts. Otherwise, look for wiring problems in the output tube circuit.

    BTW, did you just get this amplifier? Did it always work for you, and/or did you perform any mods before the VTA board?

    Tony

    Bob Latino
    Admin

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

    Re: run away bias and hum

    Post by Bob Latino on Fri Nov 25, 2011 11:24 am

    If the AC voltage to chassis ground on pins 4 and 6 of the rectifier hold steady at 430 volts AC, then the power transformer is OK. A slowly dropping DC voltage usually indicates a bad quad cap. You said you replaced the quad cap but the new one you put in could be leaking. One of the symptoms of a leaking quad cap is a slowly dropping or a constantly low DC voltage as measured off pin 8 of the rectifier (with the rectifier in there).

    See if you can beg, borrow or steal a cap tester and test each section of the quad cap (with the amp OFF!) and measure the capacitance of each section. Just ONE bad quad cap section can drag down DC voltage.

    With the rectifier in there you should get about 500 VDC off pin 8 of the rectifier to chassis ground and it should hold steady.

    Bob

    wedg714

    Posts : 74
    Join date : 2011-11-23

    Re: run away bias and hum

    Post by wedg714 on Sat Nov 26, 2011 11:31 am

    should the wiring orientation of the quad cap be the same as the original orientation or what dynakits diagram shows? they are different.

    wedg714

    Posts : 74
    Join date : 2011-11-23

    Re: run away bias and hum

    Post by wedg714 on Sat Nov 26, 2011 11:38 am

    i'm refering to the 80/40/30/20 quad cap sold by dynakits

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: run away bias and hum

    Post by anbitet66 on Sat Nov 26, 2011 11:57 am

    Orient the capacitor as Dynakitparts says to. It will put the 40uF section first, so the rectifier won't see too much capacitance on start up. The next section after the choke will be the 80uF section to give better power reserve for loud passages of music.

    I'd go back a couple of steps. The quad cap is new right? If it is wired correctly, put only the rectifier in and measure the voltage on the first stage of your new quad cap.

    Second, check the bias voltage from the circuit that feeds the output tubes their bias. If the bias is low or missing, the current the output tubes draw will run away. Is this circuit using a diode or selenium rectifier? Is it polarized to give a NEGATIVE voltage on the outputs?

    Try these steps, and report back.

    Tony

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