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    ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

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    Scot_B

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    Location : North Dallas, Texas

    ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Scot_B on Fri Dec 27, 2013 5:09 pm

    This morning my ST-70 blew a fuse on power-up.  After some trouble shooting,  I found the rectifier would blow the fuse on its own - without other tubes in the chassis ,   on my tube tester V3 power tube read shorted ,  and the vta driver outside (right channel) tube read way down in the weak zone.

    After replacing each, and re-biasing all 4 corners.  I find that the V3 (front right) power tube bias will jump from .40 vdc to about 3.25 vdc, (and hum) after 5 minutes or so of operation.

    This is consistent, in that if I let it set for a few minutes, and power it back on,  all 4 tubes come up to normal bias – then after about 5 minutes the V3 tube bias jumps.. the other tubes stay about the same.

    It is not a gradual change, it jumps!

    I have rolled the tubes around , even changed to a different quad of power tubes and the issue stays with the V3 position.

    Any thoughts on what I might check next?

    Zimmer64

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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Zimmer64 on Fri Dec 27, 2013 6:35 pm

    Maybe the bias resistor on pin 8 is damaged. Try to measure its value. If I remember correctly it should be 10 ohm.

    Michael

    Scot_B

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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Scot_B on Fri Dec 27, 2013 6:41 pm

    the 10 ohm resistor checks out,  

    As I stare at the chassis, a few thoughts are circling; 1) last thing I listened to last night was classical opera, something I hadn’t done before, I don’t recall the fat lady singing,  2)  I really  need a backup amp, silence is deafening.  

    Also, it appears to me that I have isolated this to the V3 circuitry,  I think I proved it to myself now by removing all tubes from the amp except the rectifier and V3, and saw the same thing happen; re-biased to .40vdc and after a minute or two,  Static and bias jumps to just over 3vdc. Hmmm .   Now what is unique to just this little bit of circuitry that doesn’t affect the rest of the amp…     I’m more of a mechanic than an engineer.

    I need a strategy to get another day of guilt free trouble shooting, I’m going to have to take the wife shopping tonight – this may hurt.

    Maybe I’ll move a PIO tomorrow.

    Bob Latino
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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Bob Latino on Fri Dec 27, 2013 6:46 pm

    If the problem is isolated to V3, then either the 10 ohm bias resistor on V3 is bad (as Michael has said) OR there is a non-contact issue with one of the pins on that tube socket more likely pin 5 or pin 6. Pins 5 and 6 have a 1000 ohm resistor across the two pins. Try cleaning the pins and/or bending the two sides of each pin on that socket in a little with a small flat jeweler's screwdriver.

    Bob

    ValleySounds

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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by ValleySounds on Fri Dec 27, 2013 6:57 pm

    I was having the same troubles you are describing. I had a resistor go up in smoke on me while trying to diagnose the problem. After it cracked open and was burned up it still read 8.9 ohms WHEN COOL. Apply some heat from the solder iron and it went open circuit.
    Try reflowing the solder joints on both ends of the resistor and test it while it is hot to see if the value drifts.

    Scot_B

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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Scot_B on Sat Dec 28, 2013 3:26 pm

    Thanks for the input,  I tried heating the 10 ohm bias resistor, and measuring while hot.. still checks out (10.1 ohm).

    I cleaned and pinched the tube contacts. I have re flowed the connections on V3 and the ends of signal and power into V3, as well as sections on the driver board  that were unique to V3.  Didn’t change my symptoms.  The Bias voltage jumps a couple minutes into power up…

    I had to back up, and start over.  I immediately find my B+ is 506 vdc on the driver board, a bit beyond the recommended 380-400vdc , it has always been on the low side of 380vdc when checking in the past.

    I just tried to plug a higher value resistor on the quad cap in place of the 2.2K between the 3rd and 4th section ,  thinking I could get another  drop the voltage to recommended..  I’ve put a couple combinations up to about 6K and it doesn’t change the b+ out..  

    Given this was working, with correct voltage,  is this an indication the quad cap is failed .

    peterh

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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by peterh on Sat Dec 28, 2013 4:23 pm

    If you have a high-impedance voltmeter, whats the voltage on the grid ( pin5 )? It's probably
    30-40V negative. Have the voltmeter connected while turning on, note value and bias. Then
    see what voltage you have when current rises. If voltage drops you should continue your search on
    the preboard, especially in the neighborhood of the bias potentiometer.


    Bob Latino
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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Bob Latino on Sat Dec 28, 2013 4:47 pm

    In your first post you said > "I found the rectifier would blow the fuse on its own - without other tubes in the chassis" DON'T EVER DO THIS! Without the other tubes in the chassis there will be no load on the rectifier and the the amp will give very high B+ voltages and you could take out other parts.

    Test the bias system with the bias pots set at their mid point WITHOUT the rectifier in the amp. See if you get -30 VDC to -40 VDC (to chassis ground) on both pins 5 and 6 of V3 and see that it stays that way for 15 minutes or so. If it does, then the bias sytem is OK.

    Check the PIO coupling capacitor that feeds V3 (with the amp OFF). See if it measures its correct value. If you don't have a cap tester then do a continuity test across that cap. If you get any resistance across that cap, then the cap is bad.

    Bob

    Scot_B

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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Scot_B on Sat Dec 28, 2013 7:34 pm

    Thanks for the guidance guys,  good lessons .

    I measured the grid voltage (pin 5), tubes in, no rectifier.  I measure about -42 vdc at midpoint ( range of -56.5 to -29.6 at the extents of the bias pots ) on all 4 corners.   The voltage hasn’t changed over the past 15 minutes, particularly V3.  I’ll watch it for a bit

    I don’t have a capacitance checker to verify the PIO,    is the continuity test done across the leads while in the board, or does it need to be removed to do it right?   And wouldn’t I expect to see to see some type of changing resistance reading as it charges off the ohm meter

    Scot_B

    Posts : 8
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    Re: ST70 bias jumps from .40 vdc to 3.25 VDC after 3~5 min warm up on V3 tube position

    Post by Scot_B on Sun Dec 29, 2013 1:47 pm

    More lessons learned,   New tubes don’t = good tubes.  No  I’ve rolled a new set of power tubes like a shell game, and somehow repeatedly put a “new – bad tube” in V3 that will short out after warming up. The tube even tests perfect.   I appreciate the help provided.

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