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    VTA ST-70 problem

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    BlueHeeler

    Posts : 5
    Join date : 2011-08-09
    Location : Washington, DC

    VTA ST-70 problem

    Post by BlueHeeler on Tue Aug 09, 2011 11:28 pm

    I just installed a VTA circuit board into my ST-70. Everything was going great for a few hours. I have the high gain board and a Foreplay III tube preamp. It was LOUD obviously, but great. I switched the preamp out for an old PAS-3 to tame the volume before I do some work on the Foreplay. Again everything was fine. Then I noticed some static. I am not sure if it was from the amp or preamp so I shut the PAS off to see which one was making noise. 15 seconds later, I remembered I had the ST-70 plugged in the switched power of the PAS. Without thinking, I turned the PAS-3 back on to reevaluate my troubleshooting method. The fuse on the ST-70 blew with some popping and protest. Not a problem I thought. Tonight I replaced the fuse and started the amp to have a quick listen before I take it apart to dial in the bias. A minute later the left front EL34 goes Chernobyl with glow and heat, the speaker is popping with no input signal as I had not even turned on the CD player. After disassembly the voltage across the 1-8 pin just climbs. The right side bias is completely stable .4 volts plus or minus a hundredth or so. The left rear will also bias, yet the voltage on the left front is out of control.

    I looked at the solder joints to the socket and they look solid. The wiring looks exactly the same as the working right side. I switched the tubes and it is the same problem at that socket. I am not sure where to start or finish rather troubleshooting. I really hope I did not kill this amp.

    Thanks for any troubleshooting starting point, guidance, or wisdom on this.


    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST-70 problem

    Post by Bob Latino on Wed Aug 10, 2011 6:22 am

    With the amp OFF ...

    1. Check the value of the 1000 ohm resistor between pins 5 and 6 on the left front tube socket.
    2. Check the coupling cap on the driver board for that left front tube socket. Make sure that there is no continuity across the cap. If you have a cap tester, see if that cap is within spec.
    3. Check the value of the outboard 220K (or it may be 270K) resistor that is next to the wire that leads to pin 6 on each output tube. This resistor is partly under the edge of the chassis cutout. You will have to measure it from the inside of the amp. Check the other outboard resistors as well. These are R29 - R32. The front left resistor is R31.

    With the amp ON but NO TUBES in there check the bias system. See if you get something like negative 30 to negative 40 VDC on both pins 5 and 6 of the left front output tube socket (as measured to chassis ground). Check the other output tube sockets as well.

    Let us know what you find ?

    Bob

    BlueHeeler

    Posts : 5
    Join date : 2011-08-09
    Location : Washington, DC

    Re: VTA ST-70 problem

    Post by BlueHeeler on Wed Aug 10, 2011 3:44 pm

    Hi Bob, I think we are on the right track.
    1. The 1000 ohm resistor between 5-6 is at 1000 ohms.
    2. I do not have a cap tester, but all of the caps tested the same for resistance.
    3. R29, R30, and R32 are testing at 270K. R31 is testing at 16K.Suspect I checked the color coding, it is the correct one.
    4. For the bias no tubes, the left front is giving 0.00 VDC. The other ones are -42, -43, -46 VDC.

    Update: YES!!!! While typing this message and double checking, I found the problem. The wire I ran from pin 6 to the board poked through the board and was touching the chassis. That is never good. Rookie mistake. Rolling Eyes

    It is quite satisfying to keep these amps running. Thanks Bob!

    With only one channel of output tubes the bias is set to 0.40 volts. With both sets in, the voltage drops to 0.30 volts. Is this right? Or should the reading be .40 with all tubes in place?


    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: VTA ST-70 problem

    Post by tubes4hifi on Wed Aug 10, 2011 9:27 pm

    bias reading should be 0.400 vdc on each tube, everytime you adjust one tube it will slightly affect all the others,
    it usually takes about 3 rounds to get them all the same, and bias will increase by turning the pot CCW, opposite of what you would think.

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