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    No bias readings in st70 right channel

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    xlr8
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    No bias readings in st70 right channel

    Post by xlr8 on Wed Jan 27, 2010 6:37 pm

    Hello all,

    I recently purchased and installed the VTA board in an st70 and cannot get a bias voltage reading on the right channel. The left channel seems just fine. I have the triode switches wired in, if that makes any difference. From what I can tell from all of the photos and schematics I have, everything is connected properly. Where should I begin to look for problems? All tubes light up, including the rectifier. I have not even checked other voltages.

    Thanks for any help folks!

    Bob Latino
    Admin

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

    Re: No bias readings in st70 right channel

    Post by Bob Latino on Wed Jan 27, 2010 7:22 pm

    Hi,

    First thing to check is to see if the bias system is working for the RIGHT channel. To do that turn the amp on with all tubes in there ..

    1. Measure the NEGATIVE DC voltage on either pin 5 or pin 6 of both output tubes on the RIGHT channel. You should get something like NEGATIVE 28 to NEGATIVE 40 on both pins 5 and 6. The actual value that you get depends on the setting of the bias pot for that tube. If you do get voltages on both tube sockets somewhere in that range the bias sytem is OK. If you don't get it there is a problem on the driver board with the right channel's bias system OR with the high voltage system for that channel.

    2. Put the amp into the pentode mode and now check the high voltages on both pins 3 and 4 of both right channel output tubes. Check to see that you get somewhere between 400 and 440 volts DC on both pins 3 and 4 of both tubes. If you get it the high voltage for those tubes is working OK. If you don't the proper voltages there is an error probably in the wiring of the triode/pentode switches and the output tubes are getting no high voltage.

    Let us know what you find out ?

    Bob

    xlr8
    Guest

    Re: No bias readings in st70 right channel

    Post by xlr8 on Thu Jan 28, 2010 4:44 pm

    Ok, today I fired the amp up and checked the negative DC voltages on pins 5 and 6 of the 2 right channel output tubes and got -38.2 on pin 5 V-6 and -38.2 on pin 6 V-6. I got -39.4 on both pins 5 and 6 on V-7 as well. The same tube pins on the left channel are only slightly higher at -43.7 on 5&6 of V-3 and -42.6 on 5 and 6 of V-2

    High voltage is a different story. On pins 3 and 4 of V-3 I get 472v. Same reading on V-2. On V-6 I get anywhere from 4-7 volts on pin 3 and 5-7 volts on pin 4. On V-7, the meter starts at .300 volts on pin 3 and slowly drops off. On pin 4 it's all over the place.

    I have checked and re-checked the wiring to the switches, and they are correct according to the diagram I printed off. Should I be looking at the power transformer as the culprit?

    Thank you!

    Bob Latino
    Admin

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

    Re: No bias readings in st70 right channel

    Post by Bob Latino on Thu Jan 28, 2010 5:05 pm

    Hi,

    Your bias system is fine ....

    Recheck the wiring for the triode pentode switches for both V6 and V7. A mixup in wiring could cause the voltages that you read. You said that you checked it - but check it again

    One other check > Check the DC voltage at the quad cap where the two RED wires from each output transformer connect to the quad cap. This is the quad cap lug that faces the back of the chassis. Check to see that you get about the same 472 volts (it may be a little higher) that you got on pins 3 and 4 on both LEFT channel tubes. If you get about 472 to 480 volts there and then get virtually no voltage on pins 3 and 4 of V6 and V7 then the RIGHT OUTPUT TRANSFORMER IS BAD and must be replaced.

    The POWER transformer is fine ..

    Bob

    xlr8
    Guest

    Re: No bias readings in st70 right channel

    Post by xlr8 on Thu Jan 28, 2010 5:23 pm

    Well, I just measured the cap like you said, and I get a slightly higher reading (477v) than the pins on V-3 and V-2, and almost 0 volts on the right channel tubes. Do you happen to sell output transformers???

    Bob Latino
    Admin

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

    Re: No bias readings in st70 right channel

    Post by Bob Latino on Thu Jan 28, 2010 5:27 pm

    xlr8 wrote:Well, I just measured the cap like you said, and I get a slightly higher reading (477v) than the pins on V-3 and V-2, and almost 0 volts on the right channel tubes. Do you happen to sell output transformers???

    Hi,

    Send me an Email at > Bob01605@aol.com or call me directly at 508-347-9120 anytime before 9 PM Eastern

    Bob

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