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Trying to repair a friend's Blue VooDoo (BV-60) and don't know just what to do now. Amp was blowing one of the Cathode fuses. Found that that tube was getting no bias voltage. Repaired three bad solder joints and got voltage to pin 5. Problem is that the bias voltage is -31 vdc and should be about -50 vdc. (I am checking all of this with the tubes pulled). Sorry but I don't know how to post the schematic but it has been on here several times. Maybe someone can post it for me. I have -63 vdc up to R111, across R111 the voltage goes to -31 vdc. R1, R3, R88, R89 and R96 values check good. Have not tried to check the cap C58. Don't want to have to pull the board unless I have to. Don't know how to check Q16 or D14. Any help greatly appreciated.
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The schematic was uploaded by Enzo on this thread here...
http://music-electronics-forum.com/t13541/When the going gets weird... The weird turn pro! -
Here's a zip of the schematicsOriginally posted by Enzo
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And here is Enzo's analysis of the auto bias:
http://music-electronics-forum.com/t21484/#post179253
edit: without looking too close, I'm wondering if the auto bias will work properly with no tubes.Last edited by g1; 09-29-2015, 11:31 PM.Originally posted by Enzo
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I think you are right. No tubes and the voltage at pin 5 is going to be -31 volts. Maybe correcting the bad solder joints around pin 5 corrected the problem. I'll stick the power tubes back in and see what happens. Thanks. As for Enzo's analysis of the auto bias circuit. I have taking his advice on problems before and he has always been spot on. I know that I at least owe him a Diet Dr. Pepper.Originally posted by g1 View PostComment
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No way. Being as I am old and dumb I'll just call him Mr. or Sir. (That is if he keeps on helping me). By the way, While I am thinking about it...Hats off to the posters on this board. I have never failed to get much need direction here when I asked for it. When you live way out in the sticks as I do and the nearest tech is 35 miles away (and he doesn't know much more than I do) this is a huge help.Originally posted by 52 Bill View PostComment
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Too late, my wife beat you to it.Hey, are you calling Enzo FAT?
And..um... I am.Education is what you're left with after you have forgotten what you have learned.Comment
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Well, still have not found the problem. Put the tubes back in and I get -31 vdc on pins 5. Checking some voltages I get -63 vdc on R94 on the end towards R111. The other end of R94 I get 0 vdc. I measured 9K ohms from that end (towards R95) to ground. I am getting no voltage on the center leg of Q16. I went ahead and pulled the board. I have not found a path to ground from R94. Everything seems to say that the bias circuit is not working(Duh)...I checked the diodes and they check good. Have not checked the zener diode yet. Will check it later. Need some suggestions as to where to go now.Comment
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And what is the voltage on the other side of R111 ?Originally posted by Enzo
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-31 vdc.......Checked D14, Zener across Q16. It is shorted.. Would that cause these issues?. It would be a path to ground. That and the solder joints are the only things I have been able to find amiss in the whole biasing circuit..Been a long time since I worked with Kirchhoff Laws...Guess I need to freshen up on them.Originally posted by g1 View PostComment
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Yes, a short across Q16 will greatly reduce the bias voltage. You now have a 4.7K resistance from C58 (bias cap) to ground.Originally posted by Enzo
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Just an update on this post. I replaced D14 and the problem with the bias circuit is solved. I have a set of 6L6's in the amp and the bias supply voltage at pin 5 is -48 volts. Took voltage drop measurement across R90 and looks like this amp is biased at about 23 watts. I still have it opened up if there is any info that some would like and that I can supply would be happy to do it. Thanks again for the hand holding help.Comment
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Well, I need more help with the BV-60. Put it all back together and it has a very loud hum. The Hum is without anything plugged in. It is on both the channels. Plugged an input into the effects return jack and the amp plays good and sounds good. No Hum. The controls of clean channel do not affect the hum. The controls of channel 2 do affect the hum. The control for the reverb affects the hum. Pull the first preamp tube and the hum is still there. Pull the second pre amp tube and the hum goes away. I have reflowed the solder joints on the second pre amp tube, the footswitch jacks and the effect jacks. I was going to reassemble the amp and decide to do a little more checking around the footswitch circuit. Found D6 and D7 bad. Had replaced them once already. Also found the resistance side of OC1 open. The diode side test good. I'm going to reassemble the amp knowing that D6, D7 and OC1 are bad and see If the hum is still there. Any suggestions as to what could be damaging D6, D7 and OC1. Also what is causing the hum.Comment
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How about a helping hand?
Where, on the schematic, are D6, D7 & OC1?Comment
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