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    MK III woes...

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    grizzinca

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    Join date : 2012-01-01

    MK III woes...

    Post by grizzinca on Sun Jul 14, 2013 5:28 pm

    http://s1303.photobucket.com/user/grizzinca/media/image_zpseb0cae5f.jpg.html?sort=3&o=1#
    Greeting everyone. As you can see, I'm slowly making a mess here. All is not lost I hope. Could use some help. Not sure where yellow/green from power tranny goes. As well as red/yellow. If you can see any other miscues feel free to point them out. Confusing but worth it

    hawaii.ken

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    Re: MK III woes...

    Post by hawaii.ken on Sun Jul 14, 2013 5:50 pm

    The red/yellow wire should go to the center (ground) lug and the green/yellow should go where the red/yellow is now. Make sure you have the .02mfd ceramic capacitor to ground and lug 1.

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 14, 2013 7:21 pm

    Thanks for the help. Missed the obvious

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 14, 2013 8:11 pm

    After all wiring was finished, I followed instructions that came with the driver board from Mr Latino. The voltage at the output tubes quickly went past .70 volts. I cut the power, but nothing said about where to check for the problem. Any help? Thanks

    Bob Latino
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    Re: MK III woes...

    Post by Bob Latino on Sun Jul 14, 2013 8:30 pm

    Hi,

     I think that you got the board from Roy Mottram and not from me .. If it is a VTA Mark III driver board ? > Did you install a resistor (or a jumper wire) in R39 ?

    If you did then (1) pull out the rectifier (2) Set the bias pots at their mid position (3) Turn the amp on and measure the negative DC voltage on pins 5 and 6 to chassis ground on both output tubes and see if you get something like - 45 to - 65 volts DC. If you don't then the bias sytem is not working ..

    Bob

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 14, 2013 9:15 pm

    Did the test. Getting negative 1 volt at pin 5 and nothing at pin 6...both tubes read the same.

    Bob Latino
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    Re: MK III woes...

    Post by Bob Latino on Sun Jul 14, 2013 9:20 pm

    The bias system is not working ... See if you can post a photo of the inside wiring. Maybe one of us can find the issue for you ..

    Bob

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 14, 2013 9:30 pm

    http://s1303.photobucket.com/user/grizzinca/media/image_zps090e3276.jpg.html?sort=3&o=0
    I hope this works. I appreciate everyone's help. Might need a night off, take a breather. Thanks again

    Bob Latino
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    Re: MK III woes...

    Post by Bob Latino on Sun Jul 14, 2013 10:11 pm

    I believe that you have the two bias capacitors in backwards. The two negative poles on each bias cap should face the EDGE of the drive board,

    Bob

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 14, 2013 11:53 pm

    Placed the capacitors the correct way, managed to get proper voltage at pin 5 & 6. Put the rectifier tube in, couldn't cut the power quick enough as I was checking at the output tubes and the fuse blew. I won't give up till the house burns down!!!! Next?

    Bob Latino
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    Re: MK III woes...

    Post by Bob Latino on Mon Jul 15, 2013 4:43 am

    In your photo I do not see a 1000 ohm resistor between pins 5 and 6 on each output tube. Without that resistor, each output tube won't get any bias.

    Bob

    tubes4hifi
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    Re: MK III woes...

    Post by tubes4hifi on Wed Jul 17, 2013 10:46 pm

    I'll 2nd Bob's last comment, especially based on your voltage checks at pins 5 and 6, that's not even possible!
    you should have the outputs of the PCB wired to pin 6, then thru a 1K to pin 5.  
    That's nothing different than an original MK3, like the diagram Bob posted way above.
    Also, since you had those two caps (4 total if you've got 2 amps) they probably are bad now, caps don't like reverse voltage!
    They should be replaced with new ones.  100uF 100v.   Positive side to edge of PCB, (caps are marked with a Negative stripe).
    Also - very important, the yellow-green wire now goes to the striped end of the diode on the PCB, not to the terminal strip.

    jjones3318

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    Re: MK III woes...

    Post by jjones3318 on Thu Jul 18, 2013 10:45 am

    tubes4hifi wrote:...Positive side to edge of PCB, (caps are marked with a Negative stripe).
    Bob Latino wrote:...The two negative poles on each bias cap should face the EDGE of the drive board

    ??

    Bob Latino
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    Re: MK III woes...

    Post by Bob Latino on Thu Jul 18, 2013 10:55 am

    I think that Roy was thinking of the VTA ST-70 board in which the two POSITIVE poles on those two 100 uF @ 100 volt caps do face the EDGE of the board and the negative stripe on the cap faces the center of the board.

    On the VTA Mark III board, however, the two POSITIVE poles of those 100 uF @ 100 volt caps face the CENTER of the board and the negaitive stripe on the cap faces the edge of the board.

    Bob

    tubes4hifi
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    Re: MK III woes...

    Post by tubes4hifi on Thu Jul 18, 2013 12:14 pm

    oh yes, that's correct

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Thu Jul 18, 2013 8:29 pm

    Can't tell you how all this helps. Sure appreciate everyone taking the time to look at my post. I plan on taking the weekend to finish and hopefully report success. Thanks again.
    Mike

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 21, 2013 3:03 pm

    It occurred to me that there were 2 different boards offered. I purchased the octal driver board using 6sn7 tubes. Does this board wire differently than the other board?

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sat Jul 27, 2013 6:47 pm

    [url=grizzinca/media/image_zps42adcf06.jpg.html][/URL]]grizzinca/media/image_zps42adcf06.jpg.html][/URL][/url]. Still having issues. Blew fuse about 10-15 seconds after powering up. Bad quad cap? The instructions that came with the VTA board and the diagram that came with the 2 quad caps seem to contradict each other. Seems I have everything as per all info I find on the forum and instructions received with theatre. Any help would be appreciated.

    tubes4hifi
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    Re: MK III woes...

    Post by tubes4hifi on Sat Jul 27, 2013 10:36 pm

    a few things I can't see clearly in your photo. Where is power supply ground? Is ground connected to the black speaker terminal?
    Is ground connected to the PCB? Is the NFB (yellow speaker wire) going to the correct hole in the PCB?
    And most importantly, I don't see any speaker wires connected to the speaker terminals
    AND THE most important, are any of those speaker terminals touching the chassis? They must NOT touch the chassis!
    Normally that speaker board would be mounted on the outside of the chassis to insure that,
    but it can be mounted the way you have it if you use a few fiber washers on the outside to insure none of them are touching the chassis.

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 28, 2013 10:23 am

    Making progress. Not blowing fuses anymore. However, following instructions and removing the driver tubes, installing output and rectifier tubes to check voltage from pins 5 and 6 to ground, the 6.8k resistor on the quad cap went up in smoke. Didn't get a chance to check voltage. Help?

    tubes4hifi
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    Re: MK III woes...

    Post by tubes4hifi on Sun Jul 28, 2013 10:47 am

    obviously there was a short somewhere (like maybe those speaker posts) which caused the fuse to keep blowing and finally for the resistor to give out,
    so it will need to be replaced.

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sun Jul 28, 2013 11:17 am

    Thanks. Back at it.

    grizzinca

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    Join date : 2012-01-01

    Re: MK III woes...

    Post by grizzinca on Sun Jul 28, 2013 8:05 pm

    Made all repairs and fixes I could find. Removed the quad cap and tested it with a mm as per instructions found in the forum. Found the 20 mfd section (circle) to be bad. Could this cause the 6.8k resistor to blow? Also checked all the resistors on the driver board and all checked ok. Resistors on tube sockets were also fine.

    grizzinca

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    Re: MK III woes...

    Post by grizzinca on Sat Aug 10, 2013 9:05 pm

    Hello all. Been a few days, finally got the mkIII's finished, tested and into my system. Thanks for all your help. I ended up using the best advice of all...step back and start over. I ended up starting COMPLETELY over. Now I seem to have a fairly common mkIII issue. Hum coming through the speakers. From both amps. Other than the trannies and the chassis's, ALL parts are new. Including both quad caps (seems to be the leading culprit) . From what I've read in the forum, sounds like maybe a grounding issue? Will try to get more recent pics uploaded tonight or tomorrow, but in the mean time is there any advice on where to double (triple) check? Again, the hum is in both amps, so I suspect I've made the same mistake in both amps. Thanks

    Bob Latino
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    Re: MK III woes...

    Post by Bob Latino on Sat Aug 10, 2013 9:19 pm

    Do you have interconnects plugged in to your Mark III's and either shorted on the other end of the interconnects OR plugged in to your preamp? If the input jacks are left open, you will get some hum from these amps. The definitive test for amp hum is below.

    1. Disconnect the amp from your preamp

    2. Have the interconnect plugged in to the amp only and on the OTHER end of the interconnect short between the center pin and outer shield with a jumper wire or the side of a screwdriver. What you hear now with the input shorted is the residual hum of the amp. If the hum has gone away then your preamp or interconnects are at fault OR (if your amp has a three wire cord and I do believe that I see a 3 wire cord in your photo) you may have a "ground loop". Try floating the ground on both amps with a 3 to 2 wire "cheater plug". The cheater plug will defeat the ground loop IF a ground loop is the issue that is causing the hum.

    Bob

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