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    Dyna MKIII fallen short of upgradeing caps.

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    scott6058

    Posts : 37
    Join date : 2009-01-21

    Dyna MKIII fallen short of upgradeing caps.

    Post by scott6058 on Wed Mar 11, 2009 9:00 pm

    Hi Bob
    I had to do it myself and failed.
    I replaced the quad can with an Authenticap 30 20 20 20 600v surge and simply recaped the pc with auri audience .1 and .22 mfd also I replaced the selenium with the 1n. The first one I brought up slow and the biased it and all was fine. So I decided to work on the second amp did the same and all again seems fine. I listened to the first amp for about 10 minutes and it sounded very good.
    When I finally put everything in its place I lost power to my gz34 and my rear output kt88. Only one 6an8 and one front output kt88 glow. I also replaced the coupling capacitors under the chassis with 47mfd 160v properly.
    Please lend me any advice I triple checked my wiring and connections. Best regards Scott

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dyna MKIII fallen short of upgradeing caps.

    Post by Bob Latino on Thu Mar 12, 2009 6:18 am

    Scott,

    First swap the tubes from the good amp into the amp that won't play to see if you can eliminate the fact that it could be a tube issue. If the same tubes won't light up then check the tubes sockets. On the KT88 that won't light up see if you can get about 6.3 volts AC across pins 2 and 7.

    There is a voltage check list in the Mark III manual. Do a comparison voltage check between the amp that works and the amp that doesn't work. If you find any major differences between the two amps consider that a clue as to what could be wrong with the amp that doesn't work.

    Bob

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re Dynaco MKIII Problem

    Post by scott6058 on Thu Mar 12, 2009 10:06 am

    Sorry Bob I know it's extremely hard to diagnose these amps over the internet but your help is much appreciated. I did some tube swapping but now both amps have the same problem. The gz24 and the rear kt88 both don't seem to get power. I cleaned the sockets and have 6.5 volt at pins 2 and 7 at the front kt88. I checked all my connections and solder joints several times. I'm lost I hope I didn't ruin the transformers nothing got hot or smoked. I'll fuss with it again. Any other ideas?
    Best regards Scott

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Do a voltage check

    Post by Bob Latino on Thu Mar 12, 2009 11:07 am

    Scott,

    If the amps will stay on and not blow a fuse then do a voltage check on each amp as outlined in the Mark III manual and see if anything seems radically different from the proper voltage.

    If you can get about 6.3 volts AC across pins 2 and 7 of each output tube socket then the tubes should light up. That doesn't mean that something else could be wrong with the tube - it just means that the tube's heater is OK.

    Bob

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re: Dyna MKIII fallen short of upgradeing caps.

    Post by scott6058 on Fri Mar 13, 2009 4:46 pm

    Hi Bob
    I have 6.9 volts on pin2 and 7 one KT88 and the GZ34 but the GZ fails to glow as the same with the rear KT88 it has 5.5 volts and doesn't warm up either. Other test voltage test points don't barely have values. I took the cover off my good st70 to practice my testing skill and it spec out fine. It seems odd that I'm not getting any voltage from my new quad can capacitor for the old ones had to go. I'm lost any suggestions can I test my transformers? I swapped many known good tubes and cleaned the socket as best I could.
    Sorry to be a bother.
    Thanks Scott

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dyna MKIII fallen short of upgradeing caps.

    Post by Bob Latino on Fri Mar 13, 2009 5:20 pm

    Scott,

    The GZ34 doesn't use pins 2 and 7 for the heater only the two KT88's use pins 2 and 7 for the 6.3 volt AC heater voltage. Is your GZ34 good? Try another one in there or use a 5U4 as a replacement.

    On the GZ34 check

    Pin 4 to ground - about 490 volts AC
    Pin 6 to ground - about 490 volts AC
    Pin 2 to ground - about 475 - 490 volts DC
    Pin 8 to ground - about 475 - 490 volts DC

    If you get nothing on pins 4 and 6 or only get voltage on one pin but not the other the power transformer is bad.

    If you get no voltage on pin 8 to ground - the rectifier is probably bad. If you get no DC voltage on pin 8 of the rectifier tube then the quad cap will never have any voltage because it derives it's DC voltage from pin 8 of the rectifier tube.

    If you can > take a digital photo of the interior wiring and Email it to me as an attached file to > Bob01605@aol.com > I may be able to pick up on something by looking at your interior wiring.

    Bob

    scott6058

    Posts : 37
    Join date : 2009-01-21

    re MKIII problems

    Post by scott6058 on Fri Mar 13, 2009 8:24 pm

    Thanks for everything Bob.
    I will send some pics tomorrow.
    Do have a great weekend and I am going to walk away from this till then I simply had enough.
    Best Regards Scott

    danf

    Posts : 58
    Join date : 2009-01-19

    A question for Scott

    Post by danf on Fri Mar 13, 2009 8:46 pm

    Hi Scott,

    Are you sure that you didn't plug the KT88 into the GZ34 socket and the GZ34 into the KT88 socket? That would reproduce this problem I think.

    Dan

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Dyna MKIII fallen short of upgradeing caps.

    Post by scott6058 on Sat Mar 14, 2009 11:34 am

    Dear Danf and Bob
    You guessed it and you were right. I misplaced the GZ34!
    I feel like an idiot.
    The Dyna MKIII's biased perfectly and sound very nice.
    I still prefer the sound of the my st70 for some odd reason.
    Sorry for all the confusion. This forum is the best.
    Scott

    danf

    Posts : 58
    Join date : 2009-01-19

    We've all done this!

    Post by danf on Sat Mar 14, 2009 1:47 pm

    Hi Scott,

    No need to feel dense - we've all done this trick once or twice. It's lucky that this swap doesn't damage the tubes.

    I haven't spent much time listening to stock Mark III's but I have found that similar circuits (using single triode split-load phase-inverters) sound muddy to me. The VTA replacement boards (using dual triode cathode-coupled phase inverters) work well for me, and if you want to invest more time and money in your Mark IIIs, you could consider these. The individual bias controls on the VTA board also allow lower distortion. I left a message about the VTA board on the forum. Of course, you could just enjoy your ST70...

    Dan

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