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    First Timer...Troubleshooting

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    DrZ123

    Posts : 29
    Join date : 2014-01-03

    First Timer...Troubleshooting

    Post by DrZ123 on Thu Jan 23, 2014 12:25 pm

    I am a first timer and just finished building the St-120.  Upon initial startup, all tubes light up, but then after about 15 seconds I get arcing in the rectifier and the fuse blows.  Tried new rectifier, same result.  Have been troubleshooting with Bob to no avail after re-soldering all connections and checking wiring.  Here are photos of the build, maybe someone here can help spot the problem...








    kygeezer

    Posts : 51
    Join date : 2012-11-30

    Re: First Timer...Troubleshooting

    Post by kygeezer on Thu Jan 23, 2014 3:09 pm

    Not sure from the photos, but are the Russian PIO caps insulated? If not the metal body could be shorting on the PC board.
    Good luck. This is a great forum and you will get the help you need to solve the problem.

    DrZ123

    Posts : 29
    Join date : 2014-01-03

    Re: First Timer...Troubleshooting

    Post by DrZ123 on Thu Jan 23, 2014 3:35 pm

    kygeezer wrote:Not sure from the photos, but are the Russian PIO caps insulated?  If not the metal body could be shorting on the PC board.
    Good luck. This is a great forum and you will get the help you need to solve the problem.

    I insulated the wires of the Russian PIOs, and made sure they are not touching anything. Is there something else to do?

    Bob Latino
    Admin

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    Re: First Timer...Troubleshooting

    Post by Bob Latino on Thu Jan 23, 2014 3:57 pm

    The Russian PIO's don't have to be insulated on the outside. As long as they are installed as shown in the manual, they will be set up from the board about 1/4 inch or so and the outer cases won't touch anything.

    You could try starting the amp up WITHOUT the three driver tubes in place but with the rectifier in there to eliminate a possible short inside one of the driver tubes. The amp will not play of course but if it will stay on and the four output tubes will bias - then - there is a possiblity of a short in one of the driver tubes.

    Bob

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: First Timer...Troubleshooting

    Post by arledgsc on Thu Jan 23, 2014 5:10 pm

    And perhaps add the diode mod to the rectifier tube socket.  Gets expensive fast blowing rectifiers.  I would perhaps use a WZ68 solid state rectifier until you are sure everything checks out.  If it makes you feel any better my GZ34 blew on first power up with it installed and I had everything wired correctly.  It is kind of a rite of passage with the ST-120 so at least you now have your wings.

    Oops!  You don't have the relay board so don't recommend the WZ68.


    Last edited by arledgsc on Fri Jan 24, 2014 9:54 am; edited 1 time in total

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: First Timer...Troubleshooting

    Post by arledgsc on Thu Jan 23, 2014 5:20 pm

    Upon closer inspection of your pictures double check the quad cap wiring.  Do I see a short, red wire from the first cap stage to ground?   Take a look at the last picture.

    DrZ123

    Posts : 29
    Join date : 2014-01-03

    Re: First Timer...Troubleshooting

    Post by DrZ123 on Thu Jan 23, 2014 5:40 pm

    arledgsc wrote:Upon closer inspection of your pictures double check the quad cap wiring.  Do I see a short, red wire from the first cap stage to ground?   Take a look at the last picture.

    I will check that when I get home but I THINK that is the ground wire from a twist tab of the quad cap, but I will certainly double check!

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: First Timer...Troubleshooting

    Post by sKiZo on Thu Jan 23, 2014 6:07 pm

    Only thing I'd add is to double check all the wires to make sure there's no burn through or shorts where they cross - lift and separate, as the Playtex people say. Double check real close on all components on the VTA board to make sure nothing's shorting to chassis as that can get real close in there, and do the old wiggle test at ALL the connections to makes sure you don't have any weak or missed solder points. Snip any component tails close to the joint to eliminate those as possible shorts, and look for solder whiskers. Make sure there's no cuts where the transformer wires go thru the chassis. Also might be a good idea to do continuity checks on all wires as it's not unheard of for the copper to crack inside the insulation and leave an open circuit.

    Got a good magnifier? Your problem might be too small to see with the Mark One Eyeball ...

    Process of elimination and patience mostly.

    And ya ... a copper cap rectifier might be a good thing for any startup (the WX68 heavy duty) and the rectifier diode mod isn't a bad idea either. All else fails, a dim bulb test or ramping up with a variac might be in order. The variac left in line with the amp would be a good investment for today's typical line voltage anyway.

    DrZ123

    Posts : 29
    Join date : 2014-01-03

    Re: First Timer...Troubleshooting

    Post by DrZ123 on Thu Jan 23, 2014 7:34 pm

    Just checked and that wire is indeed from the ground to the twist tab on quad cap, not an actual terminal.

    I will keep searching, anyone in NYC around to see in person?

    Laminarman

    Posts : 110
    Join date : 2009-12-30

    Re: First Timer...Troubleshooting

    Post by Laminarman on Thu Jan 23, 2014 8:47 pm

    I found building mine that a flip down head loupe and a halogen light (built in), probing with a blunt hooked plastic thingy resembling a dental probe did wonders to find any problems, and I did have some burned through insulation from soldering technique which I replaced. The probe actually was from a gun cleaning kit I got at Gander Mountain of all things. Works wonders to clean grime out of a Glock or find a short near a quad cap  Very Happy 

    sKiZo

    Posts : 1310
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    Location : Michigan USA

    Re: First Timer...Troubleshooting

    Post by sKiZo on Thu Jan 23, 2014 9:10 pm



    2x power, $23 at Amazon ... looks like a decent light. LEDs an evarthang ... the 5x spot is a nice feature, and it gets decent reviews.

    Good addition to any bench ...

    Carson LM-20 2X LED Illuminated Magnifier & Desk Lamp

    DrZ123

    Posts : 29
    Join date : 2014-01-03

    Re: First Timer...Troubleshooting

    Post by DrZ123 on Thu Jan 23, 2014 9:11 pm

    I am a dentist, I have loupes Wink

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: First Timer...Troubleshooting

    Post by arledgsc on Fri Jan 24, 2014 10:38 am

    After first double checking if you can find a tech then perhaps that is best course of action.   Like Bob said you may have a bad output tube or perhaps just lousy luck with rectifiers.  Other than wiring or tubes a shorted capacitor is about it for possible faults - typically.  But your wiring and kit components are new and you have to prove and assure yourself each are good otherwise.  In general rectifiers are worked hard in the ST-120 and power up is the most stress on them charging up the filter capacitors.  The caps, even when healthy,  look like a short circuit at the first instance of charging.

    But the gang of knuckleheads around here are eager to help.   Welcome to the forum, ask questions if you need, and let us know what you find.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: First Timer...Troubleshooting

    Post by anbitet66 on Fri Jan 24, 2014 1:10 pm

    OOOOOOOOOOOOOO!  Knucklehead???  Hey, I resemble that remark!!!

    DrZ,

    15 seconds is just about the time needed for the 5AR4 to warm up and begin conducting current.  If it blows up at that point, I would strongly suspect a short circuit.  What you can do is measure the resistance from pin 8 of the rectifier to chassis ground and note the resistance.  I'm not sure what test equipment you have, but that should not be a major concern.  A trip to Harbor Freight or Radio Shack and the expense of buying a cheap meter would work ok.   A resistance of zero to a few ohms would be great to find under these circumstances, but a low reading (10's of ohms to maybe a couple hundred) means the tube is being overloaded.  Then try disconnecting components to find where the short (or overload) originates from.

    Been doing this for over 40 years.  Started by taking apart things when I was too young to remember or know better, with the requisite results of having to throw out a lot of otherwise nice but now broken stuff.  As I got older, stuff actually began to work (sometimes better) when I reassembled it.  Kits are usually cut and dry.  You are not the first to experience problems, and with respect to Bob, you won't be the last.  If you are in NYC, I am maybe a stone's throw away.

    Let me know if you need help.  I may be able to assist.  Looking at photo's doesn't work for me most of the time.  I like to see the "patient" up close and personal.

    Good luck - Tony

    DrZ123

    Posts : 29
    Join date : 2014-01-03

    Re: First Timer...Troubleshooting

    Post by DrZ123 on Fri Jan 24, 2014 1:44 pm

    anbitet66 wrote:OOOOOOOOOOOOOO!  Knucklehead???  Hey, I resemble that remark!!!

    DrZ,

    15 seconds is just about the time needed for the 5AR4 to warm up and begin conducting current.  If it blows up at that point, I would strongly suspect a short circuit.  What you can do is measure the resistance from pin 8 of the rectifier to chassis ground and note the resistance.  I'm not sure what test equipment you have, but that should not be a major concern.  A trip to Harbor Freight or Radio Shack and the expense of buying a cheap meter would work ok.   A resistance of zero to a few ohms would be great to find under these circumstances, but a low reading (10's of ohms to maybe a couple hundred) means the tube is being overloaded.  Then try disconnecting components to find where the short (or overload) originates from.

    Been doing this for over 40 years.  Started by taking apart things when I was too young to remember or know better, with the requisite results of having to throw out a lot of otherwise nice but now broken stuff.  As I got older, stuff actually began to work (sometimes better) when I reassembled it.  Kits are usually cut and dry.  You are not the first to experience problems, and with respect to Bob, you won't be the last.  If you are in NYC, I am maybe a stone's throw away.

    Let me know if you need help.  I may be able to assist.  Looking at photo's doesn't work for me most of the time.  I like to see the "patient" up close and personal.

    Good luck - Tony

    Hey Tony, where are you located?

    I have a nice little electronic multitester and can do some more tests later. But I think basically we have determined there is a short somewhere, where it is is the issue!

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: First Timer...Troubleshooting

    Post by anbitet66 on Fri Jan 24, 2014 1:53 pm

    If you look under my avatar, you see I'm a Long Island 'er.  Near enough to cause muy trouble. Laughing

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: First Timer...Troubleshooting

    Post by sailor on Fri Jan 24, 2014 3:07 pm

    I hope most people know you can replace the rectifier tube completely with 2 cheap 1N4007 [1 amp./1000 volts] soldered to the tube socket. About 10 for a dollar. The only reason many people don't use them is because of the lack of high voltage delay and of course the difference in sound caused buy the sag when a tube rectifier can't keep up with the load. We audiophiles do love our distortion it makes the music sound so detailed.

    Brap

    Posts : 83
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    Age : 61
    Location : Lisle, Illinois

    Had the same problem --- initially

    Post by Brap on Fri Jan 24, 2014 4:13 pm

    Blew a rectifier too and did quite a bit of head scratching. Unplugged everything and checked the quad cap to ground resistance and all was OK. Then really started head scratching. Came to find out that the lead from the quad cap to the rectifier solder joint was not quite up to snuff. Re-did both and back in business. Had some insulation goobering that up.

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: First Timer...Troubleshooting

    Post by Sal on Sat Jan 25, 2014 8:20 pm

    I have talked with DrZ123 over the phone today and we found out where the short was. He replaced the wire that was shorted and was able to bias the tubes and now has a working ST-120!

    Sal

    DrZ123

    Posts : 29
    Join date : 2014-01-03

    Re: First Timer...Troubleshooting

    Post by DrZ123 on Sat Jan 25, 2014 8:28 pm

    Sal is the man. Free cleaning for him!

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: First Timer...Troubleshooting

    Post by PeterCapo on Sat Jan 25, 2014 9:52 pm

    Way to go, Sal!

    Sal

    Posts : 221
    Join date : 2009-02-05
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    Re: First Timer...Troubleshooting

    Post by Sal on Sat Jan 25, 2014 10:00 pm

    DrZ123 wrote:Sal is the man.  Free cleaning for him!

    I'll be right over....

    {

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: First Timer...Troubleshooting

    Post by arledgsc on Sat Jan 25, 2014 10:07 pm

    I'll be right over....
    Yikes!   Dr. Z better pack up and move.

    Sal is the man!  Excellent work and big pat on the back.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: First Timer...Troubleshooting

    Post by anbitet66 on Sat Jan 25, 2014 10:46 pm

    Good going Sal.  Congrats DrZ on now having a working amp.  I hope you have good luck with it.

    Tony

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: First Timer...Troubleshooting

    Post by Sal on Sat Jan 25, 2014 10:47 pm

    Thanks Guys!

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