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    RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

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    SnugDynaco

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    Join date : 2011-09-11

    RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by SnugDynaco on Sun Dec 17, 2017 2:08 pm

    Running a ST-70 can be like having a tempermental spouse.  All seems good until, BANG. affraid

    Since my build in 2014, I've habitually watched the tubes as I power on, just in case something doesn't look right.  Last week I saw the rectifier spark internally, then the fuse blew.
    While ordering a replacement GZ34 and some extra fuses, I also purchased a new set of output tubes.  With everything new in place and the bias turned down slightly, upon startup the rectifier and one channel illuminated (both tubes), but not the other channel (neither of the two tubes).  The new fuse did not blow.

    Referring to Bob Latino's troubleshooting guide, item 4 (below), this would seem to cover it.  

     
    4. Problem - Fuse blows about 10 to 30 seconds after turn on and/or sparking inside   rectifier tube  Areas to check - bad rectifier tube, bad DC high voltage power storage (usually a bad quad cap), bad (shorted inside) output tube

    The quad capacitor was original to the kit build and is now 3 and 1/2 years old.  Should I start my investigation there with the multi-meter?

    Thanks in advance for your help and suggestions.


    Last edited by SnugDynaco on Sat Dec 23, 2017 1:21 am; edited 1 time in total (Reason for editing : Title changed to show 'RESOLVED')
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    peterh

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by peterh on Sun Dec 17, 2017 3:23 pm

    SnugDynaco wrote:
    Running a ST-70 can be like having a tempermental spouse.  All seems good until, BANG. :affraid:

    Since my build in 2014, I've habitually watched the tubes as I power on, just in case something doesn't look right.  Last week I saw the rectifier spark internally, then the fuse blew.
    While ordering a replacement GZ34 and some extra fuses, I also purchased a new set of output tubes.  With everything new in place and the bias turned down slightly, upon startup the rectifier and one channel illuminated (both tubes), but not the other channel (neither of the two tubes).  The new fuse did not blow.

    Referring to Bob Latino's troubleshooting guide, item 4 (below), this would seem to cover it.  

     
    4. Problem - Fuse blows about 10 to 30 seconds after turn on and/or sparking inside   rectifier tube  Areas to check - bad rectifier tube, bad DC high voltage power storage (usually a bad quad cap), bad (shorted inside) output tube

    The quad capacitor was original to the kit build and is now 3 and 1/2 years old.  Should I start my investigation there with the multi-meter?

    Thanks in advance for your help and suggestions.




    What do you mean with "illuminate" ? Is it that the filaments does not seem to glow ?
    then investigate that, measure AC 6.3V from the power transformer, check for
    filament winding ( you have 3, one 5V for the GZ34(works) and 2 6.3V for each side( 1 works the other not). See if the windings are ok or if a bad solder joint is the problem.
    Do not, repeat not, touch quad cap or other things that does not have any
    thing to do with the missing 6.3V filament circuit. In fact , you could pull the GZ34
    until you have all EL34 filaments glowing.
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    PeterCapo

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by PeterCapo on Sun Dec 17, 2017 3:25 pm

    Doesn't sound like a problem with the quad cap because it is part of a common power supply for both channels.

    My thought is that when you changed-out the power tubes, the filament connection to the sockets might have been disturbed and you lost filament power to one side.  Try checking the filament connections, the solder, at the sockets where the tubes don't light - contacts 2 and 7 for the two power tubes.

    SnugDynaco

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    Join date : 2011-09-11

    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by SnugDynaco on Sun Dec 17, 2017 5:04 pm

    @PeterCapo : I did find a broken connection in the socket, and have repaired.

    First, without the rectifier and without speaker load, inserting the two output tubes to the side that previously failed to light/power on, the tubes now light up.  Powering down and inserting tubes for the second channel, all four output tubes have power.

    Inserting the rectifier tube and connecting the speakers to both channels, then restarting (with fingers on the power switch), the rectifier arcs inside.  Immediate shut down before the 3A fuse blows.

    I have one brand new rectifier tube left...


    Last edited by SnugDynaco on Sun Dec 17, 2017 5:05 pm; edited 1 time in total (Reason for editing : correct typo)
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    PeterCapo

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by PeterCapo on Sun Dec 17, 2017 5:40 pm

    Sounds like we're back to the beginning.  I wouldn't try another rectifier tube at this point.  This might be a good time to check the quad section capacitor - but wait a while before going inside.  It would be a good idea to bleed the quad can, in any case, provided you know how to do so safely.

    What is your AC mains reading?  You might want to monitor it at different times of day for several days.  Again, observe safe work practices around lethal voltages.

    Wait - was it a new rectifier tube that arced or was it the old rectifier tube?

    Just to mention a couple of things…

    It would be good to refrain from power-cycling the amp with little or no time between power cycles.  Allow several minutes.

    Also, it’s okay to run the amp with the rectifier tube removed, but all the other tubes should be in at the same time.
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    Peter W.

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by Peter W. on Mon Dec 18, 2017 10:39 am

    Just a few suggestions, going right back to basic troubleshooting and what to do first!

    1. With the amp off, unplugged and cold for at least 24 hours:
    a) Mark and remove all tubes. Tighten and clean the sockets carefully. Use a wooden toothpick, alcohol and a dental pick. If you have non-metallic cleaning brushes, those too. Be very careful with the miniature-tube sockets.
    b) Check *every* connection under the chassis. Use that same dental pick to pull on each wire - sometimes the connection will look good and be held in place by the insulation alone.

    This will get past bad connections and/or loose sockets, a key start for basic troubleshooting.
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    Tubes4ever

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by Tubes4ever on Mon Dec 18, 2017 12:39 pm

    PeterCapo wrote:Wait - was it a new rectifier tube that arced or was it the old rectifier tube?

    Peter asks a very important question, did you reuse the rectifier that originally arced?  Once the GZ34 arcs, it will continue to arc when power is reapplied. In other words, once it arcs it is bad and should be discarded.
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    Peter W.

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by Peter W. on Mon Dec 18, 2017 1:38 pm

    Tubes4ever wrote:
    PeterCapo wrote:Wait - was it a new rectifier tube that arced or was it the old rectifier tube?

    Peter asks a very important question, did you reuse the rectifier that originally arced?  Once the GZ34 arcs, it will continue to arc when power is reapplied. In other words, once it arcs it is bad and should be discarded.

    All true. However and at this moment, I would like to see the OP establish that ongoing problems are not mechanical in nature. Also agree that short-cycling is NOT good for the amp.

    SnugDynaco

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    Join date : 2011-09-11

    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by SnugDynaco on Tue Dec 19, 2017 7:02 pm

    @Peter Capo - The second time restarting, I mistakingly used the old sparky rectifier. A new one is now in place.

    V3, tab 1 needed to be resoldered. This was done, and all other wiring appears to be solid.

    Reconnected the speakers, preamp and CD player. Before turning everything on, I turned the bias setting slightly counter-clockwise, lowering the voltage. Powered everything on. All four output tubes illuminated/glowed, as did the new rectifier. Left this running for five minutes. Nothing looked too 'hot'; nothing sparked internally - everything appeared normal.

    However, now there is no sound, not even a hiss to my speakers. When I attempted to set the bias to 400, I got no signal from my multi-meter (set on the 2 volt dcv selection). (The multi-meter correctly measures a 1.5volt AAA battery).

    The assembly manuals included on the "Tubes4hifi" site - are those manuals for the original Dynacos and not the contemporary models? I ask due to the references to the 1.56V bias rather than the contemporary, recommended 400 bias setting, and the $7 service fee if the unit needs to be returned for service. I want the contemporary (new) manual so that I can measure voltage at each tab as was done when I finished the build, before the initial start.

    Thanks for all your time/responses to date.


    SnugDynaco

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    Join date : 2011-09-11

    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by SnugDynaco on Tue Dec 19, 2017 7:06 pm

    I am near Buffalo, NY. Are there any forum members within a six hour drive that would have the patience, time and knowledge to help me diagnose this issue? I hate to ship the unit due to the possibility of damage.

    I'll supply the chicken wings, Weber's mustard and Shalen's hot dogs if you're willing...
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    PeterCapo

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by PeterCapo on Tue Dec 19, 2017 9:23 pm

    Last time I checked, the manuals on the tubes4hifi website are for the original Dynacos.  The problems we have [apparently] fixed so far are not restricted to either the original Dynaco or the tubes4hifi version.

    But, since there seem to be ongoing issues, exactly which version of Stereo 70 do you have, please?  Is it a Latino/tubes4hifi Stereo 70 or a dynakitparts Stereo 70?  If you have the Latino/tubes4hifi version, you'd have to ask them for the proper documentation, or maybe someone who has it can e-mail it to you.

    The original Dynaco Stereo 70 used two bias pots, and the bias was set to the familiar 1.56VDC for each channel. Dynakitparts.com has a kit where two additional pots can be added to a Stereo 70 (having the original Dynaco circuit) so that each power tube's bias can be adjusted individually, in which case the bias would be adjusted to a different value. The tubes4hifi circuit uses four bias controls, and, IIRC, Bob recommends 0.4 VDC bias setting for each one of the four power tubes (EL34 or 6L6) for the tubes4hifi version.

    Do the plates of any of your four power tubes have a glowing reddish spot?

    Not sure why you have zero output.  Check obvious things like connections, switch settings and such.  See if you can isolate the problem to another piece of equipment in the chain.


    Last edited by PeterCapo on Wed Dec 20, 2017 1:07 pm; edited 1 time in total (Reason for editing : Add information.)
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    Peter W.

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by Peter W. on Wed Dec 20, 2017 8:27 am

    SnugDynaco wrote:I am near Buffalo, NY.  Are there any forum members within a six hour drive that would have the patience, time and knowledge to help me diagnose this issue?  I hate to ship the unit due to the possibility of damage.  

    I'll supply the chicken wings, Weber's mustard and Shalen's hot dogs if you're willing...

    The Dynaco Clinic *Will* still happen. Zip 19027 may be at the hairy edge of a 6-hour drive. I will let you know.

    nmchiefsfan

    Posts : 46
    Join date : 2012-03-21

    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by nmchiefsfan on Wed Dec 20, 2017 3:19 pm

    Sounds to me like no B+. Bad rectifier?

    SnugDynaco

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    Join date : 2011-09-11

    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by SnugDynaco on Wed Dec 20, 2017 10:58 pm

    I'd like to attend the workshop. Just over 6 hours according to Google maps, and have friends in nearby Lititz, PA.
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    Dynalover

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by Dynalover on Thu Dec 21, 2017 2:55 am

    Was this a genuine GZ34 or one of the modern ersatz?

    zx

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by zx on Thu Dec 21, 2017 4:58 am

    Over 40 years of owning tube amps... I never had these problems .....any one that owns a tube amp... need a Variac!.....I guess before thay get any.... tube amp or preamp.....for the cost of tubes an better sound.
    Even the china tube amps are still come in with 110v ac input....all the Ac here in the US is high...an not comeing down ...ever...good luck....hohoho
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    j beede

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by j beede on Thu Dec 21, 2017 9:16 pm

    Are you able to safely probe the amp while it is powered on? If so, for starters measure the dc voltage on pin#8 of your 5AR4. Then measure pin #5 on each EL-34. Do this with all tubes installed.

    SnugDynaco

    Posts : 26
    Join date : 2011-09-11

    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by SnugDynaco on Sat Dec 23, 2017 12:59 am

    nmchiefsfan wrote:Sounds to me like no B+.  Bad rectifier?  

    After obtaining a copy of the assembly manual, I ran all of the test numbers for initial startup and found everything was in acceptable ranges.

    Then went back an re-read Bob Latino's quick troubleshooting guide. No. 2 - even if a rectifier lights does not mean it is good. No sound and no bias would seem to confirm this.

    Having nothing to lose, I removed the first "new" rectifier and installed a second new recitifier. Lo and behold, the music has returned, and the unit now will bias.

    Both new rectifiers were supplied recently by a well known source. I will be contacting them for a replacement. The rectifiers are Sovtek GZ34.

    I still am running the older set of EL34 Mullards, and in the morning will install the new set. The older set has about 18 months of use approx. 2000 hours. Bias was beginning to drift a bit.

    Thanks for all of your advice on helping to track down the problem. I did have a loose connection at one of the pins and was able to make that repair, and learned a fair amount from this incident.


    SnugDynaco

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    Join date : 2011-09-11

    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by SnugDynaco on Sat Dec 23, 2017 1:00 am

    Dynalover wrote:Was this a genuine GZ34 or one of the modern ersatz?

    Both genuine Sovtek GZ34's
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    deepee99

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by deepee99 on Sat Dec 23, 2017 2:55 pm

    SnugDynaco wrote:
    Dynalover wrote:Was this a genuine GZ34 or one of the modern ersatz?

    Both genuine Sovtek GZ34's

    Those are not "real" GZ34s. Only Mullard made 'em.
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    j beede

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by j beede on Sat Dec 23, 2017 5:46 pm

    It's not going into a Leak Stereo 20 so a fake GZ34 should be fine.
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    deepee99

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by deepee99 on Sat Dec 23, 2017 6:44 pm

    Jim McShane's got some pretty sturdy new-issue GZ-34 types. I'd give them a whack before searching for NOS Mullards.
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    GreggW

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by GreggW on Sun Dec 24, 2017 12:02 am

    I had the same issue two years ago with a new rectifier, no music, no bias voltage. I had dropped the new rectifier, a 5AR4, but it certainly wasn't the seller's fault the tube didn't work. My rectifier of choice is a 5V3A. Though it has more voltage drop than a 5AR4, it handles a lot more current, over 400ma. The same rectifier's been in use now for more than two years.
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    PeterCapo

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by PeterCapo on Sun Dec 24, 2017 10:38 am

    If not already done, consider the "yellow sheet diode mod:" http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod
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    GreggW

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

    Post by GreggW on Sun Dec 24, 2017 2:35 pm

    Should've mentioned I did the yellow sheet diode modification. Not sure if it contributes to the life of the 5V3a. Time will tell. (I built a tube guitar amp from a kit and did the yellow sheet mod to it also.)

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    Re: RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

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