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    ST-35 won't power on....I have a theory that I want to run by you guys....

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    fedoragent

    Posts : 35
    Join date : 2013-09-14

    ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by fedoragent on Tue Dec 24, 2013 12:20 pm

    Folks,

    I have recently acquired a ST-35 from FleaBay. Previous owner claims that it worked but just shut off. This led me to think that it might have been just the fuse. Sure enough, I get to the fuse and it is melted/burnt. I replaced it with a 2 Amp fuse and with my dim bulb tester I can tell it is receiving power. However, none of the tubes light up. So, I take the meter to it to test the PT. I get 140k ohms. So, at that point, because of what the manual said, I figured the windings were good. So I moved onto the diodes, and I *think* they check out leading to the quad cap and the three resistors. (BTW, never seen those square resistors...that was new to me.) I check the quad cap, all of the capacitance readings were just straight wrong, leading me to believe that the quad cap was to blame. Then I checked the resistors, each had their ohms WELL off. I am getting 3ish volts AC from the power cord and the fuse. At the 5 terminal strip, where the diodes connect, I get 8 volts before the diode. When I get to the quad cap, I get nada.

    Two things:

    1. Previous owner had a 3Amp fuse in there and it called for a 2Amp, and I *think* there may have been too much current going through, therefore burning the fuse.

    2. If there was a failure, would the transformer in that situation be okay but the quad cap, and all of the resistors would go?

    3. Would the diodes go as well? I think they are 1N4007 or 1N4004. Dunno.

    What do you guys think and how should I proceed?

    Thanks,

    Jon

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by sailor on Tue Dec 24, 2013 12:52 pm

    I would not plug it in again until you have replaced the quad cap and I would also replace the diodes with 1N4007 or I think 1UF4007 which are the ultra fast version. There is no goo reason not to use the higher voltage rated diodes.

    fedoragent

    Posts : 35
    Join date : 2013-09-14

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by fedoragent on Tue Dec 24, 2013 1:51 pm

    sailor wrote:I would not plug it in again until you have replaced the quad cap and I would also replace the diodes with 1N4007 or I think 1UF4007 which are the ultra fast version. There is no goo reason not to use the higher voltage rated diodes.

    Kinda what I was thinking. It is the holidays so I probably won't get my parts from Dynakitparts until Thursday. But, I mean the diodes and the quad cap are a quick fix. I just want to make sure the transformer isn't toast. I think it isn't based upon my readings and based upon what the manual said. I figure if I'm getting voltage readings from the red wires from the cap and the black wires going to the fuse. I figure the PT is alright. It doesn't get hot, it doesn't hum...

    But DANG what would kill the resistors like that???

    Jon

    sailor

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    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by sailor on Tue Dec 24, 2013 5:30 pm

    The problem is when the can caps. goes bad they usually short to ground. If the transformer is turned on while still connected to the bad cap you will get a very low reading and will burn up the transformer. To get a good reading you need to unsolder the wire from the cap. When tested It should be a little on the high side [ about +10% or more] because you would have no load. Most often when a transformer starts to burn up you will either get wax melting out of it or they smell real bad because the insulation is burning.

    sKiZo

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    Location : Michigan USA

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by sKiZo on Tue Dec 24, 2013 6:45 pm

    In your first post, you mentioned the resistors being way off, and later a comment about something "killing" them? Which is it ... toast or just off?

    Reason I ask, it's not all that unusual for carbon resistors to drift WAY off - normally up - as time does it's thing. They soak of moisture and either drift or go open. Best bet is to replace ALL of those with some decent modern equivalents, and higher ratings wouldn't hertz ...

    DarthBubba

    Posts : 81
    Join date : 2012-05-05

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by DarthBubba on Tue Dec 24, 2013 10:31 pm

    fedoragent wrote:Folks,

    I have recently acquired a ST-35 from FleaBay.

    Suddenly I'm more comfortable with losing that auction  Evil or Very Mad 

    fedoragent wrote: I am getting 3ish volts AC from the power cord and the fuse.

    I would think 120VAC is expected here.  Is the power cord "open" as in, is there an internal break that prevents the wall-socket power from getting to the power transformer at all?  The rest of this exercise is pointless if the power cord is internally broken.

    Disconnect the fuse and measure the resistance of each conductor in the power cord from the fuse socket and five-tab terminal strip to the blades that plug into the wall and establish that the cord is good.

    My ST-35 power transformer primary measures four ohms, black wire to black wire (and infinite resistance to ground); the HV secondary is around 165 ohms (red wire to red wire) still connected to the rest of the power supply; each leg is 82 ohms to ground and the red/yellow wire..

    Do you have a Variac, or some safe way to bring up the AC voltage without endangering your life?  affraid

    PeterCapo

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    Join date : 2008-12-05

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by PeterCapo on Wed Dec 25, 2013 11:56 am

    Blown fuse, funky readings around power cord and power transformer - I'd be real careful.  Get a GFCI.


    Last edited by PeterCapo on Thu Dec 26, 2013 10:04 am; edited 1 time in total

    sailor

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    Join date : 2011-04-04

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by sailor on Wed Dec 25, 2013 1:49 pm

    Simple, to blow a fuse you need to pull more power than the fuse can pass. So if you blow a fuse it is probably because you have a short to ground. The most likely culprit is the can caps. If you blow a fuse and the amp stops working but you don't blow a second fuse you probably popped the diodes when the cap shorted. So you are no longer sucking to much power. In any case the first step is to change out the can cap and diodes. As far as resistors I would change all of the power resistors to wire wound where possible. They are cheap and low noise. Metal oxide for the rest.
    Take your ohms meter set on volts and make sure the can cap is discharged then set on ohms and check from pos. to neg. on each section to see if any are grounded. This is not fool proof but if it reads zero on any section it's bad.

    GP49

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    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by GP49 on Thu Dec 26, 2013 1:34 am

    DarthBubba wrote:

    I would think 120VAC is expected here.

    You WOULD think that but he's using a dim bulb tester, which connects a light bulb in series
    with the AC supply.

    I have zero experience with dim bulb testers, and don't want any.  From having been in the repair
    business for many years, I use a VariAC and a current monitor.  Dim bulb testers are just too crude
    to be of any real use beyond the most rudimentary servicing.  Bench type variable AC supplies with
    ammeters used to cost a couple of hundred dollars and up, but these days you can get a VariAC
    for about $50 and plug it into a "Kill-A-Watt" current monitor which runs around $25.  The combination
    is indispensable for anyone who intends to do any REAL servicing.

    tubes4hifi
    Admin

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    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by tubes4hifi on Thu Dec 26, 2013 9:33 pm

    a lot of meters read different of course (especially when reading transformer windings) but on a new ST35 power transformer I read 143 ohms, not 143K ohms.
    If you had a variac you could put in maybe 60vac instead of 120vac to the primaries and then check the red wires for around 200vac from each red to the red/yellow,
    or if no variac and lots of guts and safety precautions, you should read about 400vac from each red to the red/yellow wire.

    fedoragent

    Posts : 35
    Join date : 2013-09-14

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by fedoragent on Thu Dec 26, 2013 11:09 pm

    Alright boys, so here is what I did. I cut the diodes away from the 5 terminal strip. I then took readings from my DMM and verified that both Primary and Secondary are getting 320V. For kicks, i also measured at the fuse and got 59V. No smells. No meling gunk...just pure AC. Sounds to me like the PT is good. What do you guys think?

    Oh and btw, My VARIAC went to crap so I am using a dim bulb tester I hooked together. I also have a GFCI that the DBT is hooked to and the switch I am using is on the wall faaar away when I take my readings to make sure I do not goto old sparky.... I mean, I want to meet allt he jazz greats....but not THIS soon.

    Thanks guys, you are awesome.

    Jon

    fedoragent

    Posts : 35
    Join date : 2013-09-14

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by fedoragent on Thu Dec 26, 2013 11:10 pm

    Btw, the bulb in the DBT is a 60 if anyone was curious.

    tubes4hifi
    Admin

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    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by tubes4hifi on Fri Dec 27, 2013 12:07 am

    you said "both Primary and Secondary are getting 320V"

    doesn't say AC or DC volts . . . .
    If you put 60vac on the primary you should have about 200vac between each red wire and the red/yellow wire.
    If you put good diodes and good capacitors after that, you should have about 200vDC after the diodes and caps, with 60vac input.
    Or at the usual 120vac input, about 400vAC between each red wire and the red/yellow wire. And then 400vDC after diodes and caps.

    you said "at the fuse and got 59V"
    AC or DC volts, and compared to what ??? ground? the other side of the AC input line??

    The two black wires are 120vAC input to the transformer.
    The two red wires are the HV secondaries, with the red/yellow wire as a center tap, which is tied to chassis ground.

    A little bit of info can be dangerous. Too much info can be confusing. Incomplete info is useless.
    The good news is it seems it MIGHT be working, and you haven't blow up the amp or blown up yourself !  Very Happy 

    fedoragent

    Posts : 35
    Join date : 2013-09-14

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by fedoragent on Fri Dec 27, 2013 4:15 am

    tubes4hifi wrote:you said "both Primary and Secondary are getting 320V"

    doesn't say AC or DC volts . . . .  

    --- I am indeed speaking of AC volts.

    If you put 60vac on the primary you should have about 200vac between each red wire and the red/yellow wire.
    If you put good diodes and good capacitors after that, you should have about 200vDC after the diodes and caps, with 60vac input.
    Or at the usual 120vac input, about 400vAC between each red wire and the red/yellow wire.   And then 400vDC after diodes and caps.

    --- This was just a test to see if the pt was dead or usable without being attached to the diodes/rest of the circuit. My plan is to replace the diodes, the resistors and quad cap. The. i am going to run readings and tests. I will use your info as reference. Thanks so much!

    you said "at the fuse and got 59V"  
    AC or DC volts, and compared to what ???  ground?  the other side of the AC input line??

    --- At the fuse meant 59V witht the negative side of the meter tied to ground.

    The two black wires are 120vAC input to the transformer.  
    The two red wires are the HV secondaries, with the red/yellow wire as a center tap, which is tied to chassis ground.

    A little bit of info can be dangerous.      Too much info can be confusing.   Incomplete info is useless.

    --- Apologies as I was moving fast to head to dinner. In the futue I will attempt to be as complete and verbose as possible.

    The good news is it seems it MIGHT be working, and you haven't blow up the amp or blown up yourself !  Very Happy 

    --- I hope you are right!

    Of course my answers are above.

    Thanks,

    Jon

    fedoragent

    Posts : 35
    Join date : 2013-09-14

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by fedoragent on Sat Dec 28, 2013 5:12 am

    Well gang, the amplifier is up and running with a whole new power section complete with quad cap, diodes, and resistors. Sounds great. I have one thing to knock out though and that is that the left channel on the 8 ohm tap doesnt work but the 16 does. The PCBs have to be replaced from a kit for the boards I am working on tomorrow. So that is a mystery. At least i kmow the PA774 is in good and working condition.

    Thanks,

    Jon

    tubes4hifi
    Admin

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    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by tubes4hifi on Mon Dec 30, 2013 11:51 am

    bad solder connection?

    fedoragent

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    Join date : 2013-09-14

    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by fedoragent on Mon Dec 30, 2013 12:51 pm

    tubes4hifi wrote:bad solder connection?

    That is EXACTLY what it was. This kit must have been soldered by the legally blind. I reworked all the joints and saw that the 8 ohm tap had the orange wire severed. I went ahead and stripped the wire again, solderd it, created my hook and created the joint. Problem solved and is working great.

    I do not know if I should be worried about usong my 4 ohm speakers on the 8 ohm tap though. It sounds great.

    Jon

    Donkeyshins

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    Re: ST-35 won't power on....I have a theory that I want to run by you guys....

    Post by Donkeyshins on Mon Dec 30, 2013 2:34 pm

    fedoragent wrote:
    tubes4hifi wrote:bad solder connection?

    That is EXACTLY what it was.  This kit must have been soldered by the legally blind.  I reworked all the joints and saw that the 8 ohm tap had the orange wire severed.  I went ahead and stripped the wire again, solderd it, created my hook and created the joint.  Problem solved and is working great.

    I do not know if I should be worried about usong my 4 ohm speakers on the 8 ohm tap though.  It sounds great.  

    Jon

    I always assume that whoever had an amplifier before I get my hands upon it was blind, drunk or insane and treat the circuit as such. New caps, new resistors, basically anything that isn't nailed down gets replaced. It has worked very well for me up to this point and should for the immediate future.

    -D

    (the exception would be if I got an amp from Dave Gillespie, but he doesn't sell his amps too often)

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