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    My ST-70 continues to blow fuses.

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    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    My ST-70 continues to blow fuses.

    Post by wazowski on Tue Dec 28, 2010 11:37 am

    I have a older ST-70 I acquired 3 years ago. It appeared to have had some work done (updated caps on the driver board and the terminal strip).

    The unit worked fine until very recently, when it started to blow fuses just a few seconds after it was powered up.

    I followed the troubleshooting guide on page 9 of the Dynakit owner's and installation guide, as well as Curcio Audio's troubleshooting guide, and determined the cause was not the 5AR4 rectifier tube. (If you remove the rectifier tube, the fuse will not blow on power up).

    The quad cap can it where I moved next. The can was old and did not provide readings within proper spec when checked with my DVM, so I ordered a new 80/40/30/20 can from Dynakitparts.

    I installed the can as the schematics provided illustrated but neglected to test the can prior to installation.
    When I powered the unit up, It again blew the fuse.

    The resistors (22,000 and 6,800) on the cap can both check out within spec.

    All connections to the can appear solid and I could find no shorts. Readings at the terminals on the new can are not within spec. Two terminals read low OHMS, the remaining two read zero. I may have overheated the unit when soldering, or the unit may have been bad out of the box. Not sure, as I did not check it prior to installation.

    I have performed Curcio's test for the main power tranny and it passed the short test.

    I checked voltages across all points listed in Curcio's Transformer troubleshooting addendum. Voltages are all within spec.

    I am in the process now of tracing all wiring and comparing it against the schemo and the wiring diagram.

    I am also planning on checking the choke in the process.

    Does anyone have any other ideas or thoughts? I am new at this, have very little experience, but I really want to learn.


    Thanks.

    Bob Latino
    Admin

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    Re: My ST-70 continues to blow fuses.

    Post by Bob Latino on Tue Dec 28, 2010 12:29 pm

    Hi,

    TRY A NEW RECTIFIER TUBE in your amp. With no rectifier tube there is no DC high voltage. You replaced the old quad cap which is usually the cause of many older ST-70's blowing a fuse after a few seconds. About the only thing left to cause the symptoms that you mentioned. (fuse blowing after a few seconds) is a bad rectifier tube. Even though a rectifier tube lights up that does not mean it is good. Locally sometimes you can find a new GZ34/5AR4 at a "guitar store" in your area. Many guitar amps also use a GZ34/5AR4 rectifier tube.

    Bob

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Tue Dec 28, 2010 12:54 pm

    Thanks, Bob.

    I neglected to mention in my original post that I have replaced that rectifier tube...twice. The first was with a JJ, the next was with a MESA/Boogie from a local Guitar Center. No luck with either, the fuses just keep a blowin'.

    I have an old Seeburg jukebox from around the same era as the ST-70 that uses a 5AU rectifier. I read that these can be interchanged in a pinch. I popped all 3 of these into the jukebox in succession and each one worked just fine.

    It looked to me at that point that the rectifier tubes are all good.

    Bob Latino
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    Re: My ST-70 continues to blow fuses.

    Post by Bob Latino on Tue Dec 28, 2010 1:47 pm

    If you have replaced the rectifier tube and the quad cap then probably you have a bad coupling cap or some other issue on the driver board.

    See if ONE channel will NOT blow the fuse. Remove the output tubes from one channel and see if the amp will stay on running just one channel. Try both channels ALONE. If the amp will stay on with just one channel running alone then almost assuredly the problem is on the driver board.

    Bob

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Tue Dec 28, 2010 2:11 pm

    Thanks again, Bob.

    I did try this and, unfortunately the unit continues to blow fuses.

    All caps including the 4 on the driver board as well as the 2 on the terminal strip on the underside test within normal spec. They are all newer caps.

    Do you think the Choke might be the culprit?. If not, could there be a short somewhere in the driver board? How do you isolate the issue at this point?

    Mike.

    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: My ST-70 continues to blow fuses.

    Post by Sal on Tue Dec 28, 2010 2:53 pm

    Please post pictures of the top and bottom of your ST-70 if you can.

    Thanks,
    Sal

    wazowski

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    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Tue Dec 28, 2010 3:53 pm

    Sorry Sal, but I can't figure out how to post photos here. Had no problem posting on other forums. Maybe you could gimme a hint?

    Bob Latino
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    Re: My ST-70 continues to blow fuses.

    Post by Bob Latino on Tue Dec 28, 2010 3:57 pm

    Hi,

    You have to host the photos on another web site like Photobucket. This forum, although it is supposed to, cannot host a photo. It just doesn't work?

    If you have an ORIGINAL Dynaco driver board then you should probably replace the driver board and that might solve your problems. The original Dynaco driver boards were not made to last 40 - 50 years. Just replacing capacitors and resistors on the board IMHO is not enough on an amp that you are going to play on a daily basis. Everyone knows that I favor the upgraded VTA driver board for the ST-70 but if you want to keep the original Dynaco 7199 driver circuit then buy a new driver board and parts set from Kevin at Dynakitparts and replace the whole driver circuit.

    Bob

    wazowski

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    Join date : 2010-12-28
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    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Tue Dec 28, 2010 4:51 pm

    Thanks Bob. Talked to Kevin earlier today. Ordered a new quad cap can. Once I eliminate the can all together and finish checking all the wiring, I will order the driver board. I appreciate your help. Jut out of curiosity, how do you load a photo as your moniker icon?

    Bob Latino
    Admin

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    Re: My ST-70 continues to blow fuses.

    Post by Bob Latino on Tue Dec 28, 2010 5:00 pm

    As to the moniker icon > Go into your own personal profile and load an "Avatar" photo from your computer. This photo has to be small - something like 150 X 100 pixels so you will probably have to shrink the photo to the proper size in photo software like Photoshop or some other photo manipulation software.

    Bob

    Sal

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    Location : Central New Jersey Dynaco-ST70.com

    Re: My ST-70 continues to blow fuses.

    Post by Sal on Tue Dec 28, 2010 10:36 pm

    wazowski wrote:Sorry Sal, but I can't figure out how to post photos here. Had no problem posting on other forums. Maybe you could gimme a hint?

    Email me the pictures you have at salb203 at aol dot com, I'll host them and post them here.

    Sal

    Sal

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    Re: My ST-70 continues to blow fuses.

    Post by Sal on Wed Dec 29, 2010 12:58 am

    Here are the requested pictures.



    Sal

    Posts : 223
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    Re: My ST-70 continues to blow fuses.

    Post by Sal on Wed Dec 29, 2010 1:06 am

    I just noticed you have a wire connected to pin 3 of the 5AR4 tube, There should be wires on Pin 2,4,6 and 8.

    Pin 4 and 6 are the red high voltage leads from the power transformer and pin 2 and 8 are the 5 volt filament leads.

    I would also get that bias selenium rectifier electrically disconnected from the diode.

    Can you get a close up of the 5AR4 tube socket wiring and send it to me. Are you sure the original tube was a 5AR4 tube? I quickly checked my tube manual to see if there is a rectifier tube to correspond to the wiring.

    Sal

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Wed Dec 29, 2010 7:11 am

    Sal,

    You are correct, A red lead from the main transformer WAS connected to the #3 pin of the 5AR4 socket. I found this after installing the new quad cap can without correcting the problem. I relocated this wire to the #4 lead as the wiring diagram indicates, and still the amp blows fuses.

    As for the 5AR4 tube; I don't know if the tube the amp was supplied with is a 5AR4 for sure. The only markings on the tube that are visible are small and right along the bottom edge: "84H2" There are also small numbers just above those: The first could be a t or f, followed by 32.

    I will e mail photos of the tube and socket to you.

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Wed Dec 29, 2010 8:18 am

    Bob Latino wrote:Hi,

    You have to host the photos on another web site like Photobucket. This forum, although it is supposed to, cannot host a photo. It just doesn't work?

    If you have an ORIGINAL Dynaco driver board then you should probably replace the driver board and that might solve your problems. The original Dynaco driver boards were not made to last 40 - 50 years. Just replacing capacitors and resistors on the board IMHO is not enough on an amp that you are going to play on a daily basis. Everyone knows that I favor the upgraded VTA driver board for the ST-70 but if you want to keep the original Dynaco 7199 driver circuit then buy a new driver board and parts set from Kevin at Dynakitparts and replace the whole driver circuit.

    Bob
    Bob, If I post the photos on Flickr, how do I get them to show up on this forum?

    Bob Latino
    Admin

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    Re: My ST-70 continues to blow fuses.

    Post by Bob Latino on Wed Dec 29, 2010 8:23 am

    After you hit the "Post reply" button there is an icon at the top of the box that is an "Image" button. When you hit that button a box comes up and you just place the web address of your stored photo in the box. The photo will then show up in your post.

    Bob

    wazowski

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    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Wed Dec 29, 2010 10:56 pm

    ST 70 Underside view" alt="" />
    Bias close up
    5AR4 socket
    7 terminal strip 1
    underside
    rectifier tube numbers
    Rectifier tube

    GP49

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    Re: My ST-70 continues to blow fuses.

    Post by GP49 on Thu Dec 30, 2010 3:55 am

    You write:

    "All connections to the can appear solid and I could find no shorts. Readings at the terminals on the new can are not within spec. Two terminals read low OHMS, the remaining two read zero."

    If a terminal of the quad cap to ground reads zero ohms, it's wrong. Disconnect the wiring to those two and then check those two terminals on the quad cap without the wires on them. If they now measure in the "normal" range, then check EACH of the two wires you disconnected, measuring each one to ground. If you then find one of them measuring zero ohms to ground, that one has a short circuit to ground for some reason which you will then have to trace.

    Bob Latino
    Admin

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    Re: My ST-70 continues to blow fuses.

    Post by Bob Latino on Thu Dec 30, 2010 8:48 am

    Going along with what I said earlier in post # 8 on this thread and also what GP49 has said in the post above, the problem almost has to be ON THE DRIVER BOARD. The two wires from the quad cap that connect to eyelets 19 and 20 on the driver board supply B+ high voltage to the driver board. Try this > Disconnect the two wires at eyelets 19 and 20 and insulate the ends of the two bare wires. Turn the amp on. Of course the amp won't play with no B+ high voltage to the driver board but see if the amp now WON"T BLOW THE FUSE. If the amp now doesn't blow the fuse then you will have confirmed that the problem IS ON THE DRIVER BOARD. Rather than "dope out" where the problem is, the best solution is, as I said in post #8, > REPLACE THE DRIVER BOARD and all parts on the driver board. Even if you do choose to try and find the problem on the original driver board and do find it, IMHO, it's only a matter of time before something else goes wrong and you have to go in there and dope out some other problem.

    On one of the photos of your amp you show a close up of the two bias pots. The number "1376026" stamped on there means that the bias pot was built during the 26th week of 1960. Since Dynaco used parts quickly, and the ST-70 came out in 1959, you have one of the older ST-70's made. Your amp was made sometime (probably) in the second half of 1960. Your amp is 50 years old and parts don't last forever. A driver board replacement is in order here and more than likely the replacement will solve the fuse blowing issue you are having with your amp.

    Bob

    dynacophil

    Posts : 33
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    Re: My ST-70 continues to blow fuses.

    Post by dynacophil on Thu Dec 30, 2010 9:23 am

    Eylet 19 solderjoint looks to me suspect... craterlike

    as well this questionable soldering:


    wth shall this be...Krautsalad?


    Last edited by dynacophil on Thu Dec 30, 2010 10:04 am; edited 2 times in total

    kevco

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    Location : Aldan, PA, USA

    Re: My ST-70 continues to blow fuses.

    Post by kevco on Thu Dec 30, 2010 9:45 am

    Your rectifier is in fact a 5ar4 or gz34, made at the Blackburn Works by Mullard, looks to be August of 1964 manufacturing codes. That's the same tube that shipped with the Dynaco kits. 5ar4's don't get much better than that, but none of this tells you if it's still a fully functioning tube...

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Thu Dec 30, 2010 6:14 pm

    Yeah, that soldering joint does look pretty bad! Actually, that is the red/black lead from the transformer to the Selenium rectifier and I de soldered it in order to run some tests as suggested in Curcio's ST-70 transformer troubleshooting addendum. Also disconnected are the choke leads, as I tested them as well. (58 ohms across the leads). I apologize, as I should have noted that. Eyelet 19 does look a bit shaky, but seems to be solidly attached.

    As for the Mullard 5AR4 rectifier tube; I tested this tube in the amplifier of my 1962 Seeburg juke box. It operated just fine, as did my JJ GZ34, and My Mesa/Boogie GZ 34, which I purchased as replacements for the old Mullard. Thanks for identifying it. Unfortunately, I do not own a working tube tester, so I cannot check to see if it is weak.

    I have taken your advice and ordered two new bias pots, a new driver board (factory assembled), a new cap can, and a new bias section from Kevin at Dynakitparts.

    I will let you all know how I make out. Thanks much for all of your help. Have a very Happy New Year.

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Thu Dec 30, 2010 6:18 pm

    What do you all think about what appears to be a diode across the (+) and (-) terminals of the selenium rectifier? I have been through the original wiring instructions and diagrams and can find no reference to it. Do you think this might have something to do with the red lead from the transformer originally connected to the the #3 pin on the rectifier socket instead of the #4 pin, as specified in the original assembly instructions?

    GP49

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    Re: My ST-70 continues to blow fuses.

    Post by GP49 on Thu Dec 30, 2010 6:26 pm

    wazowski wrote:What do you all think about what appears to be a diode across the (+) and (-) terminals of the selenium rectifier? I have been through the original wiring instructions and diagrams and can find no reference to it. Do you think this might have something to do with the red lead from the transformer originally connected to the the #3 pin on the rectifier socket instead of the #4 pin, as specified in the original assembly instructions?


    I think nothing of it. The usual failure mode of a selenium rectifier is high-resistance, or open. if it failed open, soldering a silicon diode across it to replace it won't cause fuse-blowing unless it was done REVERSE POLARITY or if the new diode was damaged and failed (the usual failure mode of a silicon diode is SHORTED). Given how sloppy the soldering is in that Stereo 70, I wouldn't put that beyond possibility.

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: My ST-70 continues to blow fuses.

    Post by wazowski on Tue Jan 11, 2011 9:58 am

    I replaced the driver board with one from Dynakitparts along with the selenium rectifier. The 80/40/30/20 quad cap can, once out of the circuit, tested out fine, so I put it back in. Once all of this was completed, the unit worked fine.

    I went ahead then and replaced the 7 lug strip with new, including caps and resistors, along with new pre-amp input jacks, speaker terminals, power cord and fuse holder.

    The amp seems to be working fine, with all of my old tubes, including the original Mullard 5AR4.

    Bias voltage seems to be stable, unless playing at higher volume levels, then the right channel works its way upward, but the left holds steady. This is something this unit has always done, though.

    I have a PAT-4 and a PAT-4A that need some work. Any idea where I can get parts like input terminal assemblies, line cord connections, pots and switches?

    Thanks to Bob, Sal and everyone out there who helped.

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