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Dynaco ST70 power supply transformer short problem - HELP!!

Jcricket

Jcricket

not someday, but today
Subscriber
Hey Folks,
I have started working on the old dynaco st70's. I am on my 6th amp that I am overhauling. I have run into a very interesting problem. The rectifier that was in the amp had one bad side. I overhauled everything in the amp and it sounded good. It was drawing 165 watts, a little bit higher current than I normally see. Usually it is around 145 watts. I put in a a new rectifier, 5ar4, and it lasted for several days. Then it went weird and sounded terrible. I checked the new rectifier, same side was bad as the one I removed.

After several hours of studying the schematics, my opinion is the power supply transformer is the problem. Another piece of information, when this amp came to me, the choke had been smoked due to high current flowing through it. I assumed it was one of the main electrolytic caps had short circuited. It appears one of the the rectifier pins, 4 or 6, has opened.

Here is a question. If either of those pins(the anodes) were to be shorted to ground, would they act like a cathode and receive the current same as the other cathode? I did check and the heating element/cathode on pins 2 and 8 have continuity, so that part of the tube is fine.

Okay folks, any suggestions or thoughts would be very helpful.
Thanks in advance.
 
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Jcricket said:
Hey Folks,
I have started working on the old dynaco st70's. I am on my 6th amp that I am overhauling. I have run into a very interesting problem. The rectifier that was in the amp had one bad side. I overhauled everything in the amp and it sounded good. It was drawing 165 watts, a little bit higher current than I normally see. Usually it is around 145 watts. I put in a a new rectifier, 5ar4, and it lasted for several days. Then it went weird and sounded terrible. I checked the new rectifier, same side was bad as the one I removed.

After several hours of studying the schematics, my opinion is the power supply transformer is the problem. Another piece of information, when this amp came to me, the choke had been smoked due to high current flowing through it. I assumed it was one of the main electrolytic caps had short circuited. It appears one of the the rectifier pins, 4 or 6, has opened.

Here is a question. If either of those pins(the anodes) were to be shorted to ground, would they act like a cathode and receive the current same as the other cathode? I did check and the heating element/cathode on pins 2 and 8 have continuity, so that part of the tube is fine.

Okay folks, any suggestions or thoughts would be very helpful.
Thanks in advance.
Click to expand...
Does it consume B+ even with tubes removed ? That could be the can cap that is leaking.
The choke dropping insulation goo is normal and a function of heat. The choke is most likely ok.
Check the transformer B+ winding with rectifier removed, it should have equal contact with ground, and when powered on it should have AC 360Volt on each side ( maybe a little more as it's unloaded) but equal.

Examine / replace the octal socket for the rectifier. Replace the can cap (if not already done).
Get a new 5AR4
 
ohm check the HV winding. If one side is open, the rectifier works half-wave which will cause low voltage, a lot of abuse on the working rectifier plate, and high ripple current which may be enough to cook the first cap and damage the choke. A shorted cap will cause similar abuse.

I doubt its a shorted transformer, you'd have smoke coming out of it and the current draw would be way higher than 20 watts extra.

which pin on the rectifier socket is the connection to the first filter cap on? Should be pin 8. It will work for a while on pin 2 but the DC current ends up flowing through the heater and that doesn't do it any favors.
 
thanks for the feedback guys!!!

First, the original choke is charcoal, it is fried crisp. When the amp came to me, it already had a new choke and new can cap installed.

The filter cap is connected to pin 8, as it should be.

I just did some checking. The 5v filament winding measures 0 ohms as it should. The high voltage winding from leg to leg measures 0 ohms as it should. The high voltage winding measuring from one leg to ground is about 3-15 Mohms - ???????????????????
This should be close to zero as well, correct?
 
Jcricket said:
thanks for the feedback guys!!!

First, the original choke is charcoal, it is fried crisp. When the amp came to me, it already had a new choke and new can cap installed.

The filter cap is connected to pin 8, as it should be.

I just did some checking. The 5v filament winding measures 0 ohms as it should. The high voltage winding from leg to leg measures 0 ohms as it should. The high voltage winding measuring from one leg to ground is about 3-15 Mohms - ???????????????????
This should be close to zero as well, correct?
Click to expand...
Yes both HV windings should have about the same resistance to center tap ( which should be grounded in the amp)
 
Sounds like you may be running half wave rectification on that amp with one side of the HV gone...
 
Okay so with the rectifier pulled, use your meter set at the highest AC voltage setting and measure both high voltage legs. Should be about 5 hundred or more some odd volts. If nothing from center tap (chassis ground) to each leg then it sounds like the center tap is open.

If it were shorted, you would have a hot or smoking power transformer.
 
Thanks folks. I haven't checked the total b+ from leg to leg yet. The st-70 use a CLCRCRC power supply configuration. The first cap is right off the rectifier, 20uf. Next is the choke, then the second cap, 20uf.The primary b+ for the OPTs comes off the second cap.
Her is what I think happened. The second cap developed a short to ground(no longer a cap). It caused a high current flow from the primary b+ through the choke. The ground and the center tap are connected. When the short occurred in the cap, the resulting high current through the choke cooked it. This also resulted in a very high current across the center tap. It resulted in a damaged winding, almost opening the center tap.

Here is where my understanding is a little weak and I am guessing. If the center tap is mostly opened(the 3mg reading) or opened, then the voltage on the cathodes might behave more like two connectiont on a resistor, The current would flow across them in ac until one anode connection burns up. This is my speculation of what is happening.

Thoughts?
thanks again
Mark
 
at 3M its close enough to open to not matter. It won't carry any current worth talking about. At that point nothing can go bad because no current can flow. If you got sound out of it, its not open to the center tap on at least one side.

If one side was good to the center tap but not the other though, I can see that causing trouble. You'd have noticeably low plate voltage and the tubes wouldn't bias correctly though.
 
also sometimes digital meters get funny when reading through a coil, so if you have an analog meter it might be useful as a sanity check. Those mostly don't care about inductance.
 
gadget73 said:
also sometimes digital meters get funny when reading through a coil, so if you have an analog meter it might be useful as a sanity check. Those mostly don't care about inductance.
Click to expand...

I find that transformers can confuse auto-ranging multimeters. When this happens, I usually get either a ridiculously high reading, or the meter keeps "bouncing" (it goes back-and-forth between ranges), and never locks in on a reading.

If you set the range manually, to say 1000 ohms or so- usually, they will give a cogent reading.

Regards,
Gordon.
 
Okay, I'll do some measurements and get back to you folks.
Question, what would cause a rectifier to lose one side. This amp has eaten two 5ar4's(both new) in just a short time(hours). It is the same side that gets blown. Hmm............................
 
Jcricket said:
Okay, I'll do some measurements and get back to you folks.
Question, what would cause a rectifier to lose one side. This amp has eaten two 5ar4's(both new) in just a short time(hours). It is the same side that gets blown. Hmm............................
Click to expand...
The blown side carrying the major part of the current load. Bad xformer, bad socket connection, ect.
 
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After completely isolating the PS transformer from the circuit, I can not find a problem with it. I'll see if I can get the schematics uploaded.
Is it possible that the rectifier, sovtek, is just a poor quality and very susceptible to current surges? I am starting to think maybe turning it on when it is already hot? I am running in the dark here and grasping for straws so to speak.
Mark
 
IMO the Dynaco demands a lot from the rectifier. I would recommend a Mullard here instead of a Sovtek -- IIRC, I had a Sovtek short out in this application.
 
so the transformer ohms out OK when unhooked? thats a good sign

or just skip the rectifier tube entirely and go solid state with a drop resistor. These do ask too much of the rectifier tube and I'm way too cheap to want to spend 100 bucks on an increasingly rare NOS tube just to satisfy it.
 
Jcricket said:
After completely isolating the PS transformer from the circuit, I can not find a problem with it. I'll see if I can get the schematics uploaded.
Is it possible that the rectifier, sovtek, is just a poor quality and very susceptible to current surges? I am starting to think maybe turning it on when it is already hot? I am running in the dark here and grasping for straws so to speak.
Mark
Click to expand...
In post #4 you had one HV at 3Mohm . Could he problem be that the transformer connections was bad ?
Whan transformer mounted you can easily check by removing the rectifier tube and measure AC between ground
and pin 4 , should be just under 400Volt, same with pin 6 and ground. Also check the bias winding, it should give app 70Volt.
You don't need to post schematics ( if this is a dynaco st-70) , we all have copies.
As for "turning on when hot" yes this is a nono för any tubeamp, especially those with tube rectifier. Just dont!
 
I still suspect a possible socket problem, with a bad\no contact to one rectifier anode.
If it is a Dynakit, they came with two screw & nut fixed front accessory sockets that can serve as replacement swaps for defective/ charred chassis tube sockets.
 
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