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ST-70 fuses blowing .... ehhhh ....

J RR

J RR

New Member
Hey all!

I have an ST-70 I've been trying to restore for a bit now. Replaced the board, and got a new can cap in. Also redid the output connectors, and replaced the selenium with a diode.

Problem: fuses blowing. Before it kept blowing fuses, the power transformer would vibrate a whole lot. Also looked like the rectifier was a bit of a runaway, and I was able to test the amp, but it sorely lacked power.

I believe everything I put in should be fine, as long as I read the schematics right. So I'm a bit confused about the blowing fuses ... also, the fuses blow when no rectifier tube is in.

Any help in this would really rock ... because I'm tryin to kick it with this amp!
 

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You have to know something about electronics and know how to use a meter. It could be a lot of things. You can't just go and replace parts without knowing and checking the circuit first to find out which part is causing the amp to blow fuses.

Some things to check: Rectifier tube, new capacitors polarity, correct fuse rating, a shorted tube.

Read up on tube amp repair! Lots of info. :thumbsup:
 
Since the fuses blow with no rectifier, its nothing on the HV side. that leaves either bad power transformer, or a short in the heater supply as the most likely suspect area. Might be worth removing all of the tubes and trying again. If no change, unhook the 6.3v heater supply and try again. Basically the idea is to remove connections from the transformer until it either stops blowing fuses, or nothing is connected. At that point its either the last thing you unhooked, or the power transformer.
 
That makes sense ... bad transformer is my fear, and the one thing I would love not to hear, but one I've suspected as well.
 
Usually bad transformers have a unique stink of death about them, though since the Dyna has a fuse it may not have actually gotten to that point. A lot of gear (stupidly, IMO) did not come with fuses and would just cook until the transformer windings melted open or someone yanked the cord out of the wall. If you ever get one of those, there isn't much doubt your PT is fried. That and the puddle of tar running out the bottom is a pretty good clue.
 
petercapo said:
What value of fuse is blowing? You should be using a 3 amp fuse, slo-blo.
Click to expand...
Yup. I used that one. ... It blew after about 5 minutes, and the transformer vibrating pretty intensely. Another user messaged me, and I believe I might have goofed the bias direction when I put in the diode. ... probably destroying capacitors and resistors along the way.
 
Those parts are pretty tough. Reverse it and give it another go, unless you smelled smoke????
 
Bias diode looks like its stripe to the transformer, which is correct. Also looks like the caps are + to chassis ground, which is correct.
 
gadget73 said:
Bias diode looks like its stripe to the transformer, which is correct. Also looks like the caps are + to chassis ground, which is correct.
Click to expand...
Correct. But I'm supposed to be getting higher resistance measurements, measuring both directions, correct? I only get resistance measurements going one way (red lead before stripe, common after).

I snipped the wire from the diode to the PT, and turned it on -- no more shorting. All tubes seemed to function normally (except the rectifier tube obviously), and the PT was silent as can be.

Because of that and the diode measurements, I assumed the bias circuit should probably just be re-created, no? .... as an aside, I did measure each of the bias resistors at 6k ohm, and I believe they're suppose to be 10k.
 
have to use the diode check function if its a digital meter. They won't usually read diodes properly in ohm mode. It should read about 0.6 with the black lead on the stripe end and open circuit going the other way.

Diodes and caps are cheap, change them if there is a question. If any of that fails it will kill the output tubes.
 
A
gadget73 said:
have to use the diode check function if its a digital meter. They won't usually read diodes properly in ohm mode. It should read about 0.6 with the black lead on the stripe end and open circuit going the other way.

Diodes and caps are cheap, change them if there is a question. If any of that fails it will kill the output tubes.
Click to expand...
Ahh. And indeed there is a diode tester (digital meter) ... and indeed it displays around .6 (.58). So it appears the diode is solid ... I'm going to get new resistors and capacitors (and new mounting terminals) ... perhaps this will finally restore this beast. Unless anyone else on this ridiculously helpful forum has any other suggestions.

I believe the new can cap is all put together as it should as well. Resistors test out fine also. New board should be plugging away ... this is one of those projects that has gone further than it should have. Good to know my PT is good at least. Or at least it appears functioning as it should.
 
century tek said:
Are you checking your bias properly? Have you checked the rectifier tube also?
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I couldn't get voltage out of the bias point, and the unit wouldn't stay on long enough without blowing a fuse to bias properly.

I did see a spark in the rectifier tube when it was on ... perhaps it's the rectifier tube that needs replacing, but that still doesn't explain why the fuse blows without tubes in.

Since the unit operates fine with the wire clipped from the diode to the PT, I'm thinking it's the bias components (capacitors and resistors) ... although they look like they're installed correctly, I'll still root the older components out and rebuild.

I replaced the quad cap, so that should be fine ... and the circuit board is new as well ... I can't think of what else would cause blown fuses like that ... perhaps an open somewhere.
 
Check the output tubes. One could have a heater to cathode short.

Your rectifier tube should never flash.

So are you saying that with absolutely no tubes installed that the fuse still blows?

You really shouldn't be shot gunning this. You need to use a meter so you can see where the excess current draw is coming from. You can't just keep guessing and hope you will hit the bad part. This method never ends well. This is a good amplifier.
 
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century tek said:
Check the output tubes. One could have a heater to cathode short.

Your rectifier tube should never flash.

So are you saying that with absolutely no tubes installed that the fuse still blows?

You really shouldn't be shot gunning this.
Click to expand...
Correct, but only if the diode-to-PT wire is hooked up.

I snipped the wire, and it operates normally. Including the PT working normally, and without any buzzing. Which is why I'm thinking it's something in the bias circuit, which I ordered the components to rebuild.
 
The diode can easily be checked for shorts with a meter.

I was looking at your pic with the diode but did you replace it at one time? Are you sure it is in correctly? Since it is part of the biasing for the output tubes, it should have negative voltage coming from it and should be connected backwards compared to a regular positive power supply scheme.

Should mention that you should probably find a variac and DBT before going any further. It will save you on fuses.

EDIT: I see that that diode looks to be wired in the right direction.
 
century tek said:
The diode can easily be checked for shorts with a meter.

I was looking at your pic with the diode but did you replace it at one time? Are you sure it is in correctly? Since it is part of the biasing for the output tubes, it should have negative voltage coming from it and should be connected backwards compared to a regular positive power supply scheme.

Should mention that you should probably find a variac and DBT before going any further. It will save you on fuses.

EDIT: I see that that diode looks to be wired in the right direction.
Click to expand...
Indeed; I replaced the old selenium.

Everything in the circuit appears to be properly wired, which is why I'm stumped. .... The 2 resistors in the bias circuit measured at 6k, rather than 10k, and that's about the only oddity I can find. I'll probably rebuild the bias components, and if that fails, off to a far more knowledgeable tech. ... if I can find one I suppose.
 
See if something is touching chassis where it shouldnt be. Honestly I'm surprised its able to flow enough current to blow the fuse at all, and there is no way its doing it through one of the drop resistors. Whatever the short is, it must be either directly at the diode, or immediately after it. Thats basically the diode, the first cap, and the wire between them as sensible trouble spots.
 
I was JUST going to suggest looking at the insulation. Check the chassis for any burn marks
 
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