ST-35 fuse blown

Hi new here!

  St-35 fuse blown as soon as I plug it in. bad hum from transformer. Any thoughts?

Scott

.

Transformer hums for split sec then blows fuse

Does not sound good at all. One thing to check for is the DC winding resistance of the power transformer secondary. Do not p[lug it in to do this test. These would be the red wires from the transformer that connect to the diodes. Try measuring resistance from red lead to red lead. Then from each red lead to ground. Post the results here.

Depending on the results you may have a shorted winding and a bad transformer. If the results are good then we can help you isolate a problem downstream from there.

.

.

.

PeterCapo - You got me thinking so I pulled my ST35 and set up a test. I jumpered the power supply first section electrolytic cap to ground. Then measured the DCR of red to red and each red to ground. Everything tests as I thought it would. Around 170 ohms overall and close to half for each red/ground. For the bigger amps I always pull the rectifier tube but it is a pain with the diodes. No issues measuring it the same way. Please confirm it yourself and check my test .

.

My power transformer is an original Dynaco. The difference in voltage is reasonable then. As long as the full AC voltages create the proper DC voltages in the amp then all is good with either transformer.

If Scott can give us some info on his transformer readings I think we can help him out.

Pictures

Cant seem to get pictures to upload!!!

.

Step one :
- disconnect the B+ rectifier. If fuse does not blow the problem is in the B+ chain. I'd start
with the can cap.
-if fuse still blows , remove tubes
- if fuse blows with rectifier disconnected and tubes removed the mains transformer or any
of the leads to it is broken.

I checked transformer high voltage taps 172 ohms and center tap is 84 ohms and 89 ohms. I disconnected the the tubes still blows fuse. disconnected diodes and removed tubes fuse held stable. High voltage 679V center tap 337V. Tube heater voltage 6.6V diodes bad!!!

I think I got it! thanks for all you help! Has anyone on here tried to us this as a headphone amp ? And if what was the result ?

Thanks for your help

Scott

soundz915 wrote:I think I got it! thanks for all you help! Has anyone on here tried to us this as a headphone amp ? And if what was the result ?

Thanks for your help

Scott

For safety reasons i would not recommend st-35 as headphone amp. The sheer power
can easily cause irreparable hearing damage.
Use a dedicated amp with limited power. It's safe and cheaper.

Your transformer readings are good. It appears your problem is after the diodes. Remove the tubes and see if it is the power supply capacitor.

Are you proficient with the soldering iron and removing the feed to the power supply capacitor? If you can isolate the problem to the can then that is a simple replacement from Dynakitparts.com or from EBAY. Make sure you get the correct ratings/size so it fits under the cover and has the correct ratings,

I agree with PeterH. These amps work best with regular speaker loads.

What is this "Tube heater voltage 6.6V diodes bad!!!" The heaters are AC not DC so no diodes. Not sure what you mean.

If you are referring to the high voltage diodes being bad - I doubt it.

I checked all voltages on secondary windings of power transformer diode and tube. those were the voltages I got. I then checked the diodes. #1 got 9 ohms with meter pole reversed. #2 got open with meter poles reversed. Just to be clear I made test flipping + and - on both diodes. one shorted one open.

Interesting find. Not usual to have one bad diode but it sounds like you tracked it down. My concern would be what made the diode fail? Do you have an ESR meter and a capacitance meter to check the can cap? Highly recommended before you go much further.

For my ST35 I replaced the can cap anyway. Along with all the other caps. Tested all the resistors and replaced anything out of spec. Old tubes worked fine - for me.

Your getting there. Once fixed you will be pleased with the results.