I am new to the hobby and am trying to restore a ST-70 as my first project. The amp had been stored in a basement for decades, showing signs pitting and mildew on internal wiring. The original owners plugged it in to a 120 VAC wall socket, blowing the fuse and damaging the rectifier (only one filament side lights). That's where I start.
So far I have replaced the can capacitor, bias supply caps, and swapped out the selenium (plus any resistors in those areas). I got new fuses and a new 5AR4 rectifier tube.
With all tubes removed, the dim bulb tester (DBT) remains dark when supplying 120 VAC from my variac. However the power transformer seems to be supplying higher voltage than it should. I measure 400 VAC from Red to Red/Yellow. My understanding is that this value should be closer to 350 VAC.
Installing only the rectifier doesn't change the above measurement, and the DC reads about 530 V.
Now when I install the power tubes (and 8 Ohm speaker load), the DBT lights up. I assume this means I have a short somewhere. And here is where I could use advice! What are some sensible next steps?
Does it sound like my power transformer might be bad?
Should I try to get my hands on another quad of EL 34s to see if it's a tube short?
Should I desolder and pull the output transformers to test there?
So far I have replaced the can capacitor, bias supply caps, and swapped out the selenium (plus any resistors in those areas). I got new fuses and a new 5AR4 rectifier tube.
With all tubes removed, the dim bulb tester (DBT) remains dark when supplying 120 VAC from my variac. However the power transformer seems to be supplying higher voltage than it should. I measure 400 VAC from Red to Red/Yellow. My understanding is that this value should be closer to 350 VAC.
Installing only the rectifier doesn't change the above measurement, and the DC reads about 530 V.
Now when I install the power tubes (and 8 Ohm speaker load), the DBT lights up. I assume this means I have a short somewhere. And here is where I could use advice! What are some sensible next steps?
Does it sound like my power transformer might be bad?
Should I try to get my hands on another quad of EL 34s to see if it's a tube short?
Should I desolder and pull the output transformers to test there?