Are Original ST-70's known to short the OPT?
Posted: Fri Jul 31, 2009 1:58 am
Doing an ST-70 overhaul.
The owner of the amp said I could use his if I fixed it. He brought it home, fired it up, it warmed up and a white flash, no output tubes.
He figured as I did, the grid coupling caps were old and leaky.
So after much effort, the rebuild was ready to go tonight... so I thought.
Well, I found the problem why it blew up before... not capacitor...
flakey output transformers!!!!!
Last night, I had the supply done and HV tested it for 20 minutes. That
held OK.
Tonight I popped in the EL34's and biased up the left channel, great!
Biased up the right channel and as soon as the tubes began conducting,
*BZZZZRT!* and the fuse blew.
The good news, the tubes live... 50% of their screens are gone on that
pair, but they still live.
Bad news is I popped the right OPT apart and it's black with
carbon traces everywhere. Looks like she's been breaking down for a long
time. That iron looked practically new on the outside!
Other bad news is that means the left OPT is a good chance will do the
same eventually.
Is this common, or did I get the short straw?
Cheers!
The owner of the amp said I could use his if I fixed it. He brought it home, fired it up, it warmed up and a white flash, no output tubes.
He figured as I did, the grid coupling caps were old and leaky.
So after much effort, the rebuild was ready to go tonight... so I thought.
Well, I found the problem why it blew up before... not capacitor...
flakey output transformers!!!!!
Last night, I had the supply done and HV tested it for 20 minutes. That
held OK.
Tonight I popped in the EL34's and biased up the left channel, great!
Biased up the right channel and as soon as the tubes began conducting,
*BZZZZRT!* and the fuse blew.
The good news, the tubes live... 50% of their screens are gone on that
pair, but they still live.
Bad news is I popped the right OPT apart and it's black with
carbon traces everywhere. Looks like she's been breaking down for a long
time. That iron looked practically new on the outside!
Other bad news is that means the left OPT is a good chance will do the
same eventually.
Is this common, or did I get the short straw?
Cheers!