I'm into the weeds on a repair/upgrade of my ST-70 that's been in my system for 10 years. It's a Dynakit, lightly restored by the tube amp tech who sold it to me.
Here's the story:
1. Blown fuse. Replaced fuse, blew again. (3A slo-blo)
2. Replaced fuse, watched filaments start to come on, then arcing in rectifier tube and blown fuse.
3. Ordered replacement rectifier tube. Exact same result as (2)
4. Based on some internet guidance, checked resistance of quad cap leads to ground (in-situ). Found a couple basically shorted.
5. Ordered new upgrade quad cap from Tubes4HiFi. I sprang for a new VTA70 driver board while I was at it, since mine is original.
6. All parts installed. After double checking the connections, I decided to check the new quad cap in place, before applying power. Found two legs shorted to ground, just like the original. At this point I checked the resistance of the original cap, now isolated. No shorts! This is the original capacitance, but I'm wagering it was replaced - it looks new. I don't have capacitance meter to check for real.
7. Further checking showed that one of the choke leads was shorted to ground. I disconnected the choke wires from the quad cap leads, and checked to confirm. Removed from the chassis, one of the choke leads has continuity with the choke body, so this made contact with the chassis when installed. Choke resistance is around 62 ohms.
So I assume a new choke is in order (yes, there is brown goo on the bottom of the choke and the chassis bottom cover).
I'm now leery that something else may still be wrong, or might have caused the choke failure (it must be the original, though). Now that the hood is open, I'm willing to disconnect anything for a proper check. Obviously I was too lazy before. Any suggestions? Should I check the transformers? How would I do that?
Also, I have not ordered another replacement rectifier yet - hoping that one of the ones I have is still good, but they have arc'ed.
Thanks for any advice.
Jim
Here's the story:
1. Blown fuse. Replaced fuse, blew again. (3A slo-blo)
2. Replaced fuse, watched filaments start to come on, then arcing in rectifier tube and blown fuse.
3. Ordered replacement rectifier tube. Exact same result as (2)
4. Based on some internet guidance, checked resistance of quad cap leads to ground (in-situ). Found a couple basically shorted.
5. Ordered new upgrade quad cap from Tubes4HiFi. I sprang for a new VTA70 driver board while I was at it, since mine is original.
6. All parts installed. After double checking the connections, I decided to check the new quad cap in place, before applying power. Found two legs shorted to ground, just like the original. At this point I checked the resistance of the original cap, now isolated. No shorts! This is the original capacitance, but I'm wagering it was replaced - it looks new. I don't have capacitance meter to check for real.
7. Further checking showed that one of the choke leads was shorted to ground. I disconnected the choke wires from the quad cap leads, and checked to confirm. Removed from the chassis, one of the choke leads has continuity with the choke body, so this made contact with the chassis when installed. Choke resistance is around 62 ohms.
So I assume a new choke is in order (yes, there is brown goo on the bottom of the choke and the chassis bottom cover).
I'm now leery that something else may still be wrong, or might have caused the choke failure (it must be the original, though). Now that the hood is open, I'm willing to disconnect anything for a proper check. Obviously I was too lazy before. Any suggestions? Should I check the transformers? How would I do that?
Also, I have not ordered another replacement rectifier yet - hoping that one of the ones I have is still good, but they have arc'ed.
Thanks for any advice.
Jim