Hello all!
I've been working to get my Dad's old ST-70 back up and running, learning a lot on the way. I did the VTA board upgrade, added the pentode / triode modification, replaced the quad cap (twice now!), and added the upgraded input / output connectors. But was getting a good 60 Hz (likely) hum from both channels.
I cleaned up the wiring a bit, and then the hum came from one side only. But then I blew out a KT77 tube (my fault) and in the process took out the rectifier tube. I replaced both, but then started blowing fuses.
Current status: I turned on system this morning (after getting new 3A fuse) and blew out the fuse again. I started looking for shorts in my wiring, and found that all 4 quad cap connections were zero ohms to ground. I removed all connections to the quad cap, and measured resistance to ground, and saw the values gradually move up to >3 Mohms, just as I would expect (based on an older post on this forum from Bob).
I then checked each wire to ground, and found that there is an open circuit (ohms > meter maximum, >20Mohms) for all wires to quad cap except the left channel output transfer red wire (center tap), which reads 30 ohms. I suspect this is the problem, as the other output transformer's red wire to ground reads open circuit (>max resistance on ohm meter).
I read the old post (from 2009) about checking the transformer resistances to each lead, and both primary and secondaries look within stated range. But this was checking between transformer wires, nothing stated as to values with respect to ground. So between taps on the transformer, things look OK, just that the resistance to ground on the red center tap looks low.
Am I describing a failed output transformer?
Steve
I've been working to get my Dad's old ST-70 back up and running, learning a lot on the way. I did the VTA board upgrade, added the pentode / triode modification, replaced the quad cap (twice now!), and added the upgraded input / output connectors. But was getting a good 60 Hz (likely) hum from both channels.
I cleaned up the wiring a bit, and then the hum came from one side only. But then I blew out a KT77 tube (my fault) and in the process took out the rectifier tube. I replaced both, but then started blowing fuses.
Current status: I turned on system this morning (after getting new 3A fuse) and blew out the fuse again. I started looking for shorts in my wiring, and found that all 4 quad cap connections were zero ohms to ground. I removed all connections to the quad cap, and measured resistance to ground, and saw the values gradually move up to >3 Mohms, just as I would expect (based on an older post on this forum from Bob).
I then checked each wire to ground, and found that there is an open circuit (ohms > meter maximum, >20Mohms) for all wires to quad cap except the left channel output transfer red wire (center tap), which reads 30 ohms. I suspect this is the problem, as the other output transformer's red wire to ground reads open circuit (>max resistance on ohm meter).
I read the old post (from 2009) about checking the transformer resistances to each lead, and both primary and secondaries look within stated range. But this was checking between transformer wires, nothing stated as to values with respect to ground. So between taps on the transformer, things look OK, just that the resistance to ground on the red center tap looks low.
Am I describing a failed output transformer?
Steve
peterh
