I have 2 original Dynaco ST-70's.
1. I bought as a good working item along with a full Dynaco setup as my first real tube stereo. It already had the VTA kit on it, so is really the only Dynaco I know.
2. I bought a 2nd-hand stock ST-70 and I challenged myself to do the VTA mod on it. I can follow directions and solder, but am truly a novice when it comes to circuitry. Bob helped me out with a few outstanding questions and was super supportive--as it seems he always is.
After the upgrade on #2, I hooked these up to my system using them to power 2 sets of speakers--which I swapped out a bunch of different times until I found a system I liked. Over time (a matter of months), i noticed that #2 was starting to exhibit a loud 60 cycle hum. It wasn't consistent at first, but after a while made it unusable. At the exact same time, #1 started to have sporadic white / pink noise out of the left channel and a little quieter noise out of the right. Upon closer listen, I could also hear a 60-cycle hum in addition to that. The noise had gotten very loud and also made it unusable.
At this point, I needed help with troubleshooting. I have normally brought my gear to Deltronics locally (Suburbs location). They have always been very nice, but I find that I bring my gear back to them multiple times under warranty as the initial fix rarely was 100%. Vintage gear is finicky, so I get it. I realize that I want an expert to review these. I had noticed that tube4hifi offered repair service if I covered the shipping at a very reasonable bench rate. Although I knew that after shipping two of these items both ways + repair would likely mean that these items are no longer worth what I have into them, but I want to keep them and I want them to be perfect--because when they are functioning, I love the sound.
I talked with Roy and sent them over to him to troubleshoot > fix. He was super communicative and helpful. He noticed some obvious issues:
Most tubes need to be replaced (not surprising as I hadn't done this yet on either), some cold solder points and I had messed up part of the wiring on #2. He fixed them up in a week and shipped them back.
While I was waiting, I decided to bite the bullet and buy 2 new sets of matching tubes just to have everything pristine when they got back. (Mullard EL34, Tung-Sol GZ34 / 5AR4). I installed them, biased and connected to my rig.
Turned them on and instantly noticed that the hum and the noise was there on both items. Literally, nothing had changed! I sat on it as I was about to explode. After all the effort and $700 in repairs and tubes, I was listening to the same damaged amps. I replaced the speaker wire, removed the preamp - no change. I swapped out the power conditioner, brought them to a different room and a different set of speakers, no change. No input is connected at all, so it definitely is an issue with the amps or power. I am thinking less and less that the house's power or other environmental things are leading to the issue as they both have the same, yet different issues. And, I don't notice anything like this on my other gear.
I contacted Roy and he was also surprised--he actually recommended that I come here to look for help. The noise is loud and instantly obvious and he said that they were both 100% silent in his shop before they returned them to me. I trust that he is the expert and that all was tested before the return. I just have no idea what to do now.
Pic of Oscilloscope (iPhone) and vid of white noise below
Any ideas for obvious troubleshooting? What can you recommend?
I appreciate any expertise you can provide. (like I said, I am a novice w/ rudimentary skills + testing gear)