HI everyone, I am so glad to find this thread. My father (87) bought a SR 17-17 years ago and has had it in a climate controlled storage room. He gave it to me recently and I wanted to practice on it. I am teaching myself to repair vintage audio equipment. Still learning to read schematics but wouldn't feel comfortable without some kind of instructions.
I was warming it up on the Variac. I was doing it slowly and watching it over a day and a half. When I turned it up from 110v to 120 volts the 2A slo-blo fuse blew with a pop and then I heard a spewing sound; smoke came from the same corner as the single GZ-34 tube, but from inside the chassis. I had not removed the bottom yet, so I don't know the exact source of the smoke. I did quickly touch the transformers, because I have had to replace those on a Heathkit W-A5 and a Dynakit ST70. I didn't want to have to source another transformer. But, they were all cool to the touch. As they had been during the entire warm up on the Variac.
I started looking for a schematic on 2-3 websites, and couldn't find a company website.
I started pulling the tubes and checking them. The GZ34 was a Zenith and it was bad. I replaced it with a new Mullard, the only one I had.
The four EL84/7189 tubes were all good and marked "Made in Holland" but the mfg name was worn off, so what are they?
Then I tested to two 6AN8 tubes and they were both good RCA tubes.
All tubes so far had been stenciled on the chassis.
Then I started testing what I thought would be four 12AX7/ECC83 tubes as per the markings just at the base of each tube. These four tubes all have insulators on them. The two on the back right were good although I could not read the manufacturers name. The two on the back left were marked 'Made in Holland' and pegged the needle on the tube tester to the far right. Either the tubes were really excellent, or something was wrong. I looked again at the tubes and they
both were 12AT7 and not 12AX7 tubes. I looked it up and those tubes are not supposed to be interchangeable. However, in front of the two left sockets, in bold lettering it says "OR 12AT7". Meaning the 12AX7 tubes in those sockets could be replaced with 12AT7 tubes. I tested the 12AT7 tubes from the two left sockets to the 12AT7 specs and they tested good. I marked two boxes correctly and put the 12AT7 tubes in them and put them away.
So, I thought about it for a while and decided the 12AT7 marking was wrong, and the 12AX7 marking was right. I didn't have a schematic to check. It just seemed logical. I replaced the two 12AT7 tubes with good 12AX7 tubes and then used a black marker to cross out the printing that said "OR 12ATZ ".
Someone on another tread on AK told me when they wrote my initial story after this happened, that the smoke usually indicated the Canned Caps were probably blown near the power transformer, and maybe a cap that connects just outside of it. I guess my next task would be to pull that can, I don't see any stenciling on it like the other two cans have.
I must admit I am a little lost at this point. I have never rebuilt a can before. And, I am not sure what I am doing and where to go next. That mis-marking of two of the 12AX7 tubes has me a little suspicious of what I see.
Question- Could the 12ATZ tubes in two sockets where 12AX7 tubes should have been, have caused the GZ34 tube and the fuse to blow? Would it have likely have caused the caps in the can on the far left to have been damaged? And, smoke? What else should I check?
And, should I replace all capacitors, and resisters?
I can take photos of the bottom side now and post them. I took the bottom plate off and I don't see anything that looks burnt, discolored or out of the ordinary. I would really appreciate some help on this.
Thanks,
Couldbfishin
Houston, TX
