Well, the new interstage cores arrived a couple days ago from AES, so I opened up the first transformer. Disassembly was pleasingly easy, the D's just slid out
The size of the new core was perfect. With a little work I managed to get all the D's back in.
The second one went just as smoothly. So i powered it up without any tubes installed to check voltages. The plate voltages of the two amplifiers were fine, but there was no voltage on the detector or any of the filaments.
The problem with the filaments turned out to be the switch. I took it apart, which was surprisingly easy. It has a bistable mechanical contraption built around a pair of cones joined base to base with a spring around them. Attached to this is a bit of copper that bridges the two connections. Here it is first closed, then open.
Open it read "infinite" ohms and closed a couple meg ohms. I compeletly disassembled it and polished the contacts and now I have filament voltage.
Turns out that the reason that there was nothing on the detector plate was that the "tickler" coil was open. I liked the pretty blue wire on it, but I decided to rewind it.
Before:
and after:
The detritus is the remnants of the Barge Cement that I used to stabilize the coil as I wound it. It will clean off fairly easily.
So I put it back together, checked the voltages again, plugged in the tubes and was greeted by ... silence. This surprised me a little because I am quite close to the 50,000 watt clear channel KYW and I can almost receive that by sticking an antenna into a cabbage.
So I dragged out the signal generator. AF applied to the detector grid sounded in the speaker, so the new transformers and both audio sections are ok. Hooked up the scope to the detector grid and wrapped the rf output around the antenna. Messing with the tuning capacitor, I could see the modulated rf peaking on the grid and, better, hear the AF coming out of the speaker. No sign of oscillation though.
I did discover, however, that all of the binding post connections are, um, tenuous. I could make it stop working altogether by just bumping various wires. I think I need to clean and retighten every one of them. Also get a better ground and antenna hooked up. I already re-soldered all of the connections to the phone jacks, all of which were brittle and looked like cold joints. I wonder if the crystalline structure had changed over the years. I am also wondering if in spite of all my care to prevent it I managed to install the tickler backwards so that it is supplying negative instead of positive feedback. I'll check that tomorrow after I clean and retighten all those binding posts.
Paul