Why am I doing this? I see it as a challenge for my electronic skills. And: TUUUUUBES!
When I started back about two years ago, I took the radio from my granddads old workshop, where it sat for the better part of its life. As in: I remember it sitting there MY entire life.
Time has not been too kind to it, sadly. The wooden case looked very much in need for repair and the electrical stuff inside was, well... dusty.
I made sure to snap pictures of every detail before disassembling (wise decision)
Since I could not find a schematic diagram for it, I decided to just try out a "direct" approach: if I replace some wonky components (i.e. the transformer iron which was rusting away, and the fuses which were paper wrapped wires) I would give it a try and if it didn't work... it was at least going to be a nice showpiece.
Well, the visual restoration went just fine and after assembling every component, I turned it on... nothing but way too hot (in my opinion) tubes. Oh well.
Fast forward to earlier 2018. On a whim, I googled for the radio model yet again and behold: A schematic diagram! Also, several hours of watching tube equipment related repair videos, I felt up for the task.
Starting by removing all the old paper capacitors and ordering a set of newer ones, then rebuilding that thing... boy, point to point wiring can be a quite like a game of twister!
For visual reasons, I gutted the insides of the old main filter capacitor and stuck two newer capacitors into that space. Now it looks like the original is still in place and I didn't have to find a new place for the caps.
I discovered that the "on/off" switch had two positions: off and almost on. Or, infinite and 500kOhm. I'm pretty sure that's not how it should be, so I opened up the switch, pried apart the contacts and gave them a good scrubbing. Turns out they were silver, not black after all. Now the switch is infinite and 0.2 Ohms. That's more like it.
Removing all the tubes at first and then powering on, I confirmed that both the fillament and the B+ output from the transformer was OK.
Next, I installed the rectifier tube and got a nice full wave (or rather dual half wave) rectification out on the other end.
Connecting the filter capacitors and ballast again, I confirmed a nice, juicy 350V (on idle) B+ voltage. Good. Idle 350 means that it should not overshoot by much, so my 400V filter caps are OK to stay.
Up next: connecting the power output tube and the "multifunctional pre-amp tube", so I can put in a signal from the "Phono" input and validate the output stage.
I found out around here, that temperatures up to 200°C are considered normal for these tubes. No touchy! But also, do not label them with electrical tape!
After some tryouts, checking resistors, voltages and clening the tube socket contacts, I managed to get about 100Vpp on the output transformer. That's when I decided: hmmm... looks "OK", but there is no signal on the output side? Let's just connect the speaker and see if we hear anything... and behold! A 1kHz sine wave never sounded THAT good ^.^