Well here is what I have set up, the transformer is out of the radio, the tubes are on my bench and wired only to the transformer. I set it up this way to make sure nothing in the chassis was shorting out. The only thing I can think at this point is a weak transformer. The primary is drawing only about .3 amps at 115vac. The tubes out of the chassis and in the chassis measure the same. About 1.8vac with about 7 to 8 amps. I am leaning to something in the transformer. When I took it apart earlier, I noticed where one of the filament leads attached, there was some charred wrapping. I will take it apart again tomorrow and inspect it closer. Just a couple of pictures to show what I have been up to.
SincevI cannot see any charring I will take Greg's word for it. Inspect it. If does not help, then:
Take a verything off, I mean disconnect everything all the way to the xfmr leads
Size up a load that would take 10.5A @2.5V (this would have to be a power resistor of about 0.23 ohm), connect it RIGHT at the leads well. No alligator clips or thin wires.
Then connect voltmeter at the load and see the voltage. If it drops significantly from 2.5V you have a weak connection inside the xfmr. Inspect and if accessible, resolder. There is no turn short. And there is no such thing as weak winding for this kind of load, they do not weaken, , has to be the connection that is soldered to it. If not accessible, then oh, well...
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 04-03-2016, 10:36 AM by morzh.)
Skip, make sure you are measuring the transformer voltage directly at the transformer leads. If you are connecting the voltmeter probes after the clip leads or ammeter, you will see a large voltage drop as the leads have significant resistance. You could easily see a drop of 1 volt when passing 8 or 10 amps through a typical clip lead.
With a 2.5 v supply it is very critical to have a minimum of resistance in the connections and wiring, which is one reason 2.5 volt filaments were abandoned in favor of 6.3 v tubes in later sets.
Well I have success, I hope. I ended up taking the transformer apart again, this time I figured I might as well unwind it some. I discovered a few things that may or may not have made a difference.
1. The 45's filament winding was the outer most, and someone long ago had replaced it. The core was wrapped in black vinyl electrical tape and the winding looked like the ground wire form common household 3-wire electrical wiring. It looked fine otherwise.
2. The 24&27's filament winding is double wound and soldered together at both ends. It was all coated in varnish so it looked original. The burn spot in the tape is located where one of the leads and the CT lead come out the side. The CT was repaired long ago. Whom ever did the repair did not solder it back in the original spot, they were off by about an inch. Part of the original CT wire was still under all the tape. It was also wound off from center so part of the winding was right near the edge of the transformer core.
So I unwound the second filament winding as well, resoldered the CT in the correct spot. I then rewound the 24&27 filament winding, making sure it was spread evenly and centered more over the core. I then wound back on the 45's filament back on, also making sure it was evenly spaced and centered.
I had to do this twice because I used a little too much tape between winding's and the core would not fit back into the bobbin. I also broke a couple of the high-voltage leads and had to fix them. Once I got it back together I tested it and it seemed ok, no shorts. I then hooked it up the the radio, only connecting the 24&27's filament section. I placed all the 24& 27 tubes in and brought it up slowly. Low and behold it works! All filaments are lite with 2.5vac and pulling 8 amps after they warmed up.
It has been "cooking" for about an hour and the transformer is barely warm. There is a small buzz now, but the transformer is not mounted yet. I hope this fixes it, but I will keep an eye out for another transformer or have one made just in case.
Thanks for all the suggestions and help. I doubt I would have taken it back apart and unwound it with out your questions.
Other then the CT not soldered back at the original spot, and the winding's not centered on the Bobbin, I am not sure what fixed it or exactly the reason it just was weak under a load. I did also retouch the solder on both ends of the second 2.5vac winding. Maybe that helped as well.
