I worked on my EICO 720 Novice rig over the weekend. Most of the time spent was over a Wild Goose Chase. I finished up refurbishing the transmitter with replacements for all the carbon resistors with metal oxide equivalents. I also changed out the power cord with a 3-wire cord plus I replaced the two in series electolytic 40uF @ 450 volt caps with new 47 uF @ 450 volt caps. I also lubricated all the bearings. I then installed one new JJ brand GZ34/5AR4 rectifier leaving out all the other tubes.
Now it was time to do some testing. POW!!! the 3 amp fuse blew. I then started removing loads going to the power supply. Removing both the two load wires stopped the fuse from blowing. (GOOD!!! My Choke and Power Transformer are most likely OK!) I checked the two load wires and they showed no resistance as expected since the rig had no tubes in it other then the rectifier and the only parts going to ground were a couple .01 by-pass capacitors. How could this be? Why would two circuits with no loads on them blow the fuse? I screwed with the rig for 4 hours before I realized the new JJ brand Eastern European tube was most likely flashing over or oscillating. Why it would do this only when the load leads going to parts of the transmitter were connected is a mystery. (These load leads with no tubes plugged into the transmitter should not have put any additional strain on the rectifier tube.) (I have heard that rectifier tubes can oscillate. Maybe this was what was happening when I hooked up the two leads going to the rig's circuits with no tubes plugged in.) Anyway I installed a 5R4 rectifier tube temporarily and the problems disappeared even with the load wires re-connected to the power supply. I have not gone on with further transmitter testing. I spent four hours chasing my tail over all this when all it was was a bad new JJ Tesla rectifier tube. (The 5R4 tube has to go and not be used permanently as it draws more filament current plus it is too tall for the transmitter.)
So now, what to do about a bad JJ GZ34 tube? I did some research and the JJ brand GZ34 has a bad reputation for arching over. Last night I ordered one Russian GZ34 and one Chinese GZ34 tube from different suppliers. Once they are here, I'll find out if they are any better then the JJ brand. (I could go solid state with a plug in replacement, but that would put a lot of strain on the rigs transformer, choke, tubes and switches. I prefer not to do that.) For those not familiar with GZ34/5AR4 tubes, they are EXPENSIVE! Old NOS American or British/Holland made GZ34 tubes sell for mega bucks. ($30-$250.)
So that is where I am with all this right now. I suspect one or both of the two tubes I ordered will solve my problem. Just be aware JJ Tesla GZ34/5AR4 tubes have a bad reputation out of the box for arching over.
Lee, w0vt _______________________________________________ Boatanchors mailing list [email protected] https://puck.nether.net/mailman/listinfo/boatanchors
