[Boatanchors] EICO 720 Transmitter Re-Furbish Update

Lee L at w0vt.us
Mon Aug 15 14:04:49 EDT 2016

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 

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

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