...The radios we'll always remember


 Posted By: Robert Nickels (W9RAN)
Posted: 03/03/2022

On the workbench 0 Comments 03/03/2022 

My Story of the Fifth EICO 720

by Dr. Bob Heil


 

My first 720 was given to me by good friend WØERE.  I fixed a few things and got to work very well.  The search for the 730 was on and I scored two of them.  I quickly became an EICO fan.  After a fun year or so, the little jewel died - it was the function switch.  Went on search for one but to no avail but found a rough, but working 720.  Removed the function switch and I was again in business.  Through the years, I gathered two more… 

Earlier this year, I saw a factory-built beauty on E Bay and couldn’t resist.  Wow, not only did I purchase another 720, but started a great friendship with David Ishmael WA6VVL - “the restoration king”.   Dave had an entire list of what he had done to that 720 - new caps, the usual, etc.  I used the 720 for a few months but one day - it died.  Contacted Dave and he immediately wanted to see it back on his bench.  

Not only did he discover the .001@2KV plate capacitor had shorted but the failure actually blew a hole in it  (Hey - we are dealing with many parts that are close to 50 years old!)     While he had the factory built 720, he completely rebuilt it - the photos show everything he did to it. You can imagine my surprise when it was returned!   

In the 730, I only use the EL34 tubes and output transformer.   I installed a matching transformer to the grids and drive the primary from the balanced 600 ohm output of the Symmetric 528E.  Marvelous piece of equipment!   Using one of my PR 40 or PR 781 broadcast microphones, properly equalized with the three band parametric of the 528E produces some good sounding AM audio.


Restoration Summary by Dave, WA6VVL:

Hi Bob,

The following is a brief overview of your rebuild:

  • I replaced the 5R4GYA phenolic octal socket with a NOS Cinch Jones ceramic.  This required drilling out the original socket’s rivets.
  • I re-wired the B+ around C27/R16, replacing the 2-pos terminal strip with a 5-pos terminal strip - lessons learned from 720#3.  A new 6-32 clearance hole was required with the pre-determined 2-3/8” spacing.
  • I rewired the 5R4GYA socket.
  • I installed a 2-pos terminal strip for the transformer’s HV CT.
  • I replaced multiple terminal strips as required, drilling out their rivets.  Terminal strips are new from Antique Electronic Supply (AES), many cut-down as required.
  • I created a new schematic for the primary wiring.
  • I installed a P&B P/N KRP11AG (or equivalent) 115 Vac DPDT relay, a NOS Methode octal socket, and the associated primary wiring so that the function switch only controls the relay.  This required two additional 6-32 clearance holes.  I used 6-32 x 1-3/16” threaded aluminum standoff’s to mount the relay’s octal socket, but I added #6 x 3/8” aluminum spacers to the standoffs. I used the same color-code wiring that was used in 720#3, and covered the exposed pins on the octal socket w/heat-shrink tubing.
  • I installed a 1/4A 3AG fuse, mounted in a chassis-mounted 3AG fuse-holder, Radio Shack P/N 270-742, in-line with the 115 Vac line going to the function switch.  This will protect those contacts from any external fault associated with J5-pin 2 (RB).
  • I rewired T1’s primary pi-filter w/new TDK 4700pf “Y” capacitors and 3-pos terminal strips.  One of the ferrite chokes, L15, needed to be replaced from one of Bob’s parts unit, as its enamel wire was badly burned.  Bob didn’t have a replacement for L15, so I stripped off the (burned) enamel wire, cleaned the ferrite core, and wound 17T of 26GA white insulated stranded wire on the ferrite core.  That winding was then “set” with Krylon Crystal Clear Acrylic.  This will result in a lower inductance, but it s/b OK.
  • T1’s primary was re-wired in the process.  A new brown 2-conductor polarized power cord (supplied by Bob) will eventually be installed, replacing the original.
  • I removed and prepped the bandswitch S2, cleaned its terminals and then its contacts with Q-tips and DeoxIT D5.  I removed the rear support L-bracket, sanded, cleaned, and painted it w/ACE glossy black front and rear.  I cleaned the individual #4 round spacers and the switch shaft w/steel wool. 
  • I rewired L7 and L8 similar to that of 720#3, using new #8 solder terminals to mount those coils, and their associated interconnect wiring..
  • I had previously sent you a finned aluminum plate cap, with one fin removed, for the 6146.  This was retained.
  • L11, the 2.5 mH 6146 plate RFC, was cleaned, rotated 90°, and a new B+ connection added from the standoff.  This improves the installation of L17 and C15.
  • R2 was replaced with a new 25K 5%10W cement block resistor from AES..
  • I cleaned, lightly sanded, and painted the original 6CL6 shield w/ACE glossy black.
  • Cleaned the copper chassis with Wenol
  • I installed R15 (1K 10% 1W), C26 (0.001ufd 2KV ceramic disc capacitor), and an orange ground wire on the rear of the function switch and then I installed the function switch.  ALL wires going to the new function switch were replaced with 20GA insulated stranded wire.
  • All 4-40, 6-32, and 8-32 hex nuts are “sealed” using Testers #1114 yellow enamel, a “WA6VVL trademark”. 
  • Still “paranoid” about the #47 XMIT bulb’s return line, I experimented with, and installed, a 7.5 ohm 5% 1/2W MOFP resistor in series with the bulb (in series with the function switch) to minimize any unexpected fault-current through the function switch contacts.  In my opinion, the 7.5 ohm has a minimal effect on the bulb’s intensity.  During XMIT, the resistor dissipates 0.17W.  I also tested a #49 2V bulb @ 0.06A using a 68 ohm series resistor (which was actually my preference), but it just wasn’t bright enough in a side-by-side comparison with the #47 bulb and 7.5 ohm resistor.  In addition, you would need to stock a non-standard #49 bulb as a spare!!       

Dave WA6VVL.  


 


 

 


   

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