Eico 720 Transmitter with 730 AM Modulator

I had an interest in the Eico 720 transmitter ever since I was first licensed in 1963.

I believe it was mainly designed for the Novice CW market but an outboard AM plate modulator was also available. The transmitter is rated at 90 Watts input on CW and 65 Watts input on AM. The modulator is rated at 50 Watts output. I acquired this Eico 720 with 730 modulator at a local Ham flea market.

Physically they both were in reasonable shape but the cabinet of the 720 was scratched up. The cabinet of the 730 was in much better condition. I sprayed both cabinets with Mat Black Krylon paint. This definitely improved their appearance.

The 720 transmitter had a toggle switch added to the front panel. I am not exactly sure why this mod was added but it looks like it was to make switching between AM and CW easier. I removed the mod as I wanted the transmitter to be as original as possible and also was not sure of the consequences of leaving it in. Unfortunately it left a hole in the front panel that I had to plug.

I decided to get the transmitter operational first on CW and then work on the modulator. I plan on using the transmitter almost exclusively on AM. The manuals for both the transmitter and modulator are available on the Bama website. The transmitter did not come with a VFO so I used a Heathkit VF-1 VFO that I normally used to drive a Heathkit DX60B transmitter. This VFO will not work with the Eico 720 transmitter on 80 meters. After doing a visual inspection of the transmitter, replacing the line fuse with the correct value and taking care of a few other minor issues, I powered it up slowly using a Variac autotransformer. After adjusting the the transformer to the required line voltage the transmitter appeared to work OK. I did a small mod to the VF-1 VFO to allow keying of both the VFO and the 720. The VF-1 is run off a separate power supply to allow the VFO to warm up and stabilize without turning on any other equipment.

A CW contact was made on 20 meters with Mike VE2DQO in Montreal. The 720 worked very well and had no chirp. Mike could not tell that I was not using a modern rig.

As with the 720, I did a visual inspection of the 730 modulator before powering it up. I also changed the tap on the primary of the power transformer from 117 VAC to 125 VAC operation. My line voltage is always above 120 VAC and using the 125 VAC tap should make it a little easier on the tubes. The 720 power transformer did not have this option. I followed the same power up procedure as I used with the 720.

The 730 instruction manual has a procedure for setting the output tubes' bias and balance. These adjustment went smoothly. Getting the modulator to interface with the transmitter and antenna switching relay required somewhat more work than CW only operation. For smooth operation the modulator required a contact closure from the antenna relay's auxiliary contacts. The contact closure was to ground the center tap on the secondary of the power transformer to turn on the B+. Also, I needed to key the transmitter and VFO from other contacts along with being able to mute the receiver during transmit. The DPDT auxiliary contacts on my Dowkey T/R relay were pretty much used up.

The 730 modulator uses an EM84 "magic eye" tube as an over modulation indicator. I compared it's operation with the AM envelope display on an oscilloscope. It seems to do it's job.

An on air AM test with VA6RP, a local Ham, indicated that the transmitter sounded OK.

One issue with the transmitter I would like to clear up is low drive on 10 meters. I did not spend much time on this other than changing the buffer/multiplier tube which did not help. The tube quadruples for 10 meter operation. There is plenty of travel left in the drive pot when the grid current stops going up. It looks like something is saturating in the transmitter.

Both the 720 and 730 were kits and I spent some time cleaning up some unsatisfactory wiring. Also, intermittent operation of the transmitter did show up and the problems were traced to some poor solder joints.

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