The AM Forum

In replacing a burned out power transformer in an Eico 720, I`ve learned a few interesting things. The original power supply is full-wave, choke input providing about 600V B+. The power tranny is wired for CT. I did not have the specs for the original tranny, so installed a 760VCT replacement. Now, of course, I`ve got about 325V B+. The magic of CT and full-wave rectification!

The original tranny must be approx. 1400VCT! A pretty hefty piece by today`s standards.  Hard to find and even harder to find one that will fit the low profile cabinet of the Eico 720.

The radio works great now-I`m just only able to load about 85 mA of plate current. Someone suggested bridge rectification, but not sure if the new tranny would handle the current.

Mike  WB4AQL

Yep, that transformer is a real hunk-o-iron for a novice rig. With the high line voltage at my house I measured 715 volts on the plate of the 6146, getting pretty close to the RCA recommended max. Mine is only used on CW till I can scrounge up a modulator.

I just looked at Gary's transformer rewinding price and it's $165. So, yes, it's quite a nice transformer.  Too bad you can't justify having him help you out.  I was very lucky for one of my three 720's in that I had bought a replacement transformer for a 720 so when I had an opportunity to buy a 720 without one, it was a marriage made in heaven.  That is a great transmitter and worth your effort in getting it back in full operating condition but that's your decision as to whether it's worth the investment.  Perhaps you could find an EICO that is a parts unit that has a known good transformer.

Al

You need a big old TV xfmr, they can handle being used in a FWB circuit.  I use two of them in my 51 watt rig.  One is used for the 300 volt supply and filaments.  The other is used in a FWB (650v plate voltage) for the 6146 and 6550's.  Both run cool.

Fred

Good thoughts all. Will have to consider the winding fee. No doubt this is a neat tx; factory wired and very clean.

At this reduced power level, would make a good amp exciter too. A modulator deck and we might have a nice AM tx. A 720 parts rig would fill the bill!

Mike  WB4AQL

The Eico 720 was my first transmitter.  It was half of my novice station in the fall of 1967.  I bought it used.  About 7 months after I got it the transformer let out its magic smoke.  With no internet I could not find a direct replacement transformer so I appropriated a junk CRT TV transformer.  It worked great.  Today you would be hard pressed to find a crt tv let alone one going to the junk...since they don't allow it to go to the dump!

hi Mike .... I believe you can use that 760V ct xfmr just fine by using a doubler circuit ....look at a half wave doubler (c in, 2 diodes, c out ) ...repeat that for each winding and use a common c out ... I have tested this and it works fb on capacitor input circuit .... you should be able to put the choke in the ct lead to ground if the ct insulation is good enuff

1.4 kv according to Dahl.

Still made by Dahl / Hammond.  I'm sure cheaper to have Gary make yours fb again.

For the voltages shown,  look at Craigslist for an old Cb linear.   When the guys can't afford tubes,  they become a cheap source of parts as well.   I've got one I scrounged here that's 900 volts at 2 amps.  Powered 12 8908 tubes!  Don't know what I'll use it for,  but it cost 50 bucks for the carcass,  sans tubes.

--Shane
KD6VXI

And I would upgrade the primary for 122vac.
Regards,
TRS
Gary

It would be interesting to find out if Mike WB4AQL ever fixed his 720. This email thread is 5 years old.

Jim
Wd5JKO

Mike, if the 720 has the same type of accessory feed configuration like the EICO 723, you can disconnect all of the internal PS feed points. In the 723 case, just the swinging choke feed point and the AC filament connections. Then construct an outboard PS as shown here. The 723 also used 600-0-600 V at 100 mA. I found a junk box Stancor with about the same ratings, as well a surplus choke. The size of the supply is same as the 723 in width and half the depth.