720A - New Ham

[jmjatlanta]

Hello all, just went from zero to General on Saturday. Walked around the hamfest and picked up what I hope will be most of what I need.

I picked up an inexpensive 720A, and I think I may have found why it was inexpensive.

While receiving, this thing pulls about 1.5A out of the PS. Hooked to a newly created home-made dipole it can pick up a few stations. I haven't fully installed the dipole, so I'm sure it'll pick up more once I get it fully set up.

The S-Meter doesn't seem to like to move. Picking up fairly strong signals the meter stays at zero. Hitting a freq. with a strong tone on it, and it's still at zero. If I happen upon a strong phone signal where the guy talks for a bit, the meter will pick up and start bouncing around. No big deal, I can live with it like that for a while.

I then hooked it to a dummy load and keyed it briefly with the RF Power turned all the way down. The TX light on the radio lit. The PS showed an increased draw of about 1A (for 2.5A total). That seems very low to me. I increased the RF power to about 1/2 (the 12:00 position) with no additional gain on the ammeter.

My hope is that this thing won't pull much until it is hooked to a real antenna. My guess is the finals may be gone in the radio.

I'm still waiting for my license to show up in the FCC's database, so I can't test it live. I see the testing points mentioned in the manual, so I'll be checking that out. I've got limited testing equipment (dmm, o-scope, swr meter).

My question is how would you guys narrow down what is wrong? Any shortcuts (i.e. "Yeah, that radio always blew the two transistors at ...")?

Thanks in advance and I'll be passing out 73s real soon.

[John_D]

Hi and welcome to the hobby of Amateur radio and its problems. :-)

My hope is that this thing won't pull much until it is hooked to a real antenna.

It should pull up to 20 AMPS into a dummy load on full RF output.

You may find this site of use:-
http://www.usersmanualguide.com/icom...hf_transceiver

the User Manual is here:-
http://www.nodevice.com/user_manual/...s/ic-720a.html

This will interest you as well:-
http://icom.diplofix.com/forum-ICOM/IC-720

Good luck
Jon

[5B4AJB]

It sounds like you're doing everything right, maybe the finals have gone a bit "soft" - they shouldn't be too hard/expensive to replace.
Sometimes when RF PA's blow, they still partially work, giving low received signals, output etc.

The hardest thing about repairing that radio will probably be actually getting to the PA to resolder the new chip.
Find the service manual, make a big space on the bench, document everything and take your time...

[jmjatlanta]

Rookie mistake...

Had I read the destructions, I would have seen my mistake. SSB has no carrier, and the mic must include a preamp on this model.

Switching to RTTY got my SWR meter bouncing. I'm now wiring in a preamp to my mic, the circuit of which was included in my "Electronics Learning Lab" radioshack special. Hehe this thing is going to be bubble gum and shoestring before I get it on the air. But hey, the end justifies the means, right?

Once I get it all working I'll go back and tidy it up. I'm thinking of seeing if an original mic for the radio is not too expensive. If it is, I'll rebuild my preamp on a board and find a little enclosure for it.

My license showed up in the database. I am now KJ4VPJ. WooHoo!

73s and thanks for all your suggestions.

[jmjatlanta]

I wrote this this morning, but it must have gotten lost, as it isn't posted in the forum yet.

I am officially KJ2VPJ. WooHoo!

Well, I'll put the questions above in the beginner column...

The S Meter is still lazy, that's a problem for another day.

The power draw was because I was attempting to transmit SSB. Duh! No carrier. Read the manual, and it says to flip it to RTTY. Sure enough, a nice 14A draw on the power supply. Whew!

Now for my next trick...

The mic I bought is the simple MFJ dynamic handheld mic with PTT. Oops. It doesn't work with the 720A as the 720A's mic had a built-in pre-amp.

I've tried building several pre-amps with leftover stuff I have around the house, but with no tangible results. I wanted to try and use my PC as the preamp, so I ran my PC headset through my sound card, then into the 720A. I cranked the volume and the mic gain all the way, and I can get the needles to move. Popping my Ps can get me up to about 4A on the power supply. I have no idea how it sounds. My guess is distorted and full of hum. But I'll keep working on it.

The eventuality is probably digging up an original mic or finding a preamp that is clean and works. More news as it happens.

KJ4VPJ

[ei7gnb]

Well done! Any inkling to do the Extra exam yet? :P In reference to posts being lost in the ether, it was probably just waiting in out moderation queue - all new members have their first few posts moderated just to make sure they are legit in order to tackle the ever-increasing spam attempts.

Jon

[John_D]

Just get a CB mike from Ebay provided it has the built in amplifier. I've done that for my ancient TS530sp. Solder the connections as to fit your rig.

[jmjatlanta]

Hehe. Typed a reply, hit "Submit" and it booted me. It's either in the queue for moderation, or lost in the ether.

Good idea, John_D. I'm thinking of that.

For now, I wired it this way using a PC microphone with a 1/8" stereo plug:

Pin 7 of the Icom going to the top ring of the microphone plug.
Pin 1 of the Icom going to the center ring.
One 4.7uF Electrolytic capacitor between the center ring and the tip.

This setup seems to modulate the transmitter, but no contacts as yet to see how I sound. I just did it last night, so we'll see if I can raise up someone to talk to.