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Icom 735 AM TX bandwidth via balanced modulator

Posted: Tue Sep 17, 2013 2:35 pm
by wannavoodoo
I'm aware that the DSP Icom's are brick walled on TX to about 2.9kc wide on TX even when you feed them through the accessory port but what about the pre-DSP Icom's like the 735?

Does anyone know what the AM TX bandwidth is on the 735 when fed this way?

It looks like the audio when fed through the accessory port only goes through a single buffer transistor before it hits the BM.


Posted: Wed Sep 18, 2013 10:44 am
by wannavoodoo
11 views no replies!

I thought you guys knew it all! :P

I know Icom isn't preferred but I'm holding out hope that this older pre-DSP 735 will allow me to xmit 4-5kc wide via the Accessory 1 port since the SSB filter isn't in line when in AM.

I found a guy's qrz page talking about his modified 735 transmitting 5kc on AM and found some decent gates of him on youtube. I don't have a way of contacting him so I don't know if he modified his radio or just merely injects his rack into acc1 for AM.

Posted: Wed Sep 18, 2013 11:59 am
by wannavoodoo
Found more discussion, but nobody declaring what the actual TX bandwidth is when going into ACC1 for AM on the IC-735.

This post includes a link to a recording someone made of Rack gear/RE-20 mic going into ACC1 of their IC-735 on AM and being received/recorded on a Yaesu FT-102 w/ 6kc AM filter.

Audio clip (mp3) ... tach=33245

Sounds wide to me! Just want to find out how wide of AM TX it supports...

Posted: Fri Sep 20, 2013 12:18 am
by Voodoo Guru
Howdy Wannavoodoo,

Unfortunately, we have no experience at all with Icom Rigs. You will find that most eSSB (Extended SSB) Operators tend to go with other rigs. The only one that has ever been thru here was a Icom 706 MK2 G that we changed the TX Filter out on and widened it up to about 3Kc. Other then that, absolutely nothing.

Any one else out there play around with the Icom 736?

Posted: Fri Sep 20, 2013 8:53 am
by wannavoodoo
I forgot you guys are mostly SSB.

I know the 735 won't do more than 2.9kc wide SSB but I'm wanting to do hifi AM. I've ordered an ibox and some cables so I'll be giving it a go w/ rack into the balanced modulator this weekend and will report back.


Posted: Sat Sep 21, 2013 12:20 am
by Voodoo Guru
Howdy wannavoodoo,

Hmmm, well it should play very nice on AM if these following things are researched and optimized...

1 - Try to find out how the rig is producing AM. If it is a Balanced Modulator (BM) then try to find out how to best feed the Rack Audio directly to the BM thru a Capacitor and or an isolation Transformer.

2 - You will need to find out what filter the AM Mode Defaults to (if any) during transmit. This needs to be as wide as possible to pass the sidebands on either side of the carrier, ie. a 6KC Filter will give you 3kc of Audio on each sideband in AM.

3 - Find some way to disable the ALC when in AM Mode. There are various simple solutions for the ICOM to adjust the ALC so that it will not fold back your positive peaks of the AM Signal. Check this out on the InterWeb as there are several people that have some mods.

Have Fun and keep us posted!!
Take Care,

Posted: Wed Sep 25, 2013 6:27 pm
by wannavoodoo
Well, I still haven't found a definitive answer on how wide AM transmit is capable of when going in the ACC1 jack on the IC-735 but I have it setup right now like this:

Condenser mic -> Art Tube MP mic pre -> w2ihy iBox -> ACC1 on IC-735

Monitoring the Icom's AM transmit audio on a Drake R8A receiver set to 6kc wide sounds pretty hifi to me. I'll report back once I hook up some eq/processing either rack or computer based.

I have no problem reaching 100% modulation it actually looks like (on the watt meter now, haven't hooked the scope up yet, 20 watts carrier can hit 100+ watts PEP) like a little over. I am still seeing some ALC interaction when injecting audio into ACC1 so I plan to build one of those contraptions using a 9 volt battery/5k pot & rca cable to game the ALC externally so it doesn't limit my peaks at all.

But so far with just what I have hooked up this thing sounds pretty killer on AM. The thing it has going for it is there is no new school Icom DSP in line w/ ACC1 AM TX brick walling the bandwidth like on the newer DSP Icom rigs.

Thought I'd share in case anyone is ever considering an Icom 735 for AM use