FT990 problem-oscillations

[k7wxk]

New to this forum, but active on several others places. Here is my dilemma:

My FT990 has suddenly encountered a problem on 10 and 12 meters only. It oscillates when connected to my beam or my loop antenna. I have no control over the RF level (goes to max) and of course the ATU has no effect. Likewise, the external antenna tuner has no effect either.

Now, before you poopoo the antennas, let me assure you that they work fine and have been checked for SWR with a Drake TR4. The FT990 also worked fine into the antennas until just a few days ago when this condition arose. Normally, if there is any SWR present, the FT990 will reduce the power accordingly until a suitable SWR is found. Not so any more.

Here is the odd thing: The FT990 works perfectly into a 50 ohm dummy load, with full control over the RF level. It is definitely oscillating whenever it is connected to an antenna.

Any thoughts as to the cause?

Thanks for your input.

Marc

 

[wavrider]

Jumpers?

Station ground?

CMC on the coax?

If it works fine into the dummy load I would say the rig is fine.

Try just straight from the rig to the antenna, no jumpers tuners etc. etc.

Also power cords for the rig.

Had one rig drove me crazy until I shortened up the power cords, seemed it was acting as a receive antenna on 10 meters.

Maybe try an rf choke, ugly balun at the back of the rig.

 

[k7wxk]

Rider,

Thanks for the input. Tried all the above and no change. I fear it is a component common to 12 an 10 meters somewhere within the rig. It is strange that it works fine with the dummy load. Just got off the air on 10 meters with another transmitter and the antenna is fine.

Keep the suggestions coming.

Marc

 

[wavrider]

Still strange.

I have an ft-890, basically same rig different cosmetics.

Oh and welcome to the forum.

10/12 may share the same LO.

I don't have my schematic in front of me.

But still if it does not oscillate when connected to a dummy load, it should not oscillate when connected to any other load.

Both VFO's make it oscillate? or only one? Have you tried this in CW mode without the mic connected? Just using the keyer?

The rig oscillates in receive mode or only after you transmit into the antenna?

Have you tried different filter setting?

Just trying to narrow down the possibilities of what could be causing it.

PS voltage regulated or is there a bad filter cap in the PS allowing some AC ripple to get in the rig?

You may try using a different PS or a battery for test purposes.

 

[k7wxk]

Oscillates on SSB or CW (with or without mic connected), A or B VFO, transmit only. Rx is fine. Don't have a DC power cable, so haven't checked it in battery power . FT990 has a built in AC supply.

 

[wavrider]

use a multi meter set to read voltage both AC then DC.

Measure the SO 239 on the back of the rig to see if there are any voltage on it.

the dummy load presents a pure resistive load the antenna is reactive load, that may be the difference in the antenna vs the dummy load and how the radio reacts.

 

[BJ radionut]

Disconnect everything from the radio...remove the case...Remove each board/ One at a Time...clean the contacts and reseat the board...
Do this for all boards...hopefully this will cure it...
If not it's most likely parts on the filter board.... OR?
GL
All the Best
Gary

 

[k7wxk]

Rider and Gary,

Will try these tips this morning and let you know how it goes. Thanks for the input. Should I use DeOxit to clean the contacts? My experience with solid state is very limited.

Marc

Problem solved: While thinking about the self oscillation on 10 and 12, I recalled that this was mentioned in the text of a mod that I had put in the rig several years ago. So I dug out the text and re-read it. It mentioned the need for a diode on the RF board that I had somehow failed to install. So, I checked the circuit board against the picture in the text, and installed the diode. Oscillations gone. I still find it odd that the rig worked fine for all these years until just recently. Moral of the story: Don't throw any reference material away. You never know when it will come in handy.

Thanks to Rider and Gary for your input. One never knows when a friendly bit of advice may just do the trick.

Marc

 

[wavrider]

Glad you got it back in service.