HF Mystory

This one has me stumped, not a hard thing to do but I usually can figure these out so I'm hoping someone (or more) can help.

I have an Icom 706 MKII with an Icom AH4 antenna tuner with a powered speaker. This is in my truck and I have used this setup for years without issues.

Problem: when the engine is running everything works great, makes plenty of long distant contacts etc. However when I turn off the engine I immediately have very bad feedback through my speaker. Turn the engine back on and no feedback, engine off and get feedback. This is completely repeatable across various antennas both short and long. Since the body of the 706 is under the drivers seat I already have coils (forgot the correct term) and speaker wires already twisted to keep out HF interference.

There is no problem with this when using my Kenwood D700 which is next to the Icom 706

I've done all the usual things like checking voltage, wires, coax and antenna. I'm hoping someone has some ideas about where to go next?

Thanks in advance! Jim
Since the body of the 706 is under the drivers seat I already have coils (forgot the correct term) and speaker wires already twisted to keep out HF interference.
Chokes... (y)

This seems opposite of what normally happens. Normally the ignition makes the noise and all is quiet with the vehicle off.
Maybe a ground loop of some sort. Did something come ungrounded.... or maybe something became grounded that shouldn't be?
Certainly a different scenario. Good luck and let us know.
Check over at K0BG's site. it's dedicated to mobile Ops. if it's the truck in your signature line, it's old enough that someone is bound to have a answer.


Expedition Leader
I'd guess the squealing is a low voltage issue with the radio. Mobile radios are designed to typically run at 13.8 V, since that's roughly the expected running voltage in a car. So with the engine shut off you're probably down to at 12.8 V at best and perhaps even lower.

So my recommendation would be to see if you can replicate it with a variable power supply first and second check that your truck battery isn't weak or the radio needs to be repaired or adjusted due to an issue running at a slightly depressed voltage (and think about it, if you expect 13.8 V +/- 10 % that means you should not run below 12.42 V).

It could a lot of things but it boils down to some circuit to being to turn off or has lost its bias and has hit a corner case. This happens with electronics over time as component age or wear out, not to mention the environment in a car is pretty tough on things, heat, vibration, shock. It's not hard to fathom something has a broken solder joint or a cap or resistor value has drifted.
I thought the same thing Dave but per Icom the radio can run +/- 15% which would put the low side at 11.73v. And since the issue was immediate upon the vehicle being turned off the battery shouldn't be low....unless as you mention the battery is weak.
Thanks guys for the thoughts! The battery is new and I have a dual battery setup, voltage is fine and surface charge when ignition is turned off is usually high 13's which the radio doesn't pull down for a while.

Dave In Denver: I have given thought to perhaps parts of the system being old and starting to wear out. This setup is well over a decade old and has lived a hard life of off roading across 4 vehicles so you could be onto something. The part that still puzzles me is that it works fine when the truck is turned on but feedback when I turn it off. It's never done this before in any of my rigs.

PhulesAU: I will try to google KOBG when I get home tonight and see what I can find.


Expedition Leader
Yup, interested to hear what happens. Are you sure it's feedback, have you unplugged the mic? I doubt that's what it is but it would be a troubleshooting data point.
Ok I tried unplugging the mic and it only solves the problem by making it impossible to transmit. As soon as its plugged back in and I start to transmit the feedback returns.

I'm new to my area but have found a ham club that meets monthly. Hopefully next month I can attend a meeting and see if they have any ideas.

PhulesAU: I have not been able to find any website with KOBG in it. Do you know the link?

Thanks guys, keep those ideas coming!! Jim.


Expedition Leader
So the interference/feedback/squeal is only present when you key the transmitter?

I understood that when the voltage dropped the radio was making noise even on receive. If it's doing it only when you attempt to transmit I would verify your connections and probably replace the large (30A?) fuses in the power feed. If it's only during TX I'm more convinced you're chasing a power supply issue.

Also, have you asked someone to listen to your transmission to see what they hear?
PhulesAU: I've found the website and have been reading and trying to find this issue on there but no luck yet.

DaveInDenver: As soon as the rain stops I'll replace the fuse. Worth a try! I agree that it's power related but can't figure it out or find anything else that's changed in the system.

I'll keep at it, hopefully the ham club next month might have some ideas for me. Thanks guys!!
What kind of vehicle? My guess it is your evaporative control system pumping the system down and what you are hearing is the evap motor RFI. It will intermittently run while the engine is on but depending on the vehicle they we'll run when you turn the engine off. Is it just for a few seconds or what, duration?

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"dual battery system", does that involve any diodes in line? If so then add their voltage drop to the power source loss. I agree the issue is low voltage during transmit.