I have a loud buzzing on key down at times. It is not transmitting then. If I power off and on it usually goes away and sounds ok on my receiver. The finals have been replaced and power is about 4.5 watts. SWR is about 1.4 and battery is 12v. It’s random. It’s easier to fix if something stays broken. Microcontroller issue? I might have a spare but this was built several years ago. The fact that it just makes a loud buzzing sidetone with no iambic keying even though it’s selected puzzles me. After rebooting it usually works. Another transistor in the keying circuit questionable? Waveforms looked ok. May need to look again.

73s John N4HNO

One thing to check is that your power supply voltage is not drooping at keydown below a level that supports proper operation.? My QMX will squeal if the battery voltage gets too low during transmit.? Think of it as a low battery alarm :-).

?

Brien - KE7WB

?

On Sun, Feb 23, 2025 at 01:23 PM, Tech Guy wrote:

Thanks for the tip but I think the microcontroller is damaged. I guess I will order one. Very erratic. Voltage is ok. When booted it emits a tone constantly when keyed. Rebooting is the only way to stop it. Then it may work. It ?stops responding to the paddle input after a few characters. I am going to look a little more. I think I may have a spare microcontroller. It would be any easy test to swap it.

73s John N4HNO

Another idea is that RF is getting back into the rig and disturbing circuits such that buzz is produced. That’s where my money is.

I used to teach system repair. New students too often blamed the big chips they didn’t understand yet, instead of considering the many other simple possibilities. Another useless approach too often employed was the random measurement of voltage around the circuit, presumably in the hope that something might appear to show them where the fault lay. Often the probe would slip, there would be sparks, and now the circuit really did have a major fault. Measurements are to confirm or falsify a suspicion based upon an understanding of how a circuit might fail and produce the observed symptoms. It requires logic.

In this case you need to think “How does this circuit function and what would cause a buzz in this circuit during transmit?” ?The microcontroller is way down the list of suspects.

Could be RFI. See if it still does it when you are connected to a dummy load instead of the antenna.

--

73, Dan - W2DLC

When it’s keyed the tone is produced and doesn’t transmit or stop till reboot. The paddle inputs are direct to the mcu. It will send after a reboot for a few characters and just stop responding. I am thinking that an input pin or output bit may be failing. The tests were done from a good battery. The dipole is up 30 feet out 100 feet of coax and low swr. Yes other things could be wrong but swapping the mcu eliminates a lot.

-John N4HNO

John,

?

Just checking: when you say that it is a good battery are you measuring the voltage under key down/transmit conditions or just during RX.? When a battery is low, it may measure 12 volts during RX but when the transmitter is keyed the voltage would drop to where it made noises.? ?Just hate to see you tear into the rig if it is something more basic.

?

Brien - KE7WB

Thanks. It does it with a known good power supply into a dummy load sometimes. Sometimes it works for a few minutes. It’s in a die cast box with a 0.1 uF across the power connector and the power cable loops through a large ferrite. The antenna jack is fed with RG 174. It did work before. I will look some more when I get time.

-John?