Help needed with UHF handheld radio

[sainter]

Hi,

I'm trying to fix a UHF Wouxun kg-889 handheld radio, with has a this problem:

When transmitting, you can hear the transmission clearly and loud on the channel on witch you are, but you can hear the same one channel up, and down. Just not as loud.

As I understand, the carrier frequency is generated with PLL (witch is controlled by main MCU) and modulated with the output of the amplified signal from microphone via VCO. Squelch frequency is inserted via the same VCO.

As far as I understand, the modulation coming into VCO must be out or the VCO is out of wack, because when receiving, it doesn't pick up any other channel then It's set to (so carrier freq. from PPL should be good). I don't have spectrum analyzer, nor a scope the could go beyond 100 Mhz, so dealing with 446 MHz is had for me. Moreover I can't find any schematics for KG-889. Only found the modulation test pin on the pcb, but I can't probe a thing because of the high power high freq. interference coming from the radio itself.

I guess, I will have to probe without the clip but with the ground spring.

I'm  new   to electronics and half of the time, I'm just poking with the stick and hoping that It will start to work, but I want to learn. And this repair project if more fore learning then for fixing. Thank you in advance for any advise about what could be wrong, and if anybody would have a schematic for this device, or could measure the Vpp of the modulation signal, that would be great.

[OE2WHP]

So, it basically works but is to broad?

May sound stupid but have you checked the bandwidth setting?

73

[sainter]

Yes I did, narrow band.

[OE2WHP]

Have you measured if the modulation Vpp changes when you switch between bandwidth options?

[sainter]

No, will try it, as I said, I still need to find a way to distinguish the signal from transmission  induced interference. Maybe probe any other point with second channel and math-out common mode noise ?

[voltz]

Hi,
So you can hear the TX channel on a receiver one channel up and down?

Some questions:

What receiver are you using?
how much signal are you sending to it? ie. S reading on receiver
What is the channel spacing, 25Khz?

I ask this because it sounds more like a receiver issue, not an issue with the hand held. Unless is well over-modulated.

[sainter]

Yes, I can hear TX 446.01875 MHz (Channel 2 PMR446) on channel 1 and 3 (446.00625 and 446.03125) with lower sound volume, (it shows lower RX power), and 446.01875 MHz on Channel 2 with good sound volume.  Channel spacing is 12.5 kHz and narrow band is 2.5 kHz deviation as I understand. This is verified by several stations. I'm using Wouxun KG-689.

[OE2WHP]

Hmm.... smells like some firmware issue... maybe....

Have you tried to reset it back to factory defaults and then see if the problem is still there?
Do you have the programming adapter?

[sainter]

Yes, reset, reprogram.... nothing. Software only controls PLL, and the carrier frequency is ok (as far as I know). Software only controls n/w band. If it would be stuck on wide band, it would receive at a very low volume?

[voltz]

This is verified by several stations. I'm using Wouxun KG-689.

You mean other stations away from you can also hear it? If so, then yes, its over-modulated.
Otherwise the signal could be simply too strong and too close to the KG-689 to get good enough selectivity.

[sainter]

Yes, this problem remains at grater distance. But even up close, other KG-689 or KG-899 TX could not be heard on other channels.

[voltz]

If you dont have a spectrum analyser, try a SDR dongle and computer with HDSDR. Cheap.. they make a quite useful SA. That would show you the problem.

[OE2WHP]

Yes, reset, reprogram.... nothing.

just to make sure, you've done a RESET ALL not a RESET VFO only, right?

[gfiber]

Almost sound like the receiving stations are using a wider receiving filter in their receivers since the offending receive frequencies are 12.5 KHz above and below the 446.01875 unless I miscalculated. I have seen radio with dual received filter systems in them one for narrow band and one for the wider 25KHz channel spacing we did use in the States, ham radio transceivers still support the wider band receiving.