TYT TH9800 Issue

[HomerBB]

I purchased one of these quad band radios because I wanted a crossband router functioning option. I paid just over $200 new and was delighted at how well it worked when set up. Then, by a couple months of owning it the audio output failed. I had signal, and others heard me when I transmitted, but no sound. External speakers didn't make any difference.
I bought another one. It worked quite well, too. However, the same problem developed. I'm not a tech, and the three local techs we had in our area went silent key within 2 years of each other. No help locally, now.
Anyone else encounter this issue?

 

[19wrc333]

Yes, this is a known issue with the TYT TH9800.
I bought one on the cheap with this issue. It worked well, except no audio output on the 2m/70cm. As I didn't have my license at the time, it wasn't a big issue.

But.. things changed and I needed it to work. So I researched this.
There's two audio chips in this radio and these are getting more voltage as they are specced. Let me find my write up..

It's the CMX138 chip that goes bad.
Link: https://www.alldatasheet.com/datasheet-pdf/pdf/737895/CMLMICRO/CMX138A.html

There's a zener mod that patches the problem, see: https://groups.io/g/th9800/topic/96992408#102

But.. me.. I just run the radio at 12.6 Volt now with the new mod chip. I didn't do the zener mod, but had someone replace the chip for me. After that it works fine.
--
Skyline

 

[HomerBB]

@10wrc333, thanks
I'll follow your links to educate myself. I do like the radios.

 

[19wrc333]

@10wrc333, thanks
I'll follow your links to educate myself. I do like the radios.

Yes, these radios are actually really nice.

One more note. In my case I only had one of these chips replaced and not the modification done. If you do opt for the zener mod, then beware that this apparently will negatively influence receiver sensitivity on 70cm.
Maybe I'll add the zener mod myself on mine one day. Replacing the SMD chip wasn't in my capabilities, but the zener mod should be do-able.

I suggest to sign up to the TH9800 group on google groups as there you can read more about it.

There's also this video:
that might help.

Also attached schematics and a high res pic of the board.

 

[HomerBB]

Thanks, again!
I will look into it. I'm interested only fixing my issue, no additional mods. But I'm not sure of whether I can pull it off. Really terrible with the soldering. Afraid of the surface mounted stuff. I do have the necessary tools to do it, just never developed the skills. Too busy with antennas.

 

[19wrc333]

Thanks, again!
I will look into it. I'm interested only fixing my issue, no additional mods. But I'm not sure of whether I can pull it off. Really terrible with the soldering. Afraid of the surface mounted stuff. I do have the necessary tools to do it, just never developed the skills. Too busy with antennas.

I hear you.
While I'm fairly OK with soldering, replacing a 28 pin SMD chip requires experience in SMD soldering. If you never did that before, then I recommend to not try that as your first SMD experiment. There's too many pitfalls.

That's why I looked around for somebody experienced in the matter. I was lucky enough to find a friendly radio amateur that was willing to replace the chip on my behalf.

Please note that there's two of these chips in the radio. Depending on the fault you might have to replace one or both chips.

 

[HomerBB]

I'll have to order the required parts, then start a search among the many hams in this area.
I don't feel so much like the lone ranger with a silver bullet, now. . .