Tytera TYT TH7800 Transmitting Problems

[bwhte]

I have two TYT TH7800's. I have had one set up as my base for 1 - 1 1/2yrs with no problems, it has worked like a champ, until yesterday. Yesterday I went to use it and started having issues on the 2 meter. I could hear just fine and if someone was real close to my residence I could be heard, but move out 1/2 to a mile and nothing but static, but would show a full power bar. Checked swr and a full scan showed 1.2-1.4 on the two meter and even better on the 70cm. Had a buddy do some other checks on a meter he had and on it and when keyed up it would shoot up to 15 on high and on low it was still reading 11 on the 2 meter, the 70cm only showed 7-8. This radios problems are with the 2 meter, the 70cm can be used with no issues.

My second radio had never been used it has only been programed for use, it was my backup. I hooked it up and the power band is great but there is a little static and very slight muffled audio. Nothing understandable can be made out, my buddy said it was like I was talking with my hand over the mic. This radio the audio problems are on both 2 meter and the 70cm. Both radios can be listened to with no problems but when keyed to transmit is where the issues are with both radios. Any ideas? I went through the settings to make sure all was good there, even did a factory reset on the newer unit with no change in performance.

 

[kc1jts]

The TH-7800 overdrives the 2 CMX138a chips with too high a voltage. They are speced at 3.0. to 3.6v max, TYT is driving them at roughly 5 volts. As a result after a time they fail. These chips do all the input and output audio signal processing, including mic amplification, compression, as well as CTCSS and DCS management. There is a set of pads in the radio where 3.3v zener diodes should have been placed (check the schematic for the 7900), but these are omitted on the 7800. I suspect these chip(s) have failed and need to be replaced. They are available from Ali Express for a reasonable price, but are a ***** to replace. You will need to add the zener diodes and replace the 100 ohm resistors on the board with other values to regulate the 5v supply back down to 3.3ish volts. Not sure of the exact value for the replacement resistor, but standard value of 82 ohm is too big, 62 seemed too small. Might need to use a lower valued zener, like 3.0v to get it dead on. My understanding is that reducing the voltage will slightly reduce the receive sensitivity, but I haven't been able to confirm as I am still waiting for replacement chips for the radio I have on the bench currently.

Good luck.

Bill

 

[MTS2000des]

All the more reason to spend a few more bucks on a quality Japanese radio made with quality products and a warranty.

 

[AK9R]

All the more reason to spend a few more bucks on a quality Japanese radio made with quality products and a warranty.

...and a U.S. distributor who honors the warranty.