Uniden Grant XL No Transmit

Wrong Microphone, Shorted leads in the microphone plug or a bad transmit switching transistor.

if this happens without a mic attached, it is common for the tx/rx switching transistor to fail

There are a couple of things that could cause the radar to power off when going into the transmit mode.

The first thing to eliminate is input power. When in the standby mode the radar draws less current than in the tx mode so if you have low voltage for whatever reason, that could cause a shut down. Check all connections and measure the voltage to the unit while it is on.

A problem with the antenna motor or gear box could also be the problem. The antenna doesn't rotate unit the unit is switched to the tx mode. You would need to remove the dome cover (with unit off) and verify that the array will turn freely by hand. Typically, a bad motor will not cause a shut down but a physical obstruction could.

Focus on the power issue because that's the most common problem. Be aware that you need to measure the voltage as close to the radar display as possible and watch the voltage as you switch the radar from standby to transmit for a quick dip in voltage.

I can tell you that mic wires can simulate that problem when they accidently touch or ground out. Sometimes a ground is loose or capacitor leaks. Check standing wave ratio. SWR 1.5 or less prefer 1.2 or 1.3 read. Antenna cables replacement and new ends. Dirt in mic switch. Mine did this with a power mod capacitor
near final(without proper discharge 100 ohm resistor). I turned back power to 1 1/2 watts to 4 swing a Tx tuning slug *redpaint on it. I let it talk on low key an when i speak it jumps to 4 or 5 watts on meter. When a mod has been done some people don't know that capacitors discharge and it will leak back current. Thats why on a Cobra/Uniden company radio the instruction properly call for a 100 ohm resistor to be placed in parallel with Capacitor near final on bottom of circuit w/ capacitor on top spkr side(i call it top its on spkr side bottom when radio is upright (super mod kit). Or replace jumper J1 to factory deadkey of 4 watts and see no swing in talk except to 4.5 watt slightly(factory tune)

Re-align the radio connecting the meter to the correct test points. If you re align the radio properly it should solve the problem.

Sounds like a "short" if it affects modulation + power. Try to inspect board with magnifier for solder bridges, and bad resistors. Change a few resistors near back with new ones. take out slugs near back finals like L10 or L20's not sure if its same as Uniden Pc68xl. but try that. I found turning L19 up to top gave great Modulation. clip D9, Sread coils 1/16 inch in back. Try to get lower carrier. Carrier does not mean anything much. Modulation is what you need. With less Carrier it may cool down that chip. It may be too wound out.

Adj VR1 next to VR2 they are in the center of the main motherboard. VR1 will allow you to adj the S meter in AM mode only.

First check voltage of IC 4 (mb 3756) pin #8 in TX mode should be 8.0 volts. Check the voltage of the center leg of Tr36 (Final) and of Tr38 (Driver) RX=13.78 volts DC, TX=4.5 to 6.5 volts..

without looking at it I'd say your voltage regulator unless you're lucky to just have a bad soilder

Hi and welcome to FixYa,

Initially, pls confirm that there is modulation (transmit audio). The reason I asked is if IC4 (TA7222P) is blown, then the radio would have no audio amplifier which would explain no receive audio. Additionally, the final and pre-final (TR15 & 15) gets is modulated TX+12 through D8 from T1 which in turn is the output of IC4 through C48.

Perhaps a schematic diagram may be of further help.

Good luck and Thank you for using FixYa. Happy Holidays.