No TX. Everything else works great. Changed final, driver,ic4 voltage reg,mode switch and a few others. No TX? TP7 and TP8 have no voltage? 0.11volts. I no it should be 12.8volts. Pa mode works. Also radio next to it will move the meter. Any help? Thanks, TJ
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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.
can you be more specific please ? TX meter or RX s meter ? check power supply voltage is stable, obviously on RX its essential to use a constant signal, of course, if you're doing it without test gear & just doing a quick 'tweak' comparing with a good know s reading off a known set - a s9 signal is usually the standard point they calibrate to. although on a CB set its not a greatly important. on TX you may first want to check RF power out is constant first, so a dummy load (in case aerial swr is changing while on TX) needs to be used. If actual RF power is changing, suspect drying electrolytic capacitors, these sets are 30 years old, so its not unlikely. dying driver or PA transistors or dry joints are also possible. or if everything checks out, possible voltage regulation probs, so check 2SA1012 AM reg isn't dying and the other ****** too. If you're lucky, its just the variable resistors giving up.
did you change IC4=TA7222AP ?
check input ic4 pin1 is it supply voltage?=12vdc.
check C48 couple cap,also check C39 replace
with a 25 watt and C42 also C45.
check R130 1.1 ohm. also R121 180 ohm.
if pin 1 on ic4 is low volt or missing,check TR23
maybe this will help some
You've replaced the pre driver, the drivers and finals. Set bias and still low output. The pre driver is the same transistor used as the tx exciter transistor on other pcb chassis on bottom of radio. Follow the gray coaxial wire from the pre driver on the RF circuit board back to the tx/rx i.f. Circuit board. The gray coax cable plugs right next to the tx exciter transistor on this pcb. Again it's on a different pcb all together and on the bottom of the radio and also. Replace it! And check for open bias or emitter to ground chip resistor next to it. You will get your drive back. Very few rigs have I seen the actual Finals blown in the mosfet or the bipolar version. Just the driver stage with a week pre driver and the SMT "pre" pre driver on the same RF pcb. And the tx exciter on the other pcb.
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.
D23 is the reverse protection diode and it shorting is the primary
reason your fuse blew. As you posted, power was applied in reverse,
hence other components would also fail. Pls check:
1. D22 - 9.1V Zener Diode
2. TR23 (2SC1096) 9 Volts regulator
3. D19 - 5.1 Zener Diode
4. IC4 (TA7222) - Audio Amp
5. TR14 (2SC2029B/10) - Finals
6. TR15 (2SC2028B/20) - Driver 7. Most capacitors over 220uF (like C123, C114 & C112)
Hope that this be of help/idea. Pls post back how things turned up or should you need additional information.