Even MacGyver has an off day:
So my friends nicked named me MacGyver because I can rig up just about anything. Well even MacGyver has an off day. This was evident when I reversed the polarity of the 12V power feeding my Azden PCS-3000 2M FM Radio and I saw smoke exiting the back of the radio's enclosure. I immediately removed power from the radio.
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I switched polarity checking to make sure it was right this time then applied power to the Azden PCS-3000 with my finger on the power switch. This time the radio powered up with thankfully no smoke!
Where's the sound?
After fiddling with the volume control and squelch control then checking to make sure the slide switch on the back of the radio was set to internal speaker, I discovered that no matter what I did I could not get any sound from the radio. The front display was working correctly, I could change frequencies and I could tell from the signal indicator that it was picking up broadcasts but I could not hear anything from the speaker.
Time to figure out what I "broke" by reversing the direct current polarity, first you must remove the detachable control unit. This is done by loosening the knobs on each side of the radio.
Organization is important!
When disassembling a radio, I carefully place the parts into two bins, one bin for the larger parts and a smaller bin for the screws.
Remove the control unit from the radio:
Pull the control unit strait forward, the control unit attaches electrically to the body of the radio by an edge connector.
Remove the mounting rails
There are two mounting rails, one on each side of the radio. Two counter sunk Philips screws hold them in place. Remove the screws then pull the rails off.
Top cover Removal:
Remove four screws, two on one side and two on the other; just above where the mounting rails were installed. You should now be able to remove the top cover.
Bottom cover removal:
The bottom cover is held in place with four screws, one at each corner of the cover, simply remove the four screws and pull the cover off. The speaker is attached to the bottom cover, you will need to disconnect the speaker from the printer circuit board.
Upon careful examination I found that some sort of component, possibly a polarized capacitor that bridged the 12 volt input power connection, had exploded. I also noted that the diode across that bridged the 12 volt input power connection had become unsoldered. I removed the broken pieces of the capacitor and re-soldered the broken connection to the diode.
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I could not find any other charred or broken components with a visual examination. I know that re-soldering the diode that bridged the 12 volt input power connection would not resolve the problem. Next step is to look at the schematic diagram included with the PCS-3000 manual to trace the circuit paths the speaker takes back to the audio amplifier.
First circuit board:
The speaker connects to a small circuit board mounted on the bottom side of the chassis right by the SO239 external antenna connector. It is labeled "PA" in the schematic which I surmise to mean the RF Power Amplifier board. Upon further examination, it looks like the speaker does not connect to any active components on this circuit board but just acts as a pass through to another circuit board. In the picture I drew a red box around the PA circuit board and circled the external speaker connections.
The destination circuit board:
The audio output connection leaves the PA printed circuit board via a red and black wire then connects to an external speaker jack and a slide switch that allows you to choose between an internal or external speaker. Finally it connects to the RX or Receive circuit board via a green and black connection. In the picture I drew a red box around the circuit board and circled the speaker connection.
The audio amplifier circuit:
Upon further examination, the speaker connection terminates at the output pin 9 of a Toshiba TA7222AP 5.8 Watt Power Amplifier integrated circuit, this was a very common part used in the automotive car stereo, CB,and mobile ham radios in the early 1980s. I circled the TA7222AP location on the RX circuit board.
How can I be sure the TA7222AP was the defective part?
I fired up the old Oscilloscope, turned the radio on then connected the ground connection of the Oscilloscope lead to the metal chassis. I then touched pin 4, the audio input pin, of the TA7222AP integrated circuit with the scope probe and noticed an audio signal. So if an audio signal is going into TA7222AP and not coming out there was a good chance that this integrated circuit had been compromised from the reverse polarity input voltage. I also noticed in the schematic that this integrated circuit connected directly to the 12 volt input DC power making it very susceptible to damage from reverse polarity.
First remove power from the radio, and then remove the screws holding the RX printed circuit board in place. Also note that one screw holds the heat sink tab of the TA7222AP to the chassis of the radio.
TA7222AP capacitor placement:
You should be able to turn over the RX circuit board without removing any of the cabling. Make a note of how the small electrolytic capacitor connects to the TA7222AP before starting the de-soldering process. This capacitor is polarity sensitive and needs to be connected in the same manner as it was removed.
First make a note of the TA7222AP integrate circuit orientation so that you put the replacement in the same way as the defective component was removed. Carefully use solder wick to remove the solder from the pins holding the TA7222AP integrated circuit to the circuit board. Once all of the solder is removed from the pins slowly rock the TA7222AP back and forth until it becomes free and you can remove it from the circuit board.
The replacement TA7222AP should have come in a small package with the pins of the integrated circuit stuck in antistatic foam. Sometimes parts suppliers wrap the component in tin foil as a means of preventing static.
Do not touch the pins of the replacement TA7222AP when removing it from the antistatic foam. Carefully seat the replacement TA7222AP in the same orientation as the defective integrated circuit. Solder all integrated circuit connections to the printed circuit board using a high quality rosin core solder for electronics use.
TA7222AP capacitor soldering:
Re-solder the small electrolytic capacitor to the proper pin of the TA7222AP on the circuit board.
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Prepare to test radio:
Re-seat the RX circuit board but do not yet screw it back in place. Connect the speaker wires to the proper terminals on the PA circuit board. Connect the control unit to the front of the body of the radio. Connect 12v power, making sure to double-check the polarity. Connect the microphone to the front and connect a suitable antenna to the external antenna SO239 connector.
Powering up the radio:
Apply power, it you replaced the TA7222AP correctly you should immediately hear the atmospheric hiss from the speaker, if not check the volume control level and make sure the squelch is not set too high. Also check that the speaker output select switch is set to Internal. Verify all aspects of radio operation.
Buttoning up the radio:
Assembly is just in the opposite order of how you took the radio apart. Remove power and the front control unit before the assembly process. First attach the RX printed circuit board to the chassis with the provided screws, make sure you also attach the heat sink of the TA7222AP integrated circuit to the chassis with the required screw. Make sure the speaker is properly connected to the PA circuit board then install the bottom cover with four screws. Install the top cover with four screws then attach the rails to each side of the body of the radio with two countersunk Philips head screws. Finally attach the control unit to the body of the radio. Secure with two clamps that hold the control unit to the main body by turning the knobs until finger tight.
Repairing an Azden PCS-3000 radio after a mishap is not that difficult; it just takes a little time and effort with the added benefit of getting to better know you radio!
Who Writes This Blog?
John is an IT professional from Cleveland, OH who enjoys amateur radio, ham radio, metal detecting,
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