I bought another CB off of eBay that was a heck of a steal at about $10 or $15 shipped. It's a late 80s/early 90s Uniden CB radio. It's very similar to the PRO510 and PRO520 line of modern-day radios. It's quite basic - no ANL, no PA...just volume, squelch, and 40 channels.
The Problem
Because it's not a normal day in my life without problems...
When I first turned on the radio, I was greeted with...silence. It was because I didn't have the mic plugged in, right?
Yeah, no.
No matter what I did, external or internal speaker, mic plugged in or unplugged, I got no sound. My first guess was the electrolytic capacitors but they all tested and looked good. So no, not those.
Next was the audio chip, a TDA1905. This is a 5.5W audio chip with one side of the DIP package assigned to ground. So off to probing I went.
Diagnostics
My first diagnostic step was to check if the Vcc pin and ground were shorted using the diode/continuity setting. That returned fine, OL on the multimeter. No direct path.
However, things were different when I tested the output pin to ground. I got a solid beep - zero ohms. A connection that shouldn't be there. That's where things began to go wrong.
I then checked the output power on the radio. A little over 4 watts, not bad, but it was dead keyed. No matter if I tried talking in the microphone, I couldn't get it to modulate. Another clue that the chip was bad. Finally, I got a burning hot sign - the chip's 7808 regulator was absolutely burning up! On transmit, I noticed that the regulator's metal tab was scalding to almost untouchable, and it took forever to cool back down. Most of the time it wouldn't even cool off. This was starting to get strange, but it all added up.
Just to be sure, I ran an amperage test. I wanted to see how many amps this radio would pull. Since it didn't have a fuse, I wanted to see the raw draw. I saw on YouTube that most radios like this one with audio problems blew fuses due to pulling too much current. Mine was the opposite, pulling too little current.
On receive, the PRO500D should pull about 200 to 300 milliamps at standby, almost 2 amps at full S9+ signal. I only got 92 to 150 milliamps, even at full signal. On transmit, I only got about 850 to 900 milliamps, which was good - if there was no modulation. Without a doubt, at one point, the radio could have pulled too much current and internally blew the audio chip.
Conclusions
100% without a doubt the audio chip. So I'll need to purchase a new TDA1905 and a 7808 regulator, just to be safe. Removing both of these shouldn't be a huge undertaking, and could be done within an evening.
Not many places sell the TDA1905 chip, but a select few do. I bought mine from Futurlec in Thailand, and I bought a quantity of 5. It arrived about 2 weeks later but overall I'd buy again from them. It does take patience though.
This just goes to show you what you can do with a multimeter. You don't always need an oscilloscope, a frequency generator, a spectrum analyzer...all those fancy tools aren't always necessarily needed to test radio faults.
The Result?
You'd be surprised. As was I.
I was actually 100% on the money! When I replaced the faulty 1905 chip, I went ahead and added in a 16-pin DIP socket, so in case this happened again I wouldn't need to go in and desolder the chip again. I found that using solder wick and a ton of flux did an EXCELLENT job and I'll most likely use it in the future for other projects.
So, throwing in a new regulator and audio IC worked on the first try - on receive, though. On transmit, I was still stuck with a steady carrier. No modulation, nothing. So what else could it be?
Where's THIS Problem At?
Normally, a microphone would be the main culprit, and that was the starting point for me.
I was given a basic Uniden mic in a bunch of radio stuff I had received (The same stuff that had my Motorola SM50 and Maxon SM4450). Just to test it, I put it on the Cobra 21LTD I bought in addition to the Uniden. I got modulation, but it was extremely weak. The S meter never reached full wall to wall on audio peaks, so something is definitely up. I mean, the Cobra has Dynamike cranked all the way up to 11!
So the only problem to diagnose now is to troubleshoot the transmit stages and/or the microphone. Stay tuned, as always, for an update!
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