Picking up last time where we left off, I bought a mid-1980s Uniden radio off of eBay for around $10. After a quick smoke test, I found that there was no transmit audio or receive audio. I swapped out what appeared to be a shorted 7808 8V regulator and a TDA1905 audio amp chip. The receive audio came back to life, but I was still in the dark with no transmit audio. Still, a step in the right direction.
Sources for Problems
First and foremost, I need better test equipment, and what better way to obtain this equipment than to make your own.
The best piece of test equipment is a signal tracer. The one I'd like to build has support for both RF and AF inputs. Here's the schematic below:
This will help us trace where the microphone audio (if there is any) is dying off at. Once we pinpoint where it's at, we'll replace that component.
In the schematic above, you can replace the BC-series transistors with a 2N3904/2N3906 pair. D1 can also be replaced with a Schottky diode as well if you don't have a germanium on hand. In total this entire tracer runs on only 2 AA batteries, or even AAA batteries. Since this will draw a tiny amount of current, expect the battery life to be beyond excellent. R1 determines the gain of the entire circuit, or how sensitive it is to incoming signals.
In doing some digging, I have found 2 potential issues.
#1: Coupling Capacitor
In the circuit, there is a 470uF electrolytic capacitor between the modulation transformer and the TDA1905. This is a coupling capacitor between the amplified microphone audio and the transformer. If this is bad, this is DEFINITELY why we aren't getting any transmit audio. Despite it looking fine on the outside, we could be looking at a capacitor that has a high ESR or the internal electrolytic fluid has dried up.
The easiest way to replace this is just getting another capacitor. While I'm at it, I might replace all of the electrolytic caps just for the sake of longevity. Unfortunately you can't really test electrolytics with a DMM, you'll need to use a signal tracer and listen for the audio to pass through. Or buy a capacitor tester.
#2: Modulation Transformer
Less likely (like 2% likely), but still a possibility. If the transformer becomes open on any of the windings, audio will not pass. The only way to fix this is to rewind the transformer or find where the transformer is open and resolder it.
Again, I don't think this is the problem but it's a possibility.
#3: Complete Recap
The obvious if #1 or #2 doesn't work. There's an old saying that goes one rotten apple spoils the bunch - well one bad capacitor can spoil the bunch too! It's not too difficult to recap a radio given that you know how to trace, desolder, and solder. If you don't have time to mess with the signal probe then it's probably best to just recap the radio at this point. Besides, you'll be doing you and the radio a favor!
Tune Up
No doubt this thing will need a tune up after I'm done with it. And even if it doesn't, it's not a bad idea. I now have the tools to do it (signal generator thanks to the tinySA) and it would give me some practice on realigning and repairing a CB. The fact that the realignment procedures are readily-available on CBTricks makes this radio even better for practice.
This radio will not be a candidate for frequency conversion. The SM5124 PLL chip is only useful for the 40 CB channels and can not be moved out of the 27MHz band. To do that you'd have to re-engineer the entire PLL circuitry, and I'm not willing to do that. So legal beagle radio is what this will remain :)
Diagnosis?
Capacitors.
I went through last night and replaced each and every electrolytic cap I could find. Yes, the replacements were the cheap ones from Amazon but they work well and do just what I need them to. Most if not all of the old caps were waaay out of spec and it surprised me that this radio even turned on. However, after replacing the old capacitors with new ones, I had transmit audio. And very good transmit audio at that.
So if your radio isn't working right, consider an entire recap. Chances are it just might be an out-of-spec capacitor throwing the entire circuit off.
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