We seem to have a fleet of Alinco DR-1200T's kicking around. The local APRS expert uses them, along with a TNC, for APRS nodes in the area. I've repaired, recapped, and aligned several of them over the years. A new one (to me) hit the bench today.
I popped on the service monitor and ran a few tests. It generates no audio either to speaker or headphone jack. However, I do see the S-meter doing sane things as I adjust my service monitor up and down, so it's receiving, but not generating audio. TX tests seemed good. I fed a tone into the Mic in and threw the PTT. It generates a signal of the appropriate strength - about 20 watts or so.
So, something's amiss with the audio out. My first guess is that there might be something wrong with the final audio amplifier. That seemed like a reasonable place to begin. I could check the AF-in and AF-out to see what I might see.
Consulting the block diagram:
It appears I'm looking for IC4. I see an "AF Mute" signal there too, which might be suspicious. I'll look into that.
Reviewing the (blurry) schematic:

It appears that IC4 is a "uPC 1241H". With a few googles to double-check my assumptions and reading of the schematic, it would appear that
- Pin: AF In
- Pin 2: Bypass cap?
- Pin 3: Unknown
- Pin 4: GND
- Pin 5: GND
- Pin 6: AF Out
- Pin 7: Unknown
- Pin 8: Vin
Looking at the board layout, I located the chip:

Busting open the case, I found it against the rear heat sink:

Starting with the multimeter, I read 11.6 volts on VIN. That's down a bit from the 13.6 I'm feeding it. The current draw showing on my bench multimeter doesn't seem unusually high compared to another unit I had, nor is the chip particularly warm.
So, I grabbed an amplified speaker, and touched it to Pins 1 and 4 (AF In and Ground). Sure enough, I hear static. Turning the volume knob on the radio, static volume changed. Turning squelch, static went away. Seems reasonable.
I hooked my service monitor back to the antenna port and sure 'nuff, I'm hearing tone on AF In. Volume and squelch continue to work as expected. Switching my probes to AF Out, I get nada.
I looked at the remainder of the output chain, wondering of something might be pulling the AF Out down to ground or something. I don't see much. C90 seems to connect AF Out back to Pin 7. Probably some kind of feedback thing. C91 is along the way to the remainder of the output chain.
I pulled out my ESR meter and checked all of the caps in the neighborhood. Though not highly scientific about it, all showed an ESR of about 1.5 ohms or less. Nothing stuck out as a dead short or anything.
I'm reasonably content that the amp chip is the problem. If I replace the chip and all is well, I'll probably recap the entire radio, just on principles, but I'd rather not change too many variables at once.
The chip is, of course, long out of manufacture. There seem to be plenty of options on eBay for $20, shipped, or less.
Two weeks later
The replacement amp chip arrived and I swapped it in.
There was an "improvement" when I tested. Whereas, before swapping, I could hear static on the input pin when attached to an amplifiied speaker, the output pin was silent. With the new chip installed, the output pin was definitely making louder static. Progress!
I then plugged into my service monitor, and observed the 1khz tone on the input pin (1), but the output pin (6) remained loud static. Referring to the schematic, the first capacitor after the output pin was C91.
Though I had spot-checked the capacitors in this area with my ESR meter, perhaps I missed it somehow, and it was bad. I pulled out C91 and immediately, I got very loud tone on the output pin. Checking C91, sure 'nuff, it was reading open.
I replaced C91 with a fresh cap, and tested the radio. I was providing good, loud audio now. Yay! I suspect that the cap failing probably lead to the failure of the amplifier chip.
Though I had loud audio, it was a bit "fluttery" sounding. My plan has been to recap the radio after getting the audio to work, so now seemed like the time. As I went through, I ended up finding 3 capacitors that had failed in a "leaky" kind of way.
I cleaned up the board as I went through the recap process. I also pulled the inductor (L11) whose footprint you see on the image above. Goo had leaked under it, so I wanted to make sure it didn't corrode that inductor.
Interestingly, it was the three green Sanyo caps that leaked. A few of the others looked a little swollen, but they didn't fail outright.
I buttoned the radio back up and ran it through tests on the bench. It was more on-frequency when transmitting than it had been prior to the recap. It was close enough that I didn't fiddle with it. The audio quality was decidedly better than before the recap.
Fixed. It's ready to go back into service as one of our regional APRS Digi / iGates.