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. I'm contacting the radio owner for next steps. I'll update this post if I proceed.
