Hojo and the Case of the Deviated Spectrum

 

 

 I was working on another of our passel of Alinco DR-1200T's.   This one is our spare radio which I recapped, aligned (as best I could) and set aside in case our primary APRS radio failed. Well, a different radio failed, and we attempted to use this one in its place.  Unfortunately, it didn't work.

The presented problem was "Won't TX".  I brought it into the shack and ran it through a bevy of tests.  It was transmitting and was pretty much right on frequency.  I ran a tone through it, and it made audio.  I chalked it up to "testing failure" when we tried to use it, and asked our APRS guru to give it another go.

He did, and it still didn't work.  This time, he was able to give me a little more information.  While he attempted to transmit, he had a Baofeng on the APRS frequency, and was listening for the tone.  He noted that it WAS, indeed, transmitting a tone.  However, in his mind it seemed a bit short.  He sent me a short video clip.  When I heard it, It definitely sounded like the tones were WAY off.  

So, back to the bench it came.   I hooked it up to my service monitor and confirmed I could still get a tone through it.  I busted out my function generator, and had it transmit tones from 300hz on up to 5khz.  It sounded OK to me.  I was confused.

After noodling on it for a few days, I had an epiphany.  What if the deviation is WAY off?  Perhaps my service monitor is being very forgiving with a very wide, or narrow, deviation and I still hear the sound, but when it's transmitting, it's not working well with APRS receivers?

I commenced some additional testing.  I've got an old IFR 1200S service monitor, but I've never trusted the deviation meter.  It was time to try something new.  I researched using the "MIN/MAX" method on a spectrum analyzer.

Well, the results were pretty striking.  The radio's deviation was  10khz!   I didn't manage to get a screen shot, unfortunately.  It would appear that my service monitor's deviation meter is WAY off.

So, I decided to adjust the deviation without using my service monitor.  Here's what I had to do.  First, the alignment procedure calls for a 1khz tone with a P2P voltage of 25 millivolts for deviation alignment, and 4 millivolts for the mic alignment.  My service monitor doesn't go that low, and it doesn't really provide for reading the voltage.  So, I opted to use function generator.

  

 This crusty old Instek does the job.  The amplitude of the signal was still way too high.  So, I had to attenuate it even more than the function generator's internal attenuators could do.  I bodged together a line of attenuators to get the tone's voltage down where it was needed:

 That was sufficient to get the P2P voltage down to what I needed.  I tested it with my oscilloscope:

 

Now, confident that I was feeding the signal in according to the alignment procedure, I proceeded to do the MIN / MAX method to measure the deviation on my Spectrum Analyzer.  I pumped the radio's RF output into a tapped dummy load, and fed the tap into the analyzer.

After some trial and error, I was able to measure the deviation.  This is a screen shot after I finally got the trim pot adjusted such that the deviation was the required 4.9 khz, according to the service manual.

 

 

 Once it was properly aligned, I went ahead and plugged it into my IFR 1200S.  The deviation meter was reading 3khz, with all the same settings.  That would explain why my FM Deviation alignments always seemed off.  The IFR must have a problem in the deviation meter.   That also explains why an APRS tracker I built, years ago, never managed to work.  I was using the IFR to adjust the deviation.  Whoopsie.  I guess I'll have to use this method from now on.

So, I ended up fully aligning the deviation on this radio using MIN/MAX.  The proof will be in the pudding when we hook this up and try to beacon with it.  I suspect it'll be MUCH better, since the deviation is now less than 1/2 what it was.   While I was at it, I readjusted the deviation on the other of these radios I had on hand.  The deviation wasn't AS far off - 6.9 khz where it should have been 4.9, but i was definitely worth doing.

I'll hand both radios back off to our APRS guru, and see if they're working better now.  I'm hoping that he won't have to readjust his TNCs to compensate for the changes in the radios.   However, these are "in spec" now, so if the TNC needs adjusting, it's probably best to do it there.

 

Hojo and the Case of the Silent DR-1200T

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.