Hojo and the case of the Powerless Alinco DR-1200T

Alinco DR-1200T repair

So, our ham club was using an Alicno DR-1200T for our local APRS iGate/Digipeater.       It failed a while back, and we picked up an identical radio as a replacement.  Well, the other day I grabbed the dead radio and figured "You can't kill what's already dead!" and decided to attempt a repair.

I confirmed that I could apply power and nothing would light up.  There was no current draw.

I disassembled the radio and poked around.  I confirmed that the internal power connectors seemed to be attached, and that I had power as far as a jumper near the on/off switch.  I removed the front panel, and discovered that the circuit board for the controls was pretty bad looking.  Alinco didn't go to great pains to clean the flux off this board prior to shipping.  I cleaned up a few particularly bad places, as I reviewed the radio.  Then I saw this FET.

Looking carefully, I could see that the FET at Q4 was cracked.  I pulled it off the board.

Sure 'nuff, this sucker is broken.  Looking at the schematic, this is a switching FET used with the power switch.

So, activating the power switch (SW2) completes the circuit for a voltage divider on the Gate to the FET.  R34 is a 120k resistor is acting as a weak pull-up to the gate.  When SW2 is turned on, R35 basically acts as a stronger pull DOWN on the gate.  The R34/R35 voltage divider pulls the gate DOWN to about 0.1 volts when S2 is thrown.  Pulling the gate DOWN allows current to pass through, so the FET must be a P-channel MOSFET.  

I believe the failed part is in the SOT-89 form factor, based on its dimensions.  It measures about 5mm wide by 2.5mm tall.  However, it's wider than it should be due to the crack.  Sure 'nuff, SOT-89 should be about 4.5mm wide and 2.5mm tall.

I don't have any P MOSFETs in my parts box in SOT-89, but I DO have a SOT-23 MOSFET. Looking at the specs, it can handle 12 volts.  Not sure about the current draw, but it's worth a test.  I figured I might wire in the FET with some wire-wrap wire or something, just as a rudimentary test to see if anything else was broken.

As I got started, I looked at the footprint of the two form factors  It looked like I could actually get away with sticking the SOT-23 onto the SOT-89 pads!  See the illustration below.

Here are the three pads with the failed MOSFET removed.   Note that the Drain pad is nice and long.

This is the SOT-23 FET installed over those pads.  It's not a perfect fit, but it seems to get the job done.

Well, it's all over but the testing.  Let's see if this radio will once again power up and hear APRS beacons.

Results

So, it looks like replacing this FET will bring the radio back online.  Taking a look at the Alinco web site,  you have to call them with part numbers in hand, and speak to a human (eww!).  There is a $10 minimum on parts, and a minimum $13.50 shipping charge.   Given that I can replace this radio with another from eBay for about $50, it seems a bit insane to spend $23.50 to get a FET.  Checking Digi-key, it appears I can find a pretty beefy FET in SOT-89 form factor for about $0.75.  I'll go that route, since I'm ordering from Digi-Key regularly anyway.   

For the record:

Alinco Part:  Q4:  Part Code: XE0011.   Part name:  "Chip FET, 2SK508K51T1"

Digi-Key substitute:  893-1236-1-ND

I'll update this blog when I substitute the part and confirm it works OK.

Alignment Notes

I went ahead and did an alignment on the radio, since I had it open on my bench, and I had been looking for an excuse to test my new (to me) Service Monitor.  I followed the steps as outlined in the service manual.

1. Corrected VCO P/D voltage (RX) from 2.37 volts to 2.04 volts.
2. Corrected VCO P/D voltage (TX) from 2.21 volts to 2.06 volts.
3. Corrected Frequency error from -600 hz to +20hz
4. Power output VR6 (Hi): Maxed at 26w.  In spec.
5. Power output VR7 (Lo): Improved from 3.5 to 5.0 watts.
6. RF Power Meter: Adjusted from 4 segments to 3.
7. Deviation: Adjusted from 4.7khz to 4.9khz.
8. MIC gain: Adjusted from 4.9 khz to 4.2 (SUPER sensitive)
9. Discrimination adjustment: Adjusted to maximize.  Some improvement.
10. Sensitivity: No improvements with adjustment.  -117 dbm. (spec is -123).
11. Subaudible Tone: Adjusted from 0.8 to 0.7 deviation.
12. S-Meter: Adjusted to -104db (3 dBuv).
13. Squelch Sensitivity adjusted to -113 db (-6 dBuv).