?
My BS170s failed in my QMX+. ?There was no power output showing on my Power Meter, the QMX display contrast had changed, the QMX case was getting warm, and the battery voltage icon on my QMX display was flashing. My supply was 12v limited to 2 amps.
?
Here are my steps to diagnose and repair my QMX+. I will add pictures, I hope groups.io will keep them.
?
1) Power off and remove the supply and all connections. Remove the case top, bottom, and rear, and remove the washer and screw holding the BS170s to the PC board.
?
2) Connect a dummy load and power supply.
?
3) Power up and quickly check the temperature of Q503, Q504, Q505, and Q506 with your finger and if you have one, a digital infrared thermometer gun. I checked the temperature with my finger first, and two of the BS170s were too hot to touch and two were room temperature. The two hot BS170s measured 94F and the other two measured 78F.
?
?
I powered my QMX down after about 20 seconds, because I did not want it getting too hot. After a cool down period, I powered it back up to make other measurements. The temperatures of Q507 and IC503 ?were close to ambient. I measured the voltages on Q503, Q505, Q507, Q513, and Q515 to be as follows:
Q503 and Q505
Source 2.35v
Gate 0v
Drain 0v
?
Q507
Source 2.44v
Gate3.3v
Drain 5.9v
?
Q513
Base 0.4v
?
Q515
Base 3.25v
?
4) Because of its high temperature and obvious high current draw and after powereing off and disconnecting the power supply I cut just the Source lead of Q503 and Q505. I powered up and measured the voltages on Q507. No devices got hot.
?
Q507
Drain 11.3v
Source 11.3v
Gate 6.7v
?
5) I powered back down and remove the PS. I then cut all the leads on Q503 and Q505 to remove those Mosfets. I cleaned the board with Q-tips and denatured alchohol. To remove the leads from the circuit board I held the lead on the front side with needle nose pliers and heated the back side with my 140W weller gun. When it became loose, I pulled the lead out thru the front. While heating from the back side I sucked the solder out of 4 of the 6 holes with a solder sucker. The remaining two holes were connected to the ground plane and I didn¡¯t try to clean them out.
?
6) While heating the one hole not cleaned out from the back side, I started a new BS170 into Q503 and Q505. I continued heating while lowering it down to a proper height. I then folded it over, checked the fit, and soldered all three leads of each Mosfet. I affixed all 4 Mosfets to the PC board with the screw, washer, and nut.
?
7) I soldered and trimmed the leads on the backside, and inspected them. After connecting back the PS and dummy load and powering on, Q503, Q505 and Q507 measured as follows.
?
Q503 and Q505
Drain 0v
Source 11.3v
Gate 0v
Q507
Drain 11.3v
Source 11.3v
Gate 6.7v
?
I measured the power output to be 3 watts on 20m. I also ran the PA test which was as expected.
?
8) I reinstalled the QMX in its case, connected the dummy load and PS, and ran the SSB calibration, which completed successfully.
?
Hope this helps!
NQ5P
?
?
