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Galaxy Dx-959 No Am Modulation/power/transmit

First off, MOSFETs cannot be tested the same way that one would test - let's say - a 2sc1969 bipolar transistor could be tested. Because MOSFETs need a turn of voltage threshold to even operate at. Apples and oranges.

Next, you used the service manual to set up the radio. For the most part, it will work. But when it comes to the final transistors and they are MOSFETs; then they are not adjusted the same way. Probably why you snapped them. The gate voltage must be adjusted for each; 3.65v for the driver and 3.9v for the final. This is done with the mic gain set at 0, any mode, and must be tested when keying the mic at the gate pin of each device.
**EDIT** with the biasing board left in the radio!!!!

I agree with Sonoma about replacing the AM Regulator or its companion part. If one hasn't failed then the other might have.
 
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since you have to order the part I would order Q55 also. and make sure you check the gate voltages at the final and driver so you do not pop any thing else as Robb suggested.
 
how do you guys go about measuring the voltages on the 520s? Where do use stick the negative prod on the meter to get the most accurate reading? Anywhere ground or somewhere specific?
 
a ground trace on the circuit board or a can that you can hook to. do not hook to the heat sink or the case of the radio.
 
Usually there will be a bare wire running from the 10.240mhz xtal and the xtal filter; you can to attach the neg lead there. If there isn't one; then I take a small hand grinder and nick the corners of those two parts just enough to remove the nickel plating. The brass underneath that plating can be soldered, so I solder a piece of resistor lead between those two points in there.

If you look at the back of the board near the driver and final, you will see what looks like a resistor with a white circle silkscreened on the board around it. There is one near the driver and one near the final. That is where you want to put the pos lead of your DMM to check the gate voltage for the driver and final. Or, just put the tip of the pos test lead on the MOSFET gate directly. Better to attach it to the top of each resistor - though, as this will allow you to have a free hand to make the adjustment w/o worrying about slipping off and shorting something out.
 
Yea thanks Robb. I actually ended up taking a couple pieces of thing speaker wire that were attached to each other and Tinning the ends for quick easy tap soldering it any small spot. Then just kept the leads surrounded and separated from each other and moved the quick solder around to each point i was testing. May seem like a hard way to do it but allowed my hands to be free to key the mic and turn the adjustments at will. Worked great!
 
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Ok so all the replacement parts came in today. I have a 2sb817 in at Q54 and used a 2SA1012 at Q54.

Now voltages are reading:

Q54:

B:13.03
C:.06
E:13.54

Q55:

B: 12.92
C: .06
E: 13.04

With new Q54 and Q55 I still have TX and Recieve on USB and LSB at normal wattages measured by a Yaesu YS-60. On AM I have no TX but can RX. Checked PA with external speaker and nothing. Checked Talkback turned all the way up and nothing.

So, No AM TX, PA or Talkback but perfect Sideband TX.

Any ideas?
 
Robb, I honestly feel bad that I didnt keep track so that anyone with this problem could look there as well :( . Since getting it running last night I did the whole Dual Final mod ( 3 IRF 520s now) and a couple other small things. I am currently fitting it for a heatsink and fan mount. Ive done so much work to this in the last day that I cant remember which part is which :( lol
 

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