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Dx2547 issues
- Thread starter Spec2079
- Start date
Whoa! That's odd. The "silver bridge" is a test jumper. Carries current from the modulator circuit out of the inboard-most fork, and out to the final transistor through the center one, and to the driver transistor out the left-most one.
Pulling it out should kill any and all transmit power. If you're seeing wattage with it pulled out, someone has bypassed it.
TP7 is the output to the final transistor from the test jumper. If you see voltage with the jumper pulled out, there is a bypass wire in that radio, probably on the solder side of the pc board. This will make it impossible to read bias current on the driver and final. The trimpots VR10 and VR12 serve to adjust the DC current drain from the driver and from the final transistors with no drive, set to sideband with the mike gain at zero. But you have to break the circuit to the driver and the final and insert a current meter between two test points to read current. If there is a bypass wire under the pc board, you can't read the current on a meter. Not voltage, current. You measure voltage by putting your meter probes in parallel with the circuit you'll measure. To measure current, you break a circuit and place the meter across the break. Circuit current passes through the meter.
And if you're alread hip to this part of the picture, don't be insulted. This will be news to some of the readers who see this post.
73
Pulling it out should kill any and all transmit power. If you're seeing wattage with it pulled out, someone has bypassed it.
TP7 is the output to the final transistor from the test jumper. If you see voltage with the jumper pulled out, there is a bypass wire in that radio, probably on the solder side of the pc board. This will make it impossible to read bias current on the driver and final. The trimpots VR10 and VR12 serve to adjust the DC current drain from the driver and from the final transistors with no drive, set to sideband with the mike gain at zero. But you have to break the circuit to the driver and the final and insert a current meter between two test points to read current. If there is a bypass wire under the pc board, you can't read the current on a meter. Not voltage, current. You measure voltage by putting your meter probes in parallel with the circuit you'll measure. To measure current, you break a circuit and place the meter across the break. Circuit current passes through the meter.
And if you're alread hip to this part of the picture, don't be insulted. This will be news to some of the readers who see this post.
73
Well, something is completing a circuit with the mirror board removed if you still see voltage at TP7 and/or 8.
73
73
That's what I was wondering myself, lol! If that was the case, shorted final/driver would jump to mind.
Test final and driver. One of them is almost certainly toast.
When you replace the bad ones, don't just turn it on and try it. May be other damage that can cause the replacements to go up in smoke.
With the mirror board removed, see if you get 12.5v or better at TP9 in SSB. If so, then go to AM and check again in TX w/ no modulation. Should be between 5.4v and 6v. If you are not getting around those values, you will need to go check the regulator and its associated driver (darlington pair). I am guessing that regulator is a resistor after that kind of abuse, so expect issues there.
After the regulator gets the attention it needs, with the mirror board still removed, measure the gate voltage of the replaced mosfet. If you can adjust the gate voltage below 3v, you are good. Turn it down as far as it goes so that when you put the current meter across the test points for the actual bias measurement, they are not immediately turning on hard.
Don't lose the insulators under those mosfets, and don't lose the little ceramic insulators for its mounting screw, they are very necessary. Continuity between the mosfet mounting tab and the heat sync will present the same symptoms, dead short, since the tab is connected to the drain internally.
I hope you have some heat sync compound on hand, you are going to need it.
When you replace the bad ones, don't just turn it on and try it. May be other damage that can cause the replacements to go up in smoke.
With the mirror board removed, see if you get 12.5v or better at TP9 in SSB. If so, then go to AM and check again in TX w/ no modulation. Should be between 5.4v and 6v. If you are not getting around those values, you will need to go check the regulator and its associated driver (darlington pair). I am guessing that regulator is a resistor after that kind of abuse, so expect issues there.
After the regulator gets the attention it needs, with the mirror board still removed, measure the gate voltage of the replaced mosfet. If you can adjust the gate voltage below 3v, you are good. Turn it down as far as it goes so that when you put the current meter across the test points for the actual bias measurement, they are not immediately turning on hard.
Don't lose the insulators under those mosfets, and don't lose the little ceramic insulators for its mounting screw, they are very necessary. Continuity between the mosfet mounting tab and the heat sync will present the same symptoms, dead short, since the tab is connected to the drain internally.
I hope you have some heat sync compound on hand, you are going to need it.
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