A bad Q66 raises the possibility that Q67, the transistor that drives the base of Q66 will also be damaged. Q67 is a surface-mount power transistor. It has a large (relatively) foil pad directly under it where the collector lead is soldered.
This one has visible damage.
It won't always offer a visual cue when it fails. If replacing Q66 doesn't restore AM transmit carrier, Q67 may be the culprit. If the base voltage on Q66 never falls more than 6/10 of a Volt lower than the emitter voltage, this is a sign Q67 was damaged.
Removing Q67 always seems to damage the foil pad under it. Could be our crude tools are to blame, but if the part gets too hot when it breaks down, that can destroy the foil pad under the part.
We replace Q67 using a TO-220 PNP transistor with the plastic mounting surface. Can't remember the part number right now. Same part shows up as the 9-Volt regulator in newer Galaxy/Connex radios with EPT3600 boards. Saves the trouble of putting an insulated washer under the new part. We put a teflon-insulated wire on the new part's leads. They will reach to Q66 and to the main circuit board and take the place of the croaked SMT part.
It mounts in a random extra hole in the heat sink near Q66. The brown wire (collector) and yellow wire (emitter) go into the gap under Q67.
The collector wire (brown) from the new Q67 lap-solders to the center pin (collector) of Q66. The emitter wire (yellow) goes to the base lead of Q66, the lead nearest the rear of the radio.
The blue wire from the base lead of the new Q67 will go to the component side of the main pc board.
It goes to the center leg (collector) of either Q65 or Q69. Can't remember which of those this one is.
So far this has done the job when both Q66 and Q67 both fail.
And when the foil under Q67 is damaged, you can't just replace the original part where the old one went.
Something we have adopted as standard policy when servicing this type radio is to install a separate fuse to protect the radio circuit board. You wouldn't hook up a 4-Amp mobile radio to a 25-Amp power supply with no fuse, right? But that's what the factory put inside this cabinet. A fuse for the circuit board could minimize the damage when Q66 fails, or when IRF520 transistors fail as a short.
We call it cheap insurance.
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