Yaesu FT990

[Rondallb]

Hi all. I have an FT990 that I am trying to get working. I have downloaded the owner and the technical manuals and have the schematics printed out. When I got the unit, it appeared completely dead. I found that 2 wires in the power supply had become disconnected. Once I got the power supply functional, I checked the output of the regulators and found that the 13.8 volt supply was at 14.1, the 5 volt output was at 5.5, the -12 volt output is at -12.5 volts, and the 12 volt supply is at 12.3 volts. When I reconnected everything, the 12 volt supply dropped to 0.3 volts. I determined that the display board was dropping the 12 volt supply and replaced capacitor C7014 because I found that resistor R7061 was ok but the 12 volt connection at pin 3 of JP7001 was shorting to ground. After replacing the cap, reconnecting the display to the motherboard caused the 12 volt supply to drop to 1.7 volts. I disconnected the transformer end to L7001 and found the voltage there to be 12.3 volts also. I reconnected L7001 and removed transformer T7001 and found that the voltage at the connection between L7001 and R7051 was 10.5 volts and the voltage at pin 3 of JP7001 was also 10.5 volts. Any ideas about what is causing this problem? I am not sure why this is the case. I know that both the display board and the Switch A board have issues. Any assistance would be greatly appreciated.

Thanks,

Ron

 

[(*steve*)]

Perhaps you can attach the circuit diagram so we can see what you're talking about

The initial power supply voltages sound close enough to be correct. I would not initially be alarmed by half a volt or so.

 

[shrtrnd]

Check your voltage regulators (they come AFTER the transformer). Sounds like voltage regulator problem, or a power transistor failure, dragging your voltages down.
Good luck