[Yaesu] Help re ps problem with FT-980

[Dr. Gerald N. Johnson]

In the manual I down loaded from K04BB that power supply schematic is 
shown on pdf page 133, printed manual page 93. The diode D03 HZ6C1 is 
the reference for the emitter of the comparing transistor Q03 that 
drives the driver transistor Q02 while the FET operates as a constant 
current source from the negative supply as the collector load of Q03.

Often when the main pass transistor in regulator circuit fails (most 
often when it fails open) the driver transistor melts trying to carry 
the load. In this circuit as in any closed loop controller, every 
component is suspect, especially the zener reference and the negative 
supply. As well as the comparing transistor since it gets driven hard 
when the voltage goes that high.

When replacing the pass transistor make sure it has a very thin coating 
of heat sink compound to get the best heat transfer to the heat sink. 
You just want enough to fill in microscopic irregularities in the 
surfaces, not enough to squeeze out. Its better than air, but not as 
good as solid metal so you don't want thickness to limit the heat transfer.

73, Jerry, K0CQ

On 4/4/2014 2:31 PM, Fern wrote:
> I have a friends FT-980 transceiver here that has an issue with the 24 volt power supply. It's putting out 42 volts and can't adjust the voltage. Q1 main pass transistor was shorted out and did replace same. Unfortunately there isn't any voltage references marked on the schematic. Would appreciate corresponding with anyone who has serviced same model or someone who has the reference voltages.
> Best of 73 from Fern   VE7GZ
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