Measure the supply voltage at each PA module at each of the four screw terminals, easily done with the top case off. It should be 48V - 50V.
If any one supply is at 0V then the later unit "undocumented" chip fuse has blown. Later Quadras have a 15A chip fuse on the back side of the Line Filter unit, these are not shown on the TS which seems to have never been updated. These fuses do blow with age etc. and not necessarily a PA fault.
With input and output terminated, put the Quadra into "Local" and ground the PTT line to bring the PA's into the quiescent state, measure the gate bias directly on each FET, should be around 3V ± something, note any large discrepancy to a particular device.
Check C5051, 2, 3 and 4 on each module - these are chip tantalum caps across the bias supply for each FET after the setting pot and prone to failure. They short the bias to ground thus holding that device off. If a bias line is at 0V lift the associated tantalum cap and see if bias is restored.
The caps at 4 above are changed to ordinary aluminium electrolytic types in later production units and usually changed on service for reliability, any miniature 10µF/16V device will do and best anchor it with a blob of something. The idle current must be reset.
If supply volts are there and bias too, check each PA for quiescent current by inserting an ammeter in each supply line at the board, should be ≈ 400mA; use the method in 3 above to bring the Quadra to TX.
An out of spec quiescent current will denote one (about 400mA) or both (no current) of the FETs on that supply line has failed. They are best changed as (very expensive) pairs having the same batch number on both devices and quiescent currents re-set of course.
Originally posted on the YAESUQUADRA Yahoo! Group, 23/08/2011.
Copyright © 2011 R. Notton G3ZOE. Page created by A. Farson VA7OJ/AB4OJ. All rights reserved.
Last updated 27/11/2015