Measure the supply voltage at each PA module at each of the four screwterminals, 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 fusehas blown. Later Quadras have a 15A chip fuse on the back side of the LineFilter unit, these are not shown on the TS which seems to have never beenupdated. These fuses do blow with age etc. and not necessarily a PA fault.
With input and output terminated, put the Quadra into "Local" and groundthe PTT line to bring the PA's into the quiescent state, measure the gatebias directly on each FET, should be around 3V ± something, note any largediscrepancy to a particular device.
Check C5051, 2, 3 and 4 on each module - these are chip tantalum capsacross the bias supply for each FET after the setting pot and prone tofailure. They short the bias to ground thus holding that device off. If abias line is at 0V lift the associated tantalum cap and see if bias isrestored.
The caps at 4 above are changed to ordinary aluminium electrolytictypes in later production units and usually changed on service forreliability, any miniature 10µF/16V device will do and best anchor it with ablob 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 bestchanged as (very expensive) pairs having the same batch number on bothdevices and quiescent currents re-set of course.
Originally posted on theYAESUQUADRA Yahoo! Group, 23/08/2011.
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Last updated 09/10/2022