Power Audio Section - RAM 1500 Service Manual

QUICK REPAIR PROCEDURE

POWER AUDIO SECTION:

When the audio section breaks, the usual consequences are that some final transistor or/and driver transistor
gets shortcircuited. It is possible to get also some class h conmutation mosfet transistor or diode shortcircuited.
To check it you have to use a multimeter in diode mode or resistance measure. If the final transistors are OK you
can measure around 10ohms between the base and the emitter and very high resistance between collector and
emitter. In the driver transistor you have to measure around 230ohms between base and emitter.
If you have a final transistor shortcircuited, you can measure a very low resistance between collector and
emitter of all final transistors, but the lower resistance indicates what is exactly the transistor shortcircuited,
because this has a direct shortcircuit and the other paralleled transistors have arround 0.66ohms (two serie
emittter resistor). Cut the leads of this shortcircuited transistors and continue testing if you have some more tran-
sistor broken.
Also, you may have only the driver transistor shorcircuited, then you can measure this direct shorcircuit bet-
ween collector and emitter of this transistor, and arround 10ohms betweem the collector and emiter of the all final
transistors. Also it is possible to have the driver and some final transistor broken. Cut the leads of any device
broken and continue testing until you are sure you have not any more device shorcircuited.
You have to check also the mosfet transistor and diode from the class H switch. They are placed on the same
heatsink nearest to the fan. Check you have not a shortcircuit between the drain and the source of the mosfet
and check the diode works correctly.
To change any device broken you have remove the heat sinks needed to reach it. Now remove his screw and
place the new one with thermal silicone. Clean the surface if needed. Tight the screew using a screewdriver with
a torque arroud 1.3N/m.
Its is possible you have to change some burn resistor. Check visually specially the 0.33ohms emitter resistor
and the 10ohms/1W driver emitter resistor, and if you have some dark resistor measure it and change if needed.
After change all broken devices, check you have not any shorcircuit between the outputs (heat sink), rails and
ground. It is highly recommended to test the entire amplifier with the light bulbs in series with the mains supply to
avoid further power module breakings. If the amplifier start up correctly then you may short the light bulbs and
apply signal to obtain power from the amp. Use 1kHz sinusoidal signal and 4 ohms dummy load to test all works
perfectly. Check specially the switch rail works correctly, and you have not any instability when you clip the out-
put.
If the signal input clips before as expected, perhaps you can have some problem with the SOA limit circuit.
Some small SMD transistor can be broken. Check if you have problems with the positive or negative semicycle
and test the SOA limit components, placed in front of the heat sink. This circuit controls the maximun transistor
current and you can test it working with a very low impedance load (1ohm for example) and a small input signal.
you can see this system limits the output to a secure value.
Bias adjusting: you may adjust the Bias point only if requiered. To do it, connect the output to a 4ohms
dummy load. Place a 10kHz sinusoidal signal in the input, and setting the oscilloscope at 1V/div scale, adjust the
level to have 4Vp output. You have to adjust the PT1 trimer to remove the cross distortion. Take in account you
have to adjust to have the minimun current needed to remove the crossover distortion, because if you adjust it
with excesive bias current the power amplifier will work very hot or also can become hot without signal.