Quick Repair Procedure; Mains Current Limiter; Power Supply - RAM 1500 Service Manual

QUICK REPAIR PROCEDURE

MAINS CURRENT LIMITER:

To test the power amplifier it is advisable to implement a mains current limiter to avoid any shortcircuit, in
case the unit has damaged parts that have not been fixed. The most effective and easy solution is to use a pair
of 100W light bulbs arranged in parallel, and placed in series with the mains supply. If everything is OK, when
you switch on the power supply the bulbs will light until the primary capacitors are fully charged and then they
will bright softly enabling enough current to pass through to reach a normal start up of the supply.

POWER SUPPLY:

When the switching power supply breaks, the usual consequences are that both IGBT transistors (Q2 & Q3)
explode and that the IR2110S ic (U12) from the control board (the IGBT driver) gets damaged. It may be possi-
ble that more ic's from the control board get damaged too (U15 SG3525 or U9 NE555), but this is not usual.
Once an IGBT breaks, it is absolutely necessary to change the IR2110S ic too, even if it seems to be OK. If the
IR2110S ic is not replaced, the IGBTs will damage again. If you have not the requiered SMT tools to change any
SMD component, the best option is to change the complete control board.
To change the IGBTs follow the next steps: take out the power supply unit from the amplifier by removing the
screws at the PCB. Put the power supply PCB upside down and unscrew the 4 screws that are on the IGBT
heatsink, remove both switching transistors by cutting their legs and clean the pad holes of solder. Screw another
pair of transistors to the heatsink with their corresponding mica insulators, and place the heatsink again into its
location by fastening the other two screws and soldering the transistors legs to the PCB. Above manipulations
must be done with utmost care in order to have a successful repair. Check that the transistor collector is insula-
ted from the heatsink and that this heatsink is located parallel to the other one. Change the control PCB and do a
visual test of the rest of the components.
Test too the integrity of secondary rectifiers using a multimeter in "diode mode". Test also they are insulated
from the heatsink. (D9, D10, D11, D12, D13, D14, D15 and D16).
To test the power supply it is advisable to implement a little bench to avoid any risk because of lack of insula-
tion from direct ac mains voltage in most of the circuit. We recommend to use a transformer with turns ratio 1:1
to isolate the mains from the power supply (remember that the 230 V ac is still present) but now you are isolated
from earth. Connect the power supply through the current limiter (bulbs in series with the mains) and you must
look at the control board green led to check that the 15Vdc control board supply is operating and that there is not
any shorted ic. You can measure then the voltages at the output connector, they will be lower than expected
because the voltage drops at the bulbs. The voltages are the following:
S 1500: ±49V / ±81V
S 2000: ±49V / ±98V
S 3000: ±65V / ±114V
S 4000: ±65V / ±130V
S 6000: ±54V / ±108V / ±162V
S 3004: ±49V / ±81V
S 4004: ±49V / ±98V
S 4044: ±65V / ±130V
S 6004: ±38V / ±76V / ±115V
S 6044: ±54V / ±108V / ±162V
When you are sure that the unit is OK, you can short the light bulbs with the aid of a switch and measure the
correct voltages.
Bear in mind that it is possible that one of the audio power section may be damaged. Test there is no short at
the power transistors. Verify also there is not any ground track opened. When switching the amplifier on, measu-
re that the voltage at the power audio section emitter resistors correspond to the first class H level. Check with
an oscilloscope, with the probe attached to the emitter resistors, that when applying signal the power supply will
change in steps as expected. Test both polarities.
It is highly recommended to test the entire amplifier with the light bulbs in series with the mains supply to
avoid further power supply breakings. If the amplifier start up correctly then you may short the light bulbs and
apply signal to obtain power from the amp.