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JEDIA JPA-480CP Amplifier Repair (Protection Mode Problem)
This amplifier failed whilst in use and went into its Protection mode. At the time there was detectable reason for the fault so it was removed from service and brought in for repair.
The amplifier is a 480 watt low impedance or 70/100 volt line output slave, therefor contains no pre-amplifier or mixer.
It is capable of operation on either mains or 24v DC and is typical of the type used for installations in hotels and shopping centers where operation needs to be maintained in the event of mains power failure by the use of standby batteries.
With the cover removed the first job was a visual inspection for signs of overheating or component failure.
Other than the usual build-up of dust, there was no sign of damage. The next step was to check the supply rails for correct voltages. As the amplifier is designed for battery backup, the power supply is a linear design and not switched mode.
At 36 volts, all was well. A point to mention is that the 480 watt output is specified when connected to mains and not battery. When running on 24 volts the output is typically 400 watts and should always be remembered when calculating the speaker loading so as to prevent problems when running in battery mode.
According to the circuit diagram, the Protection led is driven from a thick film module HIC240C. This module is shown supplied with a dual power supply of +15v 0v -15 volts from another hybrid module board HIC-PWR. A search on line didn’t provide much in the way of information with only a close match to the HIC240C, a HIC240C1.
Measuring the supply to pins 8-9-10 showed a problem. Instead of -15 volts we had only -5 volts on pin 8 with +16 volts on pin 9. Time to remove the power supply module.
With the board removed an external bench power supply was connected to the supply pins of the HIC240C. The result was no Protection led showing so, applying an audio input to pins 12 and 14 and connecting a monitor speaker to the amplifiers 8 ohm output… everything functioned as normal.
Next, the board was connected to an 18 volt supply and measurements taken.
The negative output was still showing low as expected. With no information on the board available the next job was to draw out the circuit and see how it operated.
The ic is a cmos hex inverter. IC1E and IC1F together with the two resistors and capacitor are connected to form an oscillator with the frequency being governed by the capacitor and 4.7k resistor. This output is fed to IC1A and B which are connected in parallel to increase the current drive.
The following pair of inverters provides the same function as the previous pair but change the phase of the signal. These two outputs each drive a common collector amplifier, NPN1 and PNP1 being one and NPN2 and PNP2 being the other. NPN1 amplifies the input signal positive half cycle and PNP1 amplifies the negative half cycle, likewise NPN2 and PNP2 provide the same for the other out of phase input. The outputs from the amplifiers are rectified, smoothed and fed through the 33 ohm resistors to the +15 and -15 volt outputs.
After checking with a scope it showed a faulty 47 mfd capacitor in the negative side of the output. As this was a surface mount vertical chip component and given the age of the rest of the capacitors, it was decided to change them all for standard radial electrolytic ones. These were them glued to secure them further and prevent problems from vibration.
The finished board after replacing the components.
When all was back together again and connected to a 18 volt supply, we had matching 18.3 volt outputs on both + and – rails.
With the power supply module refitted and the amplifier powered up, the LED display showed no faults; an audio input was connected and all functioned as expected. After connecting a dummy load to the speaker output terminals the amplifier was left to run at normal programmed level for several hours to ensure all remained stable.
Time to put it back in service!
This article was prepared for you by Ian Corry who lives near Lancaster in the UK. He is the owner of a sound and communications company supplying public address equipment and radio communication systems to events throughout the country. He has over 30 years experience in the supply, installation and maintenance of analogue and digital radio systems.
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