Atlas Sound CP700 Power Amplifier Repair (solved) - Page 1
Forum section: electronics / repair

author: larryqiann
author_rank: Newbie
timestamp: May 17, 2019, 06:52:09 pm
content: Hi [pictures to be posted when I get home]Recently I picked up a non-working Atlas Sound CP-700 audio power amplifier and was able to get it working, and I'm just posting here to share my experiences in the hope that if someone has a similar unit with a similar fault they might be able to repair it.The power amplifier is a 700 W, 2-ch (350 Wpc) model with integrated output isolation transformers supporting a variety of output impedances and voltages. There are XLR inputs of both genders and barrier strip connections for all I/Os.When plugged in, the Protection LEDs (Amber) light up and do not go off on both channels, even after the anti-pop delay.tl;dr solution: Change the NPN transistors driving the protection relays and heatsink them properly to prevent recurrence of thermal failure.After removing the cover, remove the big screws on the left side and the screws on the main heat sink holding the plate with two smaller toroidal output transformers attached. Be careful as this assembly is VERY heavy, and beware of cabling attached to this assembly.Internally the amplifier is split into two halves, one heat sink half per channel. Each side's transistor gang is connected with a cable with Pos Rail, Neg Rail, and two signal inputs.Power supply to the amplifier board is 58-0-58V, with the green wires being the 0V. Be careful not to confuse these green lines with the thermostat wiring.Two relays are provided on the main board to provide speaker protection. They are in series with the speaker outputs.The amplifier section can be tested for function by connecting audio input, connecting a lamp or LED (beware could burn the LED!) and bridging across the spades on top of the relays. Do not use a speaker as there will be a loud power on transient. Alternatively, you can turn the volume up and listen to the sound through the two output transformersOn my amplifier the signal output functioned as expected. The relays are driven by a number of inputs including power on delay (r-c delay near the front LED connections) and various protection functions. The circuit board is not very easy to trace but it is single sided.Near the relays, there are two TO-92 transistors, a P and a N. These are for driving the relays. Careful examination of the PCB reveals that they have gotten fairly warm. I removed the N type ones and tested them using a multimeter, and they weren't working.The PCB was heat-damaged, so desoldering the transistors damaged the traces. The new transistors were installed on longer wires to help heat dissipation. Solid-core 22 AWG hook-up wire was stripped and fed through the PCB, and bent along the old traces and soldered to the nearest through hole joint.The transistors were replaced with 2N3904, TO-92 standard transistors. Make sure to match the Base, Collector and Emitter labels on the board to the replacement transistor. In this case, C and E were switched. Heat shrink was used to insulate the connections from the wires to the transistors, and following testing to verify the amplifier functioned normally, brackets were cut out of an old switching power supply case to clamp them with thermal grease onto the heat sinks for the power pre-amplifier stages. This should help to avoid thermal failure of the transistors, as they became hot to the touch during operation.Further notes: The fan provided has no label. It is a 24 volt fan, and runs at 200mA. The fan normally runs at 12 volts, but may run erratically if the amplifier is cold (unofficial third speed?). I'm looking into replacing this with something quieter, as the brushless motor cogs strongly and the mount is non-resilient, causing noise to resonate through the case. The fan is not too loud if not mounted. The replacement fan should likely be about 70-100 CFM full speed, and the static pressure requirements look very minimal.Hope this was helpful to someone else with the same amplifier and the same issue.Thanks for reading