Posted 01 February 2014 - 06:15 AM
I have a CTS-1200 Amplifier that gone into fault mode.
When powering up, the FRONT two RED leds starts flashing. and all the below resistors starts SMOKING!!
Any ideas where to start?
Or what is the thing that causes this problem??
Posted 03 February 2014 - 11:04 AM
These resistors are the load resistors of the crowbar circuit. The crowbar circuit 'shorts' out the rail voltages in case of a 'catastrophic' fault in the power amplifier.
This circuit is not exactly part of the SMPS circuit but is located in the Fault detection and protection of the amplifier stages.
The task of the SCR is to effectively short out the rail voltages to protect the amplifier section from high current flows when a fault is detected. When the crowbar circuit is triggered or enabled the energy accumulated in the DC power caps are shorted out through the path comprised of the SCR and resistors; It is possible that the resistors will heat up and smoke a bit. Once the crowbar is triggered, the short is detected by the SMPS protection circuitry and the SMPS modulator is taken off line (no power to the amplifier section). Note : Once the SCR has been triggered, it will remain enabled until power is cycled.
So, you probably may have some sort of short or major fault in the amplifier section. Remove all loads from the amplifier before powerering up. You may have to remove the two (large) ferrite beads, located near the filter caps and in line with the rail voltages, to verify if the problem is in the amplifer section. Of course, verify the SCR!
Posted 04 February 2014 - 03:42 AM
thanks for the replay. I removed the two beads. And the power supply just worked.
i am facing new problem in the power supply. Voltage is not equal on outputs. Is this a regulated power supply??
Still i am trying to find out the problem in the amplifier section. I removed all of the power transistors. But still the smps cannot start if i reconnect the amp section without the power transistors.
Posted 04 February 2014 - 09:31 AM
The CTS power supply is of the SMPS type. So yes, this type of ps is... challenging!
As you noticed, once you removed the ferrite beads the smps comes back up. This indicates that the fault lies in the power supply section. Most probably, the protection circuit detected some DC at one of the output stages.
When you remove the o/p devices you can notice that the voltage measured at the output may be off. From my experience, the output with voltage is the problematic one. The voltage that you're measuring comes form the low voltage side! If you continue to investigate the output section, verify the capacitor in the CB driver stage (220pF/500V). I have found some caps that became resistive! Verify also the solder connections on these drivers and of the regulators, located under the output section's [heatsink] Silpad.
Posted 24 August 2015 - 07:06 PM
At the point, I am not worrying about protection. So I removed Q25, which send power to the SCR. Removed all the 21194, and MJE1503x. Powering up the amp, I see voltage on the rails were about +-111, and the low voltages were +22&-24, going through 7815&7915, gives +-15. Everything seemed good, nothing is burnt. But the green lights still didn't come on. In frustration, I checked everything single diodes, caps, transistors, op-amps, comparators, and resistors on the dam board.
It sure wasn't helping, until I got my hand on the schematic. Found out that the "Current Monitor" circuit had some defective resistors. It wasn't like the resistors weren't working, but the resistance values were slightly off, within tolerance. Ultimately causing the U112 to be unbalanced and output high. In result, setting U111 and U211 to high, which are responsible for putting the amp in some Stand by(Not ready) mode.
As a Trial and Error procedure, I eliminated R538 & R638, and grounded R547 & R647, which disable the Current Monitor circuit. Powered up the amp, and the green lights came on. Turned it off, I then put back all the 21194, and MJE1503x. Without Q25, I turned the amp again, but this time, I bravely added my speaker without checking the outputs. Then came the rock music came on loud and clear.
Clearly either the power supply high rails are a bit off or one/more of these resistors(R533, R534, R535, R536, R633, R634, R635, R636) went slightly off causing U112 to drive high.
I am thinking of leaving monitoring circuit out of the picture for now, test the baby for a while.