Hey guys,
I’ve got a Mackie thump18s subwoofer amp module in for repair. The initial problem was the protection mode is engaged with no sound. Someone else was in before me and managed to fix the problem. But it failed on the first gig, same problem.
I had a look and it looks like components around and probably the IC NE556 (U5) were replaced.
To start with I was probing around that area too. After an hour of troubleshooting couldn’t find any obvious issue. During this time I was turning the amp on and off so it was not on the whole time. After a break, while I had the amp on I heard a very faint spark sort of noise (the sort of noise you might hear when a component burns and gives up) the amp came out of protection mode and the HV was turned on. I figured a shorted component which was causing the protection to turn on finally gave up. But as the HV was turned on, in couple of seconds I started to see smoke. Quickly turned off the amp.
The cause of the smoke was the glue on R174, and that resistor gets very hot the moment the HV is turned on. I probed that area and I measured ridiculous level of oscillation, I have attached a screenshot. I probed the same point on Channel 2 and it also did have a similar waveform but was very small in amplitude. I have two 8 ohm resistive dummy loads on each output, removing them cause the amplitude to go even high. I had to do all the measurements with in minute of the HV turning on as the R174 gets very hot.
Any idea what could be causing this? What should my next troubleshooting point be?
The amp is set at bridge mode and I noticed R175 was not populated on Channel 2.
On a second note, when the power is turned on, there is a time delay before the protection mode is off and the time varies each time. That is not the usual behaviour for these as far as I know, and I noticed when I probe with a multimeter the output of the NE556 which is on the measuring side for the current limit, protection turns on in a second and turns back off again. Happens every time when I touch the probe to the output pin. What could the reason for this be?
I’ve got a Mackie thump18s subwoofer amp module in for repair. The initial problem was the protection mode is engaged with no sound. Someone else was in before me and managed to fix the problem. But it failed on the first gig, same problem.
I had a look and it looks like components around and probably the IC NE556 (U5) were replaced.
To start with I was probing around that area too. After an hour of troubleshooting couldn’t find any obvious issue. During this time I was turning the amp on and off so it was not on the whole time. After a break, while I had the amp on I heard a very faint spark sort of noise (the sort of noise you might hear when a component burns and gives up) the amp came out of protection mode and the HV was turned on. I figured a shorted component which was causing the protection to turn on finally gave up. But as the HV was turned on, in couple of seconds I started to see smoke. Quickly turned off the amp.
The cause of the smoke was the glue on R174, and that resistor gets very hot the moment the HV is turned on. I probed that area and I measured ridiculous level of oscillation, I have attached a screenshot. I probed the same point on Channel 2 and it also did have a similar waveform but was very small in amplitude. I have two 8 ohm resistive dummy loads on each output, removing them cause the amplitude to go even high. I had to do all the measurements with in minute of the HV turning on as the R174 gets very hot.
Any idea what could be causing this? What should my next troubleshooting point be?
The amp is set at bridge mode and I noticed R175 was not populated on Channel 2.
On a second note, when the power is turned on, there is a time delay before the protection mode is off and the time varies each time. That is not the usual behaviour for these as far as I know, and I noticed when I probe with a multimeter the output of the NE556 which is on the measuring side for the current limit, protection turns on in a second and turns back off again. Happens every time when I touch the probe to the output pin. What could the reason for this be?
