Crown CTs2000 and Cts3000 channel 2 clipping on idle

[Silvi]

I have 2 amps CTS 3000/2000 that came for repair. The 3000 had a blown smps which I manage to fix finding a couple of vias raised and burnt links and the crowbar scr blown. Connecting the amp to the power source with a current limiter (heating element) the amp came on but channel two started to indicate that there was a signal on the input. Checking the output with a scope I found out that an irregular oscillation was coming out of channel 2. After a short while (5sec) the clipping indicator lights up and the output dies.
Checking the gate drives these where both good and I also interchanged them and the fault still persists on channel 2. I also interchanged the the other two modules which when fitted on the good channel both worked good. I have a schematic which I downloaded. I tried to follow around but could not find anything faulty. The amp was fully dismantled and cleaned and also checked what I could for any apparent fault but could not find any.
Has anyone found a similar fault maybe he or she could highlight me where to look for such a fault? I spent hours looking around and did not come to any conclusion yet.
Is there any service manual available? or maybe a repair guide?

[nickb]

crown_cts2000_cts3000_sm.zip


I was a bit confused as to which amp we are talking about here. Is this the 2000?


If the gate drives are the same on the working channel and the bad that would suggest checking the FETs and power switching diodes. This is full bridge output so you might have one side working while the other does not. That will give you net DC on the output and cause it to shutdown.

[Silvi]

Thanks Nick for the reply, I found the problem it was the series heating element (current limiter) was bringing down the input voltage and channel 2 shows clipping because of that. Strangely these units pull quite a bit of juice on idle. I read somewhere else about this. Well I hooked another heater in parallel with the previous one now having a 2kW load in series with the supply of the amplifier and all worked well. Finally I plugged it directly with the isolation transformer (2Kw) and checked the input current (2A on idle) The bridge mode led on the front panel was on all the time even when putting the back switch to stereo. I dismantled the input board and checked this switch, It was not making contact on one of the sides, I tired to spray some contact cleaner but all in vain. I had to desolder it and dismantle it and carefully cleaning the tarnished contacts (such small parts inside though) waiting for a new switch will take ages. Anyway reassembly after cleaning and hey presto the switch works again. The front bridged indicator led extinguished when the switch was put on dual input mode.

The 3000 unit was also checked and found a blown mosfet on channel 2. One of the gate drives had an open 10 ohm resistor that was driving the blown fet. The totem pole transistors where checked and found good. I checked the gate drive on channel one and it seems to be working well. I still have to wait for the fpq17n40 to arrive. I guess I change the whole 4 of them as I cannot risk blowing everything due to a crippled fet. As you may all know these power fets consist of a quantity of fets diffused in one die and paralleled together. this will bring up the current of the power fet. If some of the fets on the die have blown with the surge the fet will not be able to handle the specified load. Although when checked it still seems good.

I will come back with my findings after the assembly and test