Got a Crown CE2000 Semi Fixed, Beginner looking for advice

[Loafdude]

Nice forum, lots of good info here!
I see others have had help on this amplifier here in the past so thought I would post

I got my hands on two broken CE2000 amps for free so I am using it as a learning experience and fixing my first amplifier.

I had -98v on the channel 1 output and amp was staying in protection.

I found an burned resistor (R142) in the output stage (this circuit looks like some sort of feedback loop from first power transistor ( R152) on the positive rail. Not sure how this works exactly)
I traced and tested all highlighted components 'out of circuit'
In the negative driver section I found all transistors failed and 2 of 6 power transistors failed
I have replaced all these components with working devices.
These are shown in RED on my attached schematic.



I now no longer have -98V on channel 1 output. The amp comes out of it's startup delay (protection starts on then disables itself after 5 secs)
If I apply a signal it is very distorted. Clipping light engages extremely early (5-10% of volume). Volume output is low.
If I try to increase the volume the amp go into protection.
There is no negative voltage on the output.

If I supply the same signal to channel 2 it works properly. No clip light, no protection, clean sound

My question... Where do I start?
Do I need a scope to view the signal and find where it becomes distorted?
Although I tested capacitors, I did not test their ESR. I was surprised to find C112 to be OK. Could a bad cap in the drive stage cause this (I wouldn't have thought it would illuminate the clip light)

Thanks for any insight you can provide!
I appreciate it.

Also...
The complete service manual is available here
Page 579-581 has complete wiring schematic

[olddawg]

It is generally not a good idea to replace just a couple of shorted outputs. You should replace the set if you want a reliable repair. However, I doubt if that is your problem. Have you replaced the drivers with exact replacements? If you are using a sub make sure the basing is correct. Check all resistors in the area out of circuit by lifting one lead. You have a working channel it should be fairly simple to compare voltages from on channel to the other and tell where your problem starts. Of course a scope is helpful, but you have a working channel. Use it for reference. You could also make signal tracer.

[Loafdude]

Thanks olddawg!

Thanks for the heads up on comparing the channels, I will do so tomorrow night.
Should i check voltage with reference to ground or to the negative speaker output?

I used exact components (I have two broken amps. Using the other for spare parts at the moment)
The exact same drivers were used (Both were even stamped with "2" which i believe means they have similar gain) All were reinstalled with new thermal paste.
As well the other transistors were exact. Thet were also all checked for proper function before being put in.

[Mick Bailey]

Getting the amp to power up with no DC on the output is a good start.

The components you checked out of circuit may measure OK, but could fail under load conditions. Usually the best test environment is within the intended circuit, under power. You may wish to take a cautious approach and be aware of any burning smells and overheating while you're troubleshooting the amp. A light bulb limiter is useful protection.

A 1khz sine test signal and a scope would be quite useful in checking whether the output is distorted only on one half of the waveform. You can also scope the commoned Base signals on each output transistor array to check for distortion. You need to see a clean signal here. If this is distorted then you need to go further back and determine where the distortion is occuring. A/B-ing with the other channel will help to rapidly identify what's happening. You'll want to do this within the range before the clipping light comes on and examine what happens when the level is increased.

The key to diagnosing where the problem lies is to determine which functional block of the circuit is faulty, then to troubleshoot for components within that block.