Crown Power Base 2: one channel distorted, mostly low frequency.

[PfeifferElectronics]

I have a Crown PA amplifier that is sounding distorted on one channel. Where do I begin with this?

[Mark Black]

Well I can't seem to upload the service manual & schematic right now, but from experience with Crown MT/MA/PB/CSL amps it may be as simple as a dirty bias trimpot or even dirty front panel control. There are also switches for mono/stereo modes and possibly for input sensitivity range which can become dirty/oxidized as well. Might just need to be worked back & forth or in & out a few times.

The bias trimmers are pretty touchy even when working correctly.

Beyond that I have found cracked solder joints at various places on the output boards and at the places where the driver transistors interface the output board.

Oh - the manual & schematic are available here (thanks Crown!):

http://www.crownaudio.com/media/pdf/legacy/pb2sm.pdf

http://www.crownaudio.com/media/pdf/..._j0446-5_a.pdf

[Mark Black]

Oh yeah - and there are the CMRR and ODEP trimmers on the main board as well.

Additionally, those amps usually get pretty dirty inside and can benefit from a thorough compressed air cleaning accompanied by a paintbrush or some such. If the amp has been used in the vicinity of fog machines things can get really ugly in there and require a wash-out.

Edit: Maybe you already know but testing is best done with no load & monitored by a scope. Simple tap-testing of the trimmers might reveal a touchy one without seriously screwing up the settings, but any change in the bias trimmers can have pretty large effects on amp operation & current draw.

[PfeifferElectronics]

Thanks Mark. The service manual was very helpful. I re-biased the output but one channel does not sound as full as the other. It is a little less bright and very thin; no bass. Should I re-examine my bias setting? The distortion seems to have gone away.

[PfeifferElectronics]

Yeah, I fixed the problem. Mark mentioned the other trimmers and got me thinking. I also hadn't hit the chassis with compressed air. So, I noted where all the trimmers were at on the input board and turned them back and forth a few times then returned them to where they were.... I know this isn't ideal but I don't have the equipment to set these correctly. I then blew out the chassis to remove all the dust and such. Now the amp is working beautifully. Thanks for your help Mark!