Crown Com-Tech 400 Dead Channel?

[dirkwright]

Dirkwright,

I took a few minutes and installed the LME49740's into my Comtech. I noticed one thing right off the bat... I'm getting a noticeable higher-pitched (oscillating?) hum through my 2 speakers at idle. The hum continues in the background even when I am playing music.

Other than that, the audio doesn't seem to be as clean. It seems like there is more distortion in the sound, however I cannot verify this for sure. The amp isn't indicating excessive distortion on it's distortion meters, so if there is any it's still relatively small. I will say that sound quality isn't noticeably any better than it was with the MC33079's.

If you would like me to run any tests with my multimeter, please let me know. I will leave the LME49740's in my Comtech for now until you get a chance to reply.

Thanks

I'd put the MC33079's back in there. Without instruments, you really can't tell much of what's going on with the amp.

 

[techbiker]

Do I just need to purchase a scope to find out what effect this opamp is having? Would a distortion analyzer be worth purchasing as well?

Thanks

 

[dirkwright]

Do I just need to purchase a scope to find out what effect this opamp is having? Would a distortion analyzer be worth purchasing as well?

Thanks

I'm looking at USB instruments now, including 'scopes. But at the minimum you need a 'scope and a function generator.

 

[bigbargain]

sorry to bump an old thread, but the link to the schematic in the beginning of this thread no longer works (crown I suspect pulled it - buy new, don't fix old).

Does anyone have the schematic pdf that they can post here?

Thanks!

 

[llwhtt]

Just go to the Crown website and under support you'll find "discontinued products", click on that. All of the old info is there, I think Crown is one of the better ones providing old info.

Craig

 

[techbiker]

I believe the patent on the CT 400 schematic I have expired a few months ago. It should be easy enough to post here for safe keeping!

I'm happy that people are still repairing these old com-techs. Mine's still humming along happily even with the idle current boosted 30%.

Btw, I bought a Philips PM3215 with original schematic and service manual. Not an amazing oscilloscope, however it's definitely worth buying one to diagnose audio equipment (as dirk pointed out).

 

[bigbargain]

My apologies to Crown. As Craig says, they DO still have the schematic on their web site. Thanks Crown, Craig and Techbiker!

 

[bigbargain]

So I guess there seems to be some interest and expertise for these amps. I have 2 that I got when someone was cleaning out a school auditorium's closet. They both "work" but I'm sure they need some attention. The biggest usability problem is a very loud pop (speaker threatening loud) when is shut them down. I haven't opened them up and it works to simply unplug the speakers before powering down, but I'm not going to leave it that way.

Any ideas to point me in the right direction before I start?

Mind you, I haven't done any diagnostic work yet, so sorry in advance for being a little lame - I will post back after I've opened them and done some more diagnostics.

P.s. - I've heard (and experienced) that the interconnections frequently are a source of problems, so simply pulling them all, cleaning them and re-seating them many times fixes problems. That will be my first step (before even measuring anything). Of course any insight into common problems will be helpful also.

Tech Biker - I assume you mean you increased bias. To what point and why? I assume your goal was more class A? Have you noticed a big improvement?