Crown Macro-Tech 600 problem

[pharaohamps]

I've searched but found nothing that sounds similar to what I'm seeing:

Hi, all - I have a Macro-Tech 600 amp that I'm using for bass guitar into 4x10" and 1x15" cabinets. Ch. 2 sounds great but Ch. 1 has some crackling noise, mostly on loud low notes. I get noise in stereo on CH. 1 and in bridged mono mode (uses the Ch. 1 input.)

Is this a common problem on these amps? I'm mostly used to low power SS amps (under 100W,) and tube amps, but I have copies of the service manuals + schematics from Crown.

Thanks,
Matt Farrow

[SY]

Hi, Matt, and welcome to the forum!

Have you tried using an insulated probe (I use a plastic chopstick) to tap on the channel 1 parts while the amp is on to see if there's an intermittent break somewhere?

[pharaohamps]

No, not yet. I'll poke about tonight (I have a selection of wooden dowels that I use for valve amp repairs...) and see what happens.

Thanks,
Matt Farrow

[pharaohamps]

Okay - got the amp open, chopstick at the ready, power it on with a load attached to Ch. 1, and I get a nice fat spark from the CH.1 filter cap - right between the terminals.

Caps date to '87 so I'll replace those. Definitely.

Ch.1 works well at low levels, but as the input increases, I get the Signal / IOC light illuminating / flickering as you would expect, but every time the light flickers, I hear a crackling distortion in the output.

These Crowns aren't what I'm used to - should I replace the output and driver transistors while I'm in there? And there are spots for two more transistors per side of each bridge (four per channel.) What will I get by populating those?

Thanks,
Matt Farrow

[pharaohamps]

Problem solved! I discharged the caps so I could remove them, and I noticed that the Ch.1 cap screw terminals were very loose, esp. the positive side (red wire.)

I pulled out the cap, and found that the screw itself had a lot of carbon buildup on the bottom of the screwhead, where the screw contacts the ring terminal (wired to the high side of the outputs as well as the rectifier.) The negative side did not have this carbon. I scraped off the carbon from both the screw and the ring terminal, put it back together, and it is all working 100% now.

The screw must have come loose, and the spark was from the arcing between the screw head and ring terminal. The weird thumping / distortion noise was probably from the connection vibrating around with the amp.

Now I just have to clean out the chassis and button it all back up!

[SY]

Congratulations, it was an easy one!

Now I have to figure out why this %$#&ing crossover isn't working right.

[Tony]

hi,

would be a good idea to have flatwhashers/springwashers under the screw heads to avoid the screw getting loose in the future...

[Zero Cool]

Originally posted by pharaohamps
Problem solved! I discharged the caps so I could remove them, and I noticed that the Ch.1 cap screw terminals were very loose, esp. the positive side (red wire.)

I pulled out the cap, and found that the screw itself had a lot of carbon buildup on the bottom of the screwhead, where the screw contacts the ring terminal (wired to the high side of the outputs as well as the rectifier.) The negative side did not have this carbon. I scraped off the carbon from both the screw and the ring terminal, put it back together, and it is all working 100% now.

This is a very common problem with the crown maco and micro and Power Base series of amps!