Noisy Crown Microtech 600

[bigbargain]

I run sound for live performance. I have a Crown Microtech 600 that I use as a monitor amp. I’ve worked on my home stereo amps (SS and Tube), but honestly I’m not an electronics person so I don’t understand everything, but just follow instructions and am careful. I know enough to keep myself out of trouble.

The Crown amp has developed a hiss with a little static like crackle (like a radio station tuned between channels). Otherwise it amplifies well enough. Fairly quiet, but on stage and probably in the first few rows, it is distracting.

My amateur diagnosis says leaky caps, but that’s about as far as my expertise carries me. I plan to put a meter on it’s outputs, and run the output to a scope to look for noise on the PS and between the drivers and outputs. That’s about it for how far my skills will take me.


Thoughts, or further diagnostic suggestion readily accepted.

Thanks!

 

[bear]

it has two channels.

does it do this on both channels?

With the input shorted?

_-_-

 

[bigbargain]

Only 1 channel. Guess that means the PS filter caps are fine. You're getting me thinking...

The amp is at the venue, so I can't check the inputs shorted. Will do that the next time I'm there.

Probably should have pulled the amp before I left, but I'm not sure how quick I will get it done as I'm heading out of town until next Monday, so I left it. Usable in its current state...

 

[kevinkr]

It's been more than a decade since I worked on one of these amplifiers, but the output stages are fully floating bridges (that is why the two channels are bridgeable, Crown had a patent on this rather clever topology) - the supplies are independent for each channel, and voltages involved are quite high. Lots of ribbon cables running all over the place, unplug and reseat them.. I doubt there is going to be much more you can do without a whole lot more trouble shooting experience.

These are quite hazardous to work on.

 

[bigbargain]

Thanks for the warning Kevin. I've worked on tubes before so high voltage isn't foreign, but high voltage and ignorance is bad chemistry.

So I haven't got a schematic, but was expecting something pretty traditional. I assume from your comments I cannot assume the PS caps are ok, just because the noise is only on one channel?

So, are you saying I shouldn't even think about recaping it? Haven't cracked the case, but I am assuming replacing all (worst case) would not be too expensive or difficult, and would likely fix it. Is that a flawed assumption?

 

[bear]

not likely to be the PS caps... more likely a bad connection, cracked solder on a pad or noisy transistor (freeze spray to find that). Defininely whack it, and see if it is mechanical, and re-seat those ribbon connectors.

The voltages in there are moderate, just don't go between ur finger and ground while it is on, ur fine...

one can usually tell by the type of crackle noise what it is...

_-_-bear

 

[indianajo]

So I haven't got a schematic, but was expecting something pretty traditional.

Schematic of a crown microtech 1000 is on eserviceinfo.com. You don't have to be a member to get one schematic per day.

 

[jorre2002]

Microtech info

Hi Bargain;

I have the service manual for the model 1200,maybe this can help you out.
PM me i.o.t. send you the info by e-mail.
Grtngs;
Jorre-Belgium

 

[powerflux]

tip: all old servicemanuals are available on the official crown website.

 

[bigbargain]

You guys are Awesome! Thanks (Again)

Cleaned the contacts and reseated the ribbon cables. Amp is quiet as a mouse now.

I got some (undeserved) cred points with this one.

DIYA is great.

Have a great day - I did.