Crate PA-4 Distortion

[skidmark]

I'm working on a Crate PA-4 with a distortion problem. I noticed that on some schematics I downloaded from this site it shows an opto isolator chip, OC1. My amp has no such part on the PA board. Any ideas?

Also, can someone validate that the voltage measurement chart is in fact somewhat accurate. I'm measuring about -8 volts at TP-4, the chart shows -1.78.

One last tid bit of info. The output shows a nice sine wave with no load and gets really nasty when I plug a load into the speaker. Any thoughts or coments?

Thanks Al

 

[skidmark]

Well ... No replies yet. I've checked almost every component in the PA section and haven't found anything yet. Is there anyone out there familiar wit his amp? By the way the amp is bustin into some high frequency oscillation.

 

[nigelwright7557]

Is there a relay on the output ? maybe its contacts have become burnt making a bad connection ?

Other than that I would suspect output transistors.

 

[llwhtt]

If the voltage at TP4 is that negative there is something wrong on the positive side, what's on TP3?

Craig

 

[skidmark]

The voltage at TP-3 is about the same -8 volts.

And Nigel ... no, there is no relay anywhere. I did check the output transistors using the old meter way and they checked normal. However, I've been electronics long enough to know that sometimes even the most subtle deviation in component behavior can cause problems. So I took extra special care in checking the semiconductors I've check so far. If your somewhat suspicious that it could still be the output(s) because the nature of this amp I'm more than willing to go back and check again.

Thanks for all your responses.

Al

 

[skidmark]

Don't know if it's going to have any further special meaning, but the high frequency oscillations are riding on the sine wave peaks. Primarily the negative ones.

Al

 

[llwhtt]

Check R6, open? TP10?

Craig

 

[skidmark]

I'm pretty sure I check R6 already and it measured good.

One other item R13 is pretty hot. Very slight dis colorization.

Al

 

[skidmark]

Schematic and layout attached

I just attached the schematic and component layout I've been working with. Notice the opto isolator circuit. My amp does not have this feature, whatever it is. Does anyone know what it's for? And on that note does any one have schematics and layout for the amp version without the opto isolator?

Thanks Again
al

 

[skidmark]

I also attached the schematics and component layout I'm working from. Notice the opto isolator circuit in the diagram. My amp does not have this. Does anyone have the version of the amp without the opto?

thanks Al

 

[sajti]

The optocoupler is part of the limiter circuit. It reduce the gain, when overload is exist. So it is not a serious issue if it missing in Your amplifier.

Sajti

 

[dumbsolderinbitch]

crate pa4

Not sure if this is the right amp, but..
my crate amp mixer.. has that problem .. however..

if it has effects loop in and out... try running a patch from in to out and sometimes that solves the problem. just a suggestion good luck.

 

[skidmark]

For any of you who are interested and may have come across the same noise problem in the Crate, the problem turned out to be a leaky Q8 (MPSA06) transistor.

Al