Help with SoundTech 4150 PA head, please

[Peter Blair]

Hi anyone reading this. i really appreciate you trying to help me. Not only am i a complete NOOB, but i know almost nothing about electronics. that said, i have actually managed to make some simple repairs, and save a few junk-heap destined amps.

so i have this pa head. everything works EXCEPT, if i turn the volume, the tone, or the bass pot for any particular channel to full output, the "protection" light comes on and the amp makes a loud buzz. mind you, the volume might only  be at 50%. but turn the tone or bass up full, and the buzzing starts. the main output volume might only be half way up (or less) also.

Any suggestions what to look for? i do have a small multi-tester, but that's about it for test equipment. i'm willing to replace parts as an experiment. any ideas? attached is a pdf with a schematic for the individual channels, and one for the power amp. nothing there for the EQ section (in the pre-amp?)
Thanks in advance,
Pete

[Roly]

Hi Peter, welcome.


Thanks for the cct.   :dbtu:

Sounds like some form of instability, and oscillation at control extremes, and the first thing I think of is earthing paths within the mixer.  Have these been messed with at some stage in the past?  Are they dirty or loose (connectors)?

How old is the gear?  If it's getting on this can also be caused by a failing power supply by-pass cap, but take a careful look at the grounding of the various modules/sections first.


You should also measure the following voltages and post them here;

Fuse F101 to ground.

Fuse F102 to ground.

Across C124.

Across C125.

[Peter Blair]

Hi Roly,
Thank you for your time.

The amp is probably circa 1980-ish. It's old, but in good shape. No corrosion and i see no evidence that anyone's been under the hood before. i will verify the grounding of the various components, and i will test voltages that you suggested. I'll be back to you with that. Hopefully, I'll have time to do all that today!

Thank you so much for responding.

Pete

[Peter Blair]

Hi Roly,

Status report on the SoundTech 4150;

First of all though, thank you for your time, expertise, generosity, and willingness to take a look at the schematic of my problem amp, and render some possible solutions, or at least a couple things to check out. This whole thing of amp repair is my latest hobby endeavor. It's something I've wanted to make time for, for decades. I guess better late than never, as it is said! I find it amazing and remarkable that there are people out there willing to help for nothing more than a thank you.

So far I've been able to salvage a Peavey XR700 powered mixing console, a Peavey XR500C mixer amp, and a Peavey KB100 keyboard amp. (that one still needs a little tweaking). And now, with your help, it looks like I've got this SoundTech amp back in business.

I followed you advice on checking out the grounding first. One thing I thought was kind of cool about this amp is the way the power amp was on its' own chassis that mounts to the rear of the cabinet, while the inputs and equalizer (and preamp, I believe) mount separately on its' own chassis (face-plate really) on the front of the cabinet, connected by a ribbon cable. One thing that occurred to me though, about this arrangement, that didn't dawn on me until after I read your suggestions. There's no real good ground connection between the two chassis.  I'm sure there's one in the ribbon, but how good? So I added a nice substantial ground wire from chassis to chassis. At the same time I got some real cables to run from guitar-to-amp, amp-to-speaker. All my regular cables were left at a gig on Friday night because we were coming back on Saturday. I was forced to kind of mouse-up some adapter cables in the meantime. I'm sure this wasn't helping anything. So anyway, I put it all back together and voila! Everything works! No buzzing! All seems well so far. Thank you so much Roly. I will definitely be addicted to this forum for some time to come. I truly hope that at some point in the future, I can offer somebody useful advice as well. Thank you for the warm reception.

Pete

[Roly]

Thanks for the report.

meh, t'wern't nuthin'

After doing it for a while, given a few good clues, you can often put your finger on common problems...
http://youtu.be/jMBsZC-FJNE