How to Troubleshoot Amplifiers Stuck in Protection 

It's a skill learned over time. I know it seemed like forever before it started to fall into place. Just keep at it, the understanding will come.

Thanks CW, you've made my day!

Thanks Rob! It's not work if it's something you enjoy.

​@raygianelli3612 I got screwed today. I bought two "open box" Radio Shack MPA-250B PA amplifiers. When I powered them up.. protection mode.. I'd like some guidance. There are NO SCHEMATICS to be found anywhere. This sucks! Merry Christmas to me!! Any help would be AWESOME!

Appreciate your comments, Zigg. When I see someone say they understand something I've posted I feel that I've accomplished what I set out to do.

Thanks Mark, I will keep it up. I appreciate the feedback!

Thanks Steven! Doing these videos really forces me to focus and think about this stuff in ways that are, frankly, beneficial to me. I hope it is to others as well.

Excellent idea to practice on old boards first. I can assure you my first patients did not fare well!

Thanks you, Mar. I'm sure there are many more to come!

Thanks TJET. That's what I'm shooting for, so it's nice to hear that it was understood!

Douglas, it seems like I have to learn ALL my lessons the hard way. Oh well, those lessons do stick!

@@raygianelli3612 They sure do.

James, you made my day! That's just what I'm trying to get across. I'm firmly in the "teach a man to fish" camp. My goal is to share the knowledge taught to me and the experience I've gained in order to give back to the community.

Bill, as I said in the video there's only two scenarios; either there's excessive DC on one of the output stages, or the protection circuit itself has failed. Let me know how you make out!

Thanks Roro. I've had a lot of questions about amps in protection, so I thought this video might help.

Thanks Jerry. It's always nice to hear that these videos are helping someone!

Well come on down here to South Florida. The weather's great this time of year. Summer is another story, though! Shoot me an email. My contact information is available if you click the line below my subscriber and video count. Always happy to help out a fellow tech!

👍

Good work, Robert! Not a common problem, and therefore can be one of the last things you'd check.

Thanks for the kind words! Life is getting in the way of working on this stuff. I hope to get back to it soon.

I wish it was. I got decent but never what I considered good.

Appreciate the kind words, Uri!

Buddy, AVR's are a bear to work on! Even a simple 2 channel receiver with IR remote capability is magnitudes more complex than one without. Add multi channel and surround decoding circuitry and you're in no man's land. I tend not to work on them simply due to the fact that they lose value faster than a new car driven off the lot!

@@raygianelli3612 💩 you’re not kidding about that. I can’t stand them, but every time I work on them it brings my customer closer back to 2 channel. Reversing consumer thinking from the late 90’s marketing damage is slow. Eventually they come around and reinvest into 2 channel once they realize 90% of their tv stuff comes only from one speaker.

Very interesting, Arnold. The SX-950 doesn't have one either, but by the SX-1050 one was installed. The diode won't adversely affect the protection system, since it is reverse biased until the field around the relay coil collapses, and the diode becomes forward biased, which prevents the buildup of harmful voltage. The flyback diode should not affect the brake in the PLC system, since the relay contacts are what interfaces with the brake. Make sure the diode used has a high reverse breakdown voltage.

While I'm not familiar with that particular unit, most only have adjustments for the output stages. That should hold for STK or discrete designs.

@@raygianelli3612 IDK there's a lot of discussion around those STK series Modules. But the factory that made them was destroyed in an earthquake, Any replacement you get is likely a chineese knock off (and from all reports, not as clean as the orginals. There's a kit, it's heatsink and circuit board (with discrete components) you can mount and it's suppose to work better than the knockoffs. Idk that I will do that See, the unit had multiple problems, power related as well as the protection circuit. I'm hoping I kept the originals that came out of it. (I probably did) And will put them back in once I get it up and running. as far as the circuit protection I looked right at the voltage, and said.. that's not right. I did some math and came up with 7 vs the 2V I was seeing. But It never dawned on me that this was a delay circuit. Once you pointed that out, then it all made sense. Plus, the manual (as scant as it is), is written in German. Not helpful ... lol But again, thank you.

@@Jerry-vz4ix Parts availability is going to be an issue for many items going forward. I need to work on my SMD soldering skills... which are pretty much nonexistent at the moment.

@@raygianelli3612 yep, were both of that age when eyesight isn't near what it should be. One thing I will tell you indispensable, is one of those video cameras/ microscope. Ya'd think it would be hard soldering while looking at a video monitor, but it's not. Take a little getting use to. But once you do, you'll never go back to the goggles. plus, they aren't as expenive as you'd think I picked one up for.. wanna say it was less than 150 or so. Make sure it's at least 1080p at 60hz The other thing that makes life easier (but is more costly), is hot air.. not a gun, but it is. Really small nozzle, I haven't picked one up yet. So far, just working without it. I did pull out that cap and check it. ESR was 11 ohms, the capacitance was less than half of the stated value. this thing was build back when a lot of bad caps were being sold in the supply chain. They were knock offs coming out of china, half of the problems i've been running into are those caps going bad. Eh, what can ya do. ESR meter and check em all. I guess. Course replacing that cap just revealed other problems. This amp is a basket case. Not worth money in time I have into it. But, Nothing else is on the workbench. It goes on the bench when I have slack time, or off when I have something else to do. Right now, redoing some traces that burned when it failed. So that will take a little time. If it wasn't so cool looking, I'd pull the transformer and throw it away.

@@Jerry-vz4ix Yeah, I'm looking into video microscopes, among other things. Hot air station too.

Same procedure. Trouble is either excessive DC at output stage, usually caused by a blown channel, or a trouble with the protection circuit itself. Speaker connection doesn't affect it, because the protection circuit is there to protect your speakers.

I don't know what "forward bias leaning" is either, but forward bias is the term used to denote when a transistor begins conducting.