Killed my M1A

[Ross6860]

Well, probably not the smartest move...I had my AR3a hooked up to the M1A amp (with a C1 pre-amp).

My wife rarely asks for music to be turned up, but she likes Pearl Jam and I just happened to have it playing. "Hey, turn that up a little..." OK, at about 40% volume on the pre-amp and the power amp shuts down with the blinking red light. It does not reset even with a complete power down and sitting for 30 minutes, so I assume I cooked something.

The amp is rated for 4 ohms, but I know those aren't the easiest speakers to drive.

Any hints on what to look at first? I looked at a schematic and it doesn't look like there are any fuses in it.

Thanks in advance. Hope I didn't turn it into scrap. Luckily it was pretty inexpensive.

 

[Ross6860]

You killed a perfectly good rifle! Anyway, take a close look at the schematic on the far right. It shows the various configurations for the power supply and you have at least two fuses in that amp. Gonna have to look under the hood for a peek. Now here's where it gets fun...if you've blown fuses, there's a good reason why and THAT...is the question.

At least I didn't kill an M1A1....

Just the main power supply fuse F1 in there. It's good (wouldn't even get the blinking protection light if it was blown).

Nothing obvious, like a burned or melted component, blown trace, etc.

Pretty much exhausted my component level troubleshooting skills, or lack thereof.

No confidence in any local shop, so I guess it's a parts unit for somebody, or landfill material.

 

[mbz]

Not familiar with the amp so ok to disregard. Blinking LEDs normally indicate a protection issue, do you have a multimeter? Are you reasonably
comfortable taking voltage measurements to confirm protection issue. Will try and track down the service manual.

 

[mbz]

Yeah, blinking LED means amp in protection, normally not such a big deal, but complicated since amp uses
power packs rather than discrete. If you are ok with a voltage measurement to confirm the theory.
With black MM probe connected to amp chassis (or black spkr post) measure the dc voltage at jumper J201
highlighted. Expect 0-50mV, could be 1-65Vdc. Repeat measurement for jumper J200.

 

[Ross6860]

Yeah, blinking LED means amp in protection, normally not such a big deal, but complicated since amp uses
power packs rather than discrete. If you are ok with a voltage measurement to confirm the theory.
With black MM probe connected to amp chassis (or black spkr post) measure the dc voltage at jumper J201
highlighted. Expect 0-50mV, could be 1-65Vdc. Repeat measurement for jumper J200.

View attachment 1010827

Thanks for the tips. If the service manual actually had a step-by-step troubleshooting guide I would be golden. They are obviously "written" for people that are intimately familiar with electronics and audio amplifiers.

I've managed not to kill myself over the last 40 years or so...I'm very comfortable working on energized equipment, I just have no, or minimal, "electronics" experience. I've worked on motor starters, switch gear, AC and DC power generation and distribution, and early PLC control systems.

I'll get the voltage readings and go from there, and thanks again.