Optimus MPA 125 Blowing Fuses

[Bigred]

The outputs I would bet are blown along with possible power supply damage. As Chris mentioned earlier... by just replacing the blown fuse numerous times you may have inflicked even more damage. For the future I wouldn't replace a blown fuse without ATLEAST some preliminary checking (it went for a reason). Anyways.... a good start I think would be to check Q107 thru 112
D122

 

[kevinkr]

A couple of quick thoughts, perhaps Radio Shack can provide a schematic if you contact them via their website. Also it is possible that Sams Photofacts might have a schematic and service information for this model.

One thing working in your favor is it's old enough to be all through hole components, nothing like troubleshooting something with lots of smd parts as well.

 

[anatech]

Hi Brian,
Just a thought.

What were you going to use this amp for?

-Chris

 

[60fairlane]

Hey Chris,

Primarily for an in home "studio" for lack of a better word. I'm going to be driving a couple of speakers (maybe up to four) with output from a variety of sources through a 5 channel mixer.

- Brian

 

[anatech]

Hi Brian,
This is only a single channel amp, isn't it? PA stuff generally dose not run in stereo.

-Chris

 

[Bigred]

Yeah this is a single channel. Can use it to run CD players and such but you obviously won't be getting stereo left and right separation

 

[anatech]

It may serve you better if you feel like building a stereo amp of lessor power in that case, depending of course on how much you paid for it.

You aren't going to run four speakers unless you run them on a 70 V line, or use a series parallel connection for them.

-Chris

 

[60fairlane]

Chris,

Thanks for the ?? re: application - I honestly didn't even think about stereo separation. Keeping that in mind - what do you think I should do? Does this one end up in the "bin"? Are there any components that would lend themselves to other applications? Ok, I'll quit asking questions..(for now!)

- Brian

 

[Bigred]

I guess the rest of us are invisible here. Good Luck with your pa amp

 

[kevinkr]

Chris,

Thanks for the ?? re: application - I honestly didn't even think about stereo separation. Keeping that in mind - what do you think I should do? Does this one end up in the "bin"? Are there any components that would lend themselves to other applications? Ok, I'll quit asking questions..(for now!)

- Brian

Depends on what you want to do, do you need stereo or will mono suffice? If you are interested in stereo one of the chip amp kits using the LM3886 might be just the ticket. There are several threads on the subject here. Otherwise you might still want to fix it.

Depending on power supply voltages the power transformer is definitely worth keeping as is that chassis and heatsink. You can probably package a kit amp in that box if you are a little creative.

There is undoubtedly a 70V/25V output transformer in this thing as well if it is for pa work. Does this unit have connections for high voltage lines - usually marked 25V or 70V out? For studio work this is generally not needed, and generally has rather poor bandwidth as well. The output transformer will have a bunch of wires going directly to the output connections on the back. (which I couldn't read unfortunately)

 

[60fairlane]

BigRed,

I'm sorry if it seems as though I'm communicating directly with Chris - he just has been the primary person thats been helping me (at least on a level that I can currently understand/operate). I haven't had a chance to check your recommendation yet, I will respond after I get my meter on those.

Thanks,
Brian

 

[anatech]

Hi Brian, All,
I thought it was time for a reality check, that's all.

In other words, what do you actually want to do with it Brian?

It would be silly to end up with a working mono PA amp if Brian wanted stereo.

So don't bow out fellas, your hints are valuable and I'm no more right than anyone else here. I'm going to hang back a bit.

-Chris

 

[60fairlane]

Hi All,

There is undoubtedly a 70V/25V output transformer in this thing as well if it is for pa work. Does this unit have connections for high voltage lines - usually marked 25V or 70V out?

Yes - there is a 70v output on the rear.

Ok, so here is what I'm thinking - ditch the PA direction, strip what can be stripped and build up a LM3886 or ??

I checked on ebay and there are several "chip/s" for sale. I also did a search here and waded through as many posts as I could. Should I desolder the transformers and check them? (so that I don't create any more "issues" by popping fuses etc.)

 

[60fairlane]

Also, to answer your last ? Chris - I would like this to function in "stereo"...

 

[anatech]

Hi Brian,
Now let's examine if that is a reasonable way for you to go.

First, you need to repair or clear the short so that you can measure your supply voltages. I'm hoping they are too low for a 100 W amplifier. You'll see why in a few posts.

Measure the rectifiers for shorts, same for your output transistors and drivers. With a transformer output, even a DC offset will cause extremely high current to flow, so please disconnect the transformer and try using the ballast lamp Andrew suggested to power your amp up.

Now for the big question. Assuming you can make a circuit board, or buy them, you will need to build an amplifier. It may be possible to reuse the power supply, case and heat sink. So, before we pursue the "swapping of the amp guts", you need to be sure you can do the work. In the meantime, try to find the reason for the high current so you can at least measure your supply voltages.

-Chris