Realistic MPA-100 Stereo Power PA Amplifier (2024)

Repairing a new to me. but neglected Realistic MPA-100 power amp. It was functional, sort of.

Huge heat sinks with 6 big transistors pasted to them each. Two independent volume pots and even in neglected state was way way loud.

The vu meters were not interested in moving and required dissasmmbly. Heat applied with a soldering iron on the adjustment screw, freed up the movement. Took a couple attempts and about 15 - 20 mins.

The bulbs were blown, so I made some new ones with leds and resistor. 20 mA 3v x2 led and 1k resistor / 3k resistor (positive /resistor/positive) They seem super bright and look nice, but ideally they wouldn’t be blinding. Any ideas?

It had a really bad hum, ripple problem so next to go was the old dried up power capacitors. Old 10,000uf ones out.....

New ones in....

This eliminated 100% of the hum. Dead silent. Took about 15/20 mins. Huge improvement in sound quality. Same specs, but night and day.

Next steps include replacing the other electrolyitics and finding a better lighting solution. The VU meters only move enjoyably after 3/4 volume and that is too loud for my tastes. I’m hoping someone can let me know if this is just the way it is with this unit.

Anyways, covid has me stuck inside so more to come tomorrow.

SmileyFaceEQ said:

The bulbs were blown, so I made some new ones with leds and resistor. 20 mA 3v x2 led and 1k resistor / 3k resistor (positive /resistor/positive) They seem super bright and look nice, but ideally they wouldn’t be blinding. Any ideas?

Click to expand...

To dim an LED simply requires you to give it less current. That translates to a higher value of series resistor. What power source is driving them? AC or DC? Voltage?

The meters are intended to represent output and given this is rated 100W the 0dB point on the meter would, in theory, represent 100W output.

10 (-10) would represent 10W, and 20 (-20) would represent 1W.

So, yeah, if the meters are moving a lot then it's probably pretty loud.

To make them move more at lower volume you'd have to modify the meter circuit to make it more sensitive. Then, incorporate a switch and circuit to make the range work at higher power too. Some amps have a meter range selector switch as part of the design. The scale might be 1 (normal) and -10dB (divide by 10, where 0 would represent 10W rather than 100W).

Ylli said:

To dim an LED simply requires you to give it less current. That translates to a higher value of series resistor. What power source is driving them? AC or DC? Voltage?

Click to expand...

6V DC. The led’s are 3v each at 20mA. I tried a 1k and 3k ohm resistor. Don’t want to buy and install 10 different resistors, so decided to ask. I guess I’ll jump it up to 5 and see if it knocks them down enough.

Replaced all of the electrolytic capacitors and man the meters work great and it sounds really solid, for what it is. I watched a guy test power output and dc ripple before and after power cap replacement and he gained 2 W on the output and eliminating ripple. Seems to have made the same effect on this unit as well. The meter response is much more accurate and it appears that the sensitivity is improved as well.

SmileyFaceEQ said:

It had a really bad hum, ripple problem so next to go was the old dried up power capacitors. Old 10,000uf ones out.....

New ones in....

This eliminated 100% of the hum. Dead silent.

Click to expand...

There you go boys n girls... perfect example of power supply hum

Old thread I know but I am going into mine right now and about to replace the meter lamps. Last time I had it plugged in both worked but now one is out. Thanks for the voltage spec.

I am probably going to use incandescents as I think I have the right size. It's interesting that the bulbs are not only in green filter sleeves but are also frosted. I'd like to maintain the same look which is really nice.