Top 20 Alesis RA-150 Amplifier Questions & Answers

Start with the basics. Fuses, check transformer, look for burnt or damaged parts. With this kind of thing more than one part could have failed. Indeed you could have more faulty parts than working ones! Or it could be just one thing that's gone!!!

Your first clue to going wrong with the repair is that the relay with two extra pins was the wrong type! So you need to look at the old one and read the ratings on it and search the net for one that does match it with the correct number of pins. As for the resistors you need to read the colour code of them and again search for something the same size. The colour code will tell you the type you need. If you don't know how to read the colour code search on the net for a resistor colour code calculator. As they look bad I would replace them, even if they did work, they could go dead later.

OK one thing at time first take cover off unit then check fuse if its blown there is a short do not replace fuse yet then locate bridge rectifiercheck it for short if it is unsold-er and replace it then locate output transistors and check all of them for short. if one of the devices that i told you to check is bad then when you replace it then replace fuse and try powering up unit. be sure to replace all shorted parts before replacing fuse and powering it up. DE

The first things to check are the devices on the heatsink. They do all the work so tend to go when pushed! Remember stereo amps are just two amps put together, so at least you have a good channel (which is the same as the bad one) to do tests with say an ohmeter.

ALesis doesn't make it anymore and you can buy one for about $185 online.
If you could find the board , it would be about $75-125

Hi Cristina,
You may download the service manual of your unit at this LINK.

Reset the fuses.

Check for a stereo/ mono switch in the front or back, and make sure it wasn't accidentally switched in the wrong position. Also check and make sure the wires are not crossing each other causing a short.

Either by sheer force or if you can see a hole in them, by an Allan Key.

it may be an open input side coupling capacitor in the feedback circuit around the power amp. if the amp has a limiter in it it may be stuck. here is the service manual, in any case. http://forum.vegalab.ru/attachment.php?attachmentid=75690&d=1265371278

Sounds like the volume controls are dirty. You can try just working the volume controls up and down (amp off) that sometimes helps. If they are really dirty, you will either have to get some cleaner made for switches and pots (a spray can with the little tube) or take it to a reputable stereo or PA repair shop and have them fix it for you.

Hi and welcome to FixYa, Initially, it would be doubtful if it were a component that is faulty; otherwise the fault would not be intermittent. A most likely suspect would be cold solder on the power output of channel 2. Cold solder as the term implies is a condition where the solder joints has lost its 'good electrical connection', barely touching or the solder itself has dulled with cracks that has developed. A quick fix is to re-solder the legs of any and all dull colored / grayish solder joints with particular attention to power components and those attached to heatsinks. Good luck and thank you for asking FixYa.

on bridge mode the speaker impedance should be 8ohms, less than this clipping will happen. try bridging two 4 ohms in series.   
if this happens without speaker connected, u have a power amp section that is trying to draw more power than it can handle. 
as for the second check connections first, otherwise i suspect u have a defective power amp for the right channel.

You say "it seems like". Do you mean to say it is intermitent, or did it start cutting out and then finally fail? If that is the case I would look first to replacing the relay in contention in this thread.
http://www.fixya.com/support/t224708-amp_cuts_out_after_while

It's a cheap fix and will eliminate one potential cause. The Radio Shack part is widely available.
http://www.radioshack.com/product/index.jsp?productId=2062483

My RS-300 had started dropping the left channel, but occasionally both. It is possible that the relay has failed on one side and thus you have a dead channel...I did the fix and am now quite happy with the amp

Steve

You have an overload in either the power supply section or Main amp section. Culprits in the power supply are the transformer, the Large Capacitors, rectifier, voltage regulator.
In the Main amp anything on the heat sink.
Look carefully for burnt or damaged parts. If there's more than one fuse, follow the path of the one that is blowing it might lead you to the suspect.

Try Digikey.com and Mouser.com.

If that centered oval thing is the power switch, it is LIKELY a special and you MAY have to go to Alesis to get a replacement... I hate it when they use specials...

The other thing is to see if the actual switch or mechanism MIGHT be the same as a part you can get via one of the sites mentioned above.

Normally when people bring their HI-FI to a party they tend to fix up the leads to the speakers in a flimsy manner ( as it is only temp) . With the crowd mulling around these leads(wires) could very well come loose or even short out. This leads to your Final Drive ( Output Amp) to short out. It is not an expensive repair. Check your final amp & replace any shorted/blown items. To get full satisfaction the job best be handled by the local agents of the amp.

you have more than likely damaged some transistors.
the resistors have burned because of excessive current.
there are 2 typesof transistors,bipolar and mosfet
a bipolar transistor has 3 pins.1) base 2)emitter 3)collector.
an NPN is tested with the + meter lead on the base , and a PNP transistor with the - lead on the base.
TESTING
base to collector should read .5 to .6 v drop in one direction and open in the other with a digital meter on the diode check function.
base to emitter about the same,but slightly higher than the collector.
if base to collector is .5v then base to emitter is .52 or so.it is always higher than the base collector number.
between emitter and collector will read open in both directions.
if there is a short on any lead combination,the transistor is blown.
npn transistors operate on the positive rail power supply,and pnp on the negative rail.
typically a 150 watt amp will run + - 65vdc.
the transistors running on the plus side are npn and on the negative pnp.
most pnp output transistors are green.,but some are black,
so we must go by the numbers on the transistors, eg.A1076 is a pnp,B755 is a pnp and all transistors that have a C or D eg. D424 are NPN transistors. all A and B numbers are PNP and all C and D numbers are NPN.
MOSFET transistors have 3 pins 1)gate 2)drain 3)source
the source goes to power like the collector does. the drain is like the emitter,and the gate switches it on like the base does.
testing a mosfet is a different procedure.
a mosfet with k then a number e.g. k135 is a positive device like npn.
a mosfet with j and a number e.g. J50 is a negative device like pnp.
a mosfet reads open source to drain both directions.
for a positive (k number device) the plus meter lead goes on the source.,the black meter lead on the drain. on the diode test function.
jump a wire from the source to the gate for 1/2 second and remove the jumper while leaving the other two meter leads connected to the source and drain.The mosfet should switch on and the meter should show a short.that means the mosfet switched on.About 5 seconds later,the resistance from drain to source should go open.
for a p channel mosfet,it checks the same,except that the black meter lead goes on the source and gate .
a mosfet will stay switched on longer than the voltage applied to the gate,if it is driving a high impedance device such as a meter.
a bipolar transistor will not stay on when voltage is removed from the base lead.
i hope this has been of some help.

Some of the amplifiers IC are damaged need to replace them

Its samwha brand with a 100volt working voltage 6800 microfarad capacitance value its next symbols seems to me the chemical compositions or stock code, and its maximum working temperature. Could be a low ESR type capacitor.