Problems with Alesis RA-500

[sippo]

Hello,

i have a problem with one channel of an Alesis RA-500 Amp. If i connect a signal generator and an oscilloscope to the amp everything i can see is a distorted signal that decreases from the time i start the amp to a certain point and which is lower in level compared to the other channel. If i play music with this channel the output is very low an distorted. So far i have checked all transistors, diodes, caps and resistors on the main amp pcb O.K., there is no !! voltage drop over the emitter resistors, the supply voltages for output and driver stage are allright. There´s another small pcb attached to this board which seem to include the protection circuit and a small part of signalcircuit.
Maybe someone here could help me with this or have a schematic of the RA-500.

Thanks in advance

Regards

Frank

 

[sippo]

Nobody in here who knows something about this amp ??

Frank

 

[anatech]

Hi Frank,
No diagram, sorry.

No bias could mean a shorted protection transistor or bias transistor. Discharge the caps and go hunting in diode test mode.

-Chris

 

[sippo]

Thanks for the help Chris,

now i have checked everything and finally resolder all joints just for sure but the distorted low level output still remains.I´m running out of ideas. I have asked the Alesis distributor for a replacement board but without a reply yet.

Frank

 

[anatech]

Hi Frank,
Stick your negative meter lead on the positive speaker output terminal. Measure the bases of the outputs and drivers. Record the voltages.

If you can find the differential pair, measure the emitter voltages with respect to ground.

-Chris

 

[eccentricload]

inject a source output on the differential input via R9 .

If there are still low audio, work till you reach the VAS circuits.

 

[east electronics]

this amplifier is ...a bad amplifier .... secondary psu caps arround the protection area is a common failure and also poor soldering since amplifier is under thermal stress and show soldering problems also quiet often

also the protection ic fails quiet often

 

[eccentricload]

You can also try swapping the IC on the other amp ( BA4560 ) or have it removed.

 

[oldmaelstrom2]

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[lm324]

Hi Frank,

Service Manual here:

Alesis RA500 (manual de serviço) - [Audio List]

Good Luck

Roger

 

[Enzo]

Well, let us all hope that in the four and a half years since Frank started this thread, he has resolved his problem.

 

[shred805]

i wish i would have checked this forum earlier. I purchased a ra-500 from alesis website. 4 months later right after the warranty is gone, the left channel is no longer working. I am experiencing similar problems as Frank. This amp must have been designed/constructed poorly