Got this amp with 4 A56 and 2 10ohm resistors blown due to dropping a screw driver in that area. I replaced the blown parts and replaced 494 also. Amp produces rail voltage and regulated voltage however there is no rail to rail oscillation on outputs. On one octocoupler with red on pin 8 and black on pin 5 of DMM I get 20volts on the other octocoupler I have 2 volts. Noticed the low independence light is on also.
In a vertical list, post the circuit board designation of the optocoupler, the pin 1-2 voltage and the pin 3-4 voltage for both optocouplers.
Octocoupler U230
Pin1 0 volts
Pin2 0 volts
Pin3 0 volts
Pin4 0 volts
Pin5 0 volts
Pin6 0 volts
Pin7 0 volts
Pin8 20 volts
Octocoupler 232
Pin1 0 volts
Pin2 0 volts
Pin3 0 volts
Pin4 0 volts
Pin5 72 volts
Pin6 72 volts
Pin7 72 volts
Pin8 70 volts
Pin1 0 volts
Pin2 0 volts
Pin3 0 volts
Pin4 0 volts
Pin5 0 volts
Pin6 0 volts
Pin7 0 volts
Pin8 20 volts
Octocoupler 232
Pin1 0 volts
Pin2 0 volts
Pin3 0 volts
Pin4 0 volts
Pin5 72 volts
Pin6 72 volts
Pin7 72 volts
Pin8 70 volts
My mistake. I should have looked at my photos.
I need 3-4 and 5-8. Pin 3-4 = black probe on pin 3 and the other on pin 4. Same for 5-8 (black on 5). There will be 2 voltage readings per optocoupler.
I need 3-4 and 5-8. Pin 3-4 = black probe on pin 3 and the other on pin 4. Same for 5-8 (black on 5). There will be 2 voltage readings per optocoupler.
Optocoupler U230
Pins3-4 0 volts
Pins5-8 30 volts
Optocoupler U232
Pins3-4 0 volts
Pins5-8 2 volts.
Pins3-4 0 volts
Pins5-8 30 volts
Optocoupler U232
Pins3-4 0 volts
Pins5-8 2 volts.
So far I believe U232 is not working due to not getting the correct voltage. However my eyes must be going bad or this is a 3 layered pcb and I’m not able to track down where the pins are connected at this moment.
The regulator for the optocoupler that has only 2v has failed. I think it's on the heatsink. Possibly FET327? See if you read near 0 ohms between leg 3 of that FET and terminal 8 of the optocoupler.
I get OL reading with the recommended check. The voltages on that fet when powered up are
Gate = 0 volts
Drain = 72 volts
Source = 0 volts.
Gate = 0 volts
Drain = 72 volts
Source = 0 volts.
I figured I would I’m referencing a MTX 7801 I repaired as they are similar. I’ll continue looking and reply was found.
I figured I would I’m referencing a MTX 7801 I repaired as they are similar. I’ll continue looking and reply was found.
After all day I finally figured it out and amp is producing clean sound. After a few swear words and throwing things I finally realized FET 261 is the voltage regulator for U232. After testing confirmed the mosfet was operational. I then noticed the diodes in front of the mosfet weren’t diodes but actually zeners. I tested one D263 and got a funky reading. So I tested transistor Q261 and discovered it was leaking. Upon replacement of the transistor amp came on and low independence light turned off and LED236 lite up and amp started producing sound and has been playing ever since.