MTX thunder 4200x no audio

[Perry Babin]

Was the replacement transistor new?

If not, try a new one. If it was, measure the DC voltage directly between the base and emitter (instead of from ground) when the rail voltage is low.

 

[Hexhclass]

I swap the transistor with one in the audio section,no change.
voltage between emitter and base is .41v

 

[Perry Babin]

0.41 is probably enough to make it conduct a bit. Is the voltage across the 4.7k resistor more than 0.000v DC?

 

[Hexhclass]

voltage across the 4.7k is plusing from 241.3mv to 319.23mv

 

[Perry Babin]

Email me. I have a schematic (not an MTX schematic) that may be close. You need to see how close it is to the circuit you have.

babin_perry@yahoo.com

 

[Hexhclass]

schematic is very close,just part numbers look different

 

[Perry Babin]

Find the two resistors on your board that correspond to R608 and R609 in the diagram and remove one leg of each, one at a time. After taking each one out of the circuit, check to see if the amp begins to work properly. Do this with the transistor in the circuit and with no solder bridge across the base/emitter.

 

[Hexhclass]

I remove each resistor one at a time no change.circuit is a little different am just noticing that.R609 is connected to pin 12 on the 393 and on the schematic pin 12 is connected to gnd

R608 goes to pin 11 on the 393 same as schematic

pin 12 on the 393 is also connected to d602

am starting to think the 393 is bad

 

[Perry Babin]

The 339 (a 393 is a different IC).

Pin 12 of the LM339 is ground.

If none of that made a difference, I think the LM339 may be leaky. If you're going to replace it, you could try clipping the three output legs that are tied together, (1 at a time) to confirm that the chip is the problem.

 

[Hexhclass]

yes the Lm 339

I will clip the legs to see what happen and also drop a new one in there to see what happen.

thanks for the help