MB Quart RAA2400

[mike49504]

at the opamp (393) i have voltage for half a sec then it goes to 0 volts on all pins same with the outputs, The rectifiers are off the board i tested each one and all test fine also tested all power supply fets all test fine.

Checked the voltages on the sg3525ap on the power supply driver board

pin1=0
pin2=5.16
pin3=0.2
pin4=0.24
pin5=2.06
pin6=3.69
pin7=2.04
pin8=4.91
pin9=5.70
pin10=.07
pin11=5.16
pin12=.01
pin13=12.31
pin14=5.14
pin15=12.31
pin16=5.16

i replaced one of the 7812's and still no rail voltage in this amp

wondering what else could it be??

i have voltage on the fets
leg1= 5.15
leg2= 13.08
leg3=0

Wondering what else i should check?

[Perry Babin]

Looking at the center legs of the power supply FETs (check all of them) with your scope, do you have a square wave that swings from ground to 2x the B+ input voltage?

[mike49504]

Perry,

I misplaced a transistor for this amp im trying to put it back together.

The transistor is an mpsa13 Can i sub this for something else??

[Perry Babin]

The MPSA12, 13 and 14 are very similar. The MPSA14 is used in some of the SoundStream amps so you may have those. I don't know of any other common transistor that has similar characteristics (high current rating and very high gain).

[mike49504]

Ok cool i have a old soundstream board for parts and it has the mpsa14's in it should work right??

[mike49504]

Perry,

On my notes on this amp it has fmg22r and fmg33s but not sure if i wrote the 33s down wrong.

would the 22s's work also? with the fmg22r's

[Perry Babin]

I don't know how the MPSA13 is being used but in most circuits, the MPSA14 should be interchangeable.

The 33 is in a larger case. The S version is the positive rectifier. The R version is the negative rectifier so the S and R are not interchangeable.