memphis st1500

[timwebb]

im working on one of these amps and the negative voltage reg only has -4.5v. i cant find what dragging it down.

 

[brandes.cm]

Are any parts getting warm, op-amps? Otherwise pull it from the board and test it externally with your supply.

 

[Perry Babin]

Is the 7915 operating at a significantly higher temperature than the 7815?

Did you insert a 1 ohm resistor between the output leg of the 7915 and the board and measure the voltage across that resistor to determine if the current being drawn was more than the regulator is rated for? This is covered under item #26 of the audio troubleshooting page of the DVD.

 

[timwebb]

ok i found what was pulling it down. now ive rebuilt the output driver cards and when i put one in the green and red light come on. the 1 ohm resistors check good. i replaced the 1027 and 1023 transistors and the diodes. what am i missing?

 

[Perry Babin]

Does it power up normally with no driver boards?

Does it power up and produce oscillation on the outputs with only one board? Try both boards, one at a time in each location.

 

[timwebb]

ok it powered up but a resistor on the card r208. a 120ohm resistor instantly smoked. so im pulling the 2sa183 thats connected to it.

 

[timwebb]

the part test good the resistor still burns.

 

[Perry Babin]

Does the resistor burn with the A1381 out of the circuit?

Does it burn with the 1381 in the circuit but the A1023 connected to it out of the circuit?

 

[timwebb]

Does the resistor burn with the A1381 out of the circuit? no.

Does it burn with the 1381 in the circuit but the A1023 connected to it out of the circuit? yes

 

[Perry Babin]

That would likely indicate that you have a solder bridge somewhere. The current has to pass through a higher value resistor and the 1381 with the 1023 out of the circuit. That would cause the higher value resistor to burn (assuming that it's the same size) if the transistor was defective.

 

[timwebb]

i checked the cards they are ok. i didnt find any solder bridges. so the 1381 could be bad? it connects to the c3503. and those were the only ones i didnt replace.

 

[Perry Babin]

Look at the BP1200.1 schematic diagram in the JBL folder. The circuit is almost identical to this one. As you can see, there is very little that can draw current through the 1381 with the a1023 out of the circuit. The schematic has an error. It shows a direct connection between the base and emitter of the 1381 and also shows them going to ground.

I'm assuming that the 120 resistor that you're referring to corresponds to R233 and R333 on the JBL diagram.

 

[timwebb]

my big pc died a while ago and took my hard drive with it that i had the dvd on. so i have to but it again....here is a old picture of the one i worked on a long time ago. that blue 121 120 ohm resistor is the one that keeps burning up.

 

[Perry Babin]

Where is the DVD that you loaded it from?

Do you have the same problem with both driver boards?

Do you have the same problem no matter which location it's installed in?

 

[timwebb]

one of my friends took it outa my cd book and now he cant find it.

yes. the issue is with both driver cards. and they have the same problem in both spots.

 

[timwebb]

any suggestions on what i should look for next? should i replace those transistors in case they are bad but testing good.

 

[Perry Babin]

Do you have any DC on the center legs of the output transistors?

 

[timwebb]

with the black probe on the ground i get 12v dc.

 

[Perry Babin]

I'm assuming that there were no driver boards in the amp.

IS the transformer shorted primary to secondary?

 

[timwebb]

I'm assuming that there were no driver boards in the amp.

IS the transformer shorted ?

i just checked. yes it is shorted primary to secondary.