Fosgate 1000a2 with blown output

[81300sd]

q306
CBE
12.75
10.62
1-.2

Q307
-16.73
10.64
2.33


Fairly certain I put everything in the correct place. I did one part at a time. Unsolder then solder the new part. Used the schematic to verify as I was doing it.

 

[Perry Babin]

Q306 is defective.

 

[81300sd]

Q306 changed.

Still have -54v@ output

q306 has

12.6
10.6
10.1

 

[Perry Babin]

Are R320 and R321 within tolerance?

 

[81300sd]

r320 is 1998, r220 is 739 Should be 2k

r321 is 530, r221 is 530 also. Should be 1k

Not sure what to think of that. Guess I should desolder r320 and see what its at.

 

[81300sd]

Unsoldered 320, its good.

 

[Perry Babin]

Are you measuring these with the amp off? I can't see anything in the schematic that could make the resistors read low.

When I get to the shop tomorrow, I'll look to see if I have a similar amp.

 

[81300sd]

yes, all resistors measured without power.

 

[81300sd]

just desoldered 321, its good too. Something is influencing them.

 

[Perry Babin]

Check R313, Q305 and R316. Post the voltage on Q305.

Also measure the voltage across R208 and R308 (probe on each side of the resistor with the amp powered up).

 

[81300sd]

R313 and R316 are good.

Voltage into R208 and R308 is -17.28

Voltage out of R208 is -14.62
Voltage out of R308 is -14.76


Q305

C 7.29v
B 17.60v
E 18.2v

 

[Perry Babin]

Voltages on Q301 and Q302?

Measure the voltage 'across' R306 and R307 (not referenced to ground - one probe on each end of the resistor).

Is the voltage on Q307 the same as it was?

 

[81300sd]

Q301
14.2
-22.1
-14.7

Q302
17.6
-21.5
-14.7

R306
.05v
R307
.08v

Q307 did change, it is now
16.6
14.4
6.2

 

[Perry Babin]

It appears that the output voltage is driving the differential amp so far down that it can't function. Do you have 0v on the gates of all of the output transistors? You can use chassis ground as the reference.

I'm assuming that the collector voltage on Q307 is negative.

 

[81300sd]

All the gates are 15.5v

Yes Collector voltage is negative on Q307

 

[Perry Babin]

Pull Q322 and Q324 out of the circuit and solder a jumper between the emitter and collector of Q323. Do you still have more than a few millivolts of DC on the output?

 

[81300sd]

I now have 3.4v on the output gates

7.0v on the drains on the irf640's
-97v on the drains on the irf9640's

46v on the output.

 

[Perry Babin]

Connect a jumper wire from the jumper on Q323 to the end of the 0.1 ohm resistors that is not connected to the outputs. Do you still have DC on the output?

 

[81300sd]

Output has -4v

Gates are at 0

irf640 drain @ 47
irf9640 drain @ -56

 

[Perry Babin]

I think you have a leaking IRF640. Disconnect the gates of the IRF640s the easiest way possible. Sliding the gate resistors off of their pads is likely the easiest. After disconnecting the gates, measure the resistance from the gate to the drain on each 640. Do any read anything other than open?


Do NOT apply power with the gates disconnected from the drive circuit.