Acoustic 370 Amp Power Module Troubleshooting Question

[olegeiser]

Hello All,
I am working on a Acoustic 370 amp with no sound. Found the 100 ohm base resistor(R339) fried on Q305 (RCA 40409) and replaced. I also replaced the 8 output drivers with 2N3055.Still no sound.
Could someone offer a substitute for Q303 (RCA 40408)? Also anyone who has troubleshot this problem before who might could help would be kindly appreciated....Many thanks!

Schematic here:
http://img594.imageshack.us/img594/8198/pwrmodule.jpg

[Alex R]

- the Q303 40408 is a 90v 700mA NPN transistor. I'm assuming it's the TO-205 case (see diagrams on page one of attached datasheet), and is simply soldered to the board. Is there a heatsink? If not, then any 90v 700mA (or higher) NPN will do if the leads will go in the holes. If there is a heatsink clipped around it or whatever then we'll have to find one in a suitable case, or add a heatsink to the replacement.

Check all the semiconductors around the output stage for shorts. Is it blowing fuses/shorting?

[gbono]

The 40408 was packaged in the TO-39 package - I use a 2N3044 from Digikey as a replacement.
Have you started to chase an input signal from the input connector, through the preamp and finally the power amp? assume you have a signal generator and scope.

[olddawg]

Hello All,
I am working on a Acoustic 370 amp with no sound. Found the 100 ohm base resistor(R339) fried on Q305 (RCA 40409) and replaced. I also replaced the 8 output drivers with 2N3055.Still no sound.
Could someone offer a substitute for Q303 (RCA 40408)? Also anyone who has troubleshot this problem before who might could help would be kindly appreciated....Many thanks!

Schematic here:
http://img594.imageshack.us/img594/8198/pwrmodule.jpg

Have you checked the power supply? It's usually a good idea to actually trouble shoot a problem rather than shotgunning components. It can get expensive fast. Were the finals and drivers shorted? Did you test them? It would be a real drag to put $150 worth of parts into a repair to find out that a 50 cent coupling cap has opened up or something. Do you have a scope and a signal source?

[olegeiser]

The 40408 was packaged in the TO-39 package - I use a 2N3044 from Digikey as a replacement.
Have you started to chase an input signal from the input connector, through the preamp and finally the power amp? assume you have a signal generator and scope.

2N3044 not shown at the Digikey site.Could it be 2N3440?
Problem is definitely in the power amp as I have swapped out a known good one. Today I will order the power and driver transistors,replace the coupling capacitors and go from there. How is the service and reliability of parts from Digikey? Many thanks for your help.

[nsubulysses]

As so often happens my internet searching has led me back to this familiar place

I have replaced Q303, a 40408, in a 370 but all I had to try to match the 90V 700mA 40408 was a MPSA06 which is 80V 500mA Ic. I figured I could just measure the voltage drop over the 100 ohm resistor and see if my collecter current comes close to 500mA, but I'm getting only .28V max (full clipping) over the 100 ohm resistor, not even close to 500mA. Am I incorrectly measuring the base current of the 40410?

[loudthud]

Did you measure across R325? Most of the current goes into the Base of Q311.