Newbie, amp won't power up, only 6V inside

[kanigetts]

Soundstream 204 40W/Chan power amp. It won't power up. Voltage drops to 6V when the fuse is installed. I don't have the schematic. Power chip is SG3524N. NPN Power FETs are BUZ11. Rectifiers around the transformer are FEN16CT. The OP amps are TL074CN quad JFET. Output FETs are TIP102 and 107s. I'm new to electronics repair and I don't know where to start on this. Nothing looks burnt. One resistor looked discolored but had the right resistance when I removed it. I tried to follow a dead amp tutorial but couldn't get past this point.

[Perry Babin]

What are you using for a power supply?

[kanigetts]

Thanks for the reply. I should have mentioned this in my first post. When hooked up to a 12V battery, the amp would blow the 20A fuse so I thought if I used a much weaker supply, I might be able to see what is going on. I'm using a 5A Class 2 switching power supply. It plugs into the 110AC and outputs 13VDC.

[kanigetts]

Update: I looked at your basic amp guide and followed it to find at least part of the problem. All of the 8 output FETs are bad in one way or another. Some are shorted and some leak current back. Once they have been removed from the amp, the power side seems fine. The center pin on the BUZ11s on the power side reads 12.39V and the 4 rectifiers near the transformer read 21.45V. It looks as though someone has already replaced some of the output FETs in the past. Before I replace all of them and power it back up, is there anything else I should check in case there is another fault that is causing them to fail?

[Perry Babin]

The outputs in this amp are not FETs. They're darlington bipolar transistors.

You should confirm that all of the emitter resistors are within tolerance (likely 0.1 ohms, ±5%).

If at least one TIP102 and one TIP107 is good. Reinstall them (all that are not defective), clamp them tightly to the heatsink and power it up to see if it will produce audio.

[kanigetts]

Thanks for that clarification. The resistors all read 0.1 Ohms but I can't see finer tolerance since my meter only reads to tenth Ohms. None of the TIP102 are good. I was able to buy a replacement TIP107, but without the TIP102, I can't try it out. I'll just order all of them and try out a single pair to see if I get audio. Would substituting TIP142T/147T for TIP102/107 be OK? I thought the higher current capacity might make them less likely to fail in the future. Any suggestions for improvements to any of the components would be welcome.

[kds9591]

Originally Posted by kanigetts

Thanks for that clarification. The resistors all read 0.1 Ohms but I can't see finer tolerance since my meter only reads to tenth Ohms. None of the TIP102 are good. I was able to buy a replacement TIP107, but without the TIP102, I can't try it out. I'll just order all of them and try out a single pair to see if I get audio. Would substituting TIP142T/147T for TIP102/107 be OK? I thought the higher current capacity might make them less likely to fail in the future. Any suggestions for improvements to any of the components would be welcome.

SoundStream amps are well engineered amps. It's best to use the original part numbers or the cross-referenced equivalents.

Wiping off the old dry heatsink thermo compound and using new will provide better heat transfer improving reliability. Keep us posted on your repair progress.