I have an older 500 watt subwoofer plate amp that quit working and I was wondering if there was someone here willing to help guide me through diagnosing the problem. I have some electronics experience from college but it has been several years since I've utilized anything I learned. Use it or loose it I guess.

Anyway, the subwoofer that the plate amp was in sat around for several years before I decided to hook it up one day. I used the line level inputs and it seemed to be working fine for several hours and then quit. I took the plate amp out of the enclosure and found two resistors coming from the line level inputs totally fried to the point I couldn't make out their values. They both measured nearly 100 ohm so I replaced them with new ones but there is still an issue. Unfortunately, the amplifier still doesn't produce any output.

I have an old Tektronix 465 analog scope and multimeter but am not knowledgeable enough to know what kind of voltages I should be seeing without a schematic. I am also having a hard time following the signal path so when "poking around" with the scope I tend to get lost.

I am eager to learn more about electronics repair, specifically for audio applications but really could use some guidance.

If I were to provide pictures of the amp could someone help identify what I am looking at and what I should be seeing with my test equipment?

Any help or advice would be greatly appreciated.

 

two resistors coming from the line level inputs totally fried

Resistors in the AF input path shouldn't be liable to over-current, are you certain of said components' application? (FWIW: It is barely possible that the described damage owes to a failed {shorted/leaky} coupling capacitor -- in which case there may be subsequent damage to the PS rectifiers)

BTW: As a matter of course (and based upon your OP) you may wish consider 're-caping' the PS...

A schematic, or, to a lesser extent, a photo, would be most helpful...

Good luck
Best regards
HP

 

Resistors in the AF input path shouldn't be liable to over-current, are you certain of said components' application? (FWIW: It is barely possible that the described damage owes to a failed {shorted/leaky} coupling capacitor -- in which case there may be subsequent damage to the PS rectifiers)

BTW: As a matter of course (and based upon your OP) you may wish consider 're-caping' the PS...

A schematic, or, to a lesser extent, a photo, would be most helpful...

Good luck
Best regards
HP

Thank you for the quick reply. I am not 100 percent sure of the resistors application. I will post some detailed pics on the morning.

Thanks again!

 

line level is supposed to be milliwats, not enough to burn resistors large enough to see. check for dc on the outputs you were using, and what the actual audio level is.

 

Here are a few pics of the amplifier.

Here is the in its entirety. Power supply, torodial transformer, pre amp, and power amp.
Full amp

Here are the fried resistors before I removed them. They both are fried past the point of visually being able to read the values. Both measured very close to 100 ohm.
Fried input resistors
Fried resistor close

Finally, here is the pre amp PCB removed and new 100 ohm resistors "temporarily" in place. My iron decided to die after removing old resistors.
Pre amp

I am fairly certain the problem remains in the pre amp. I am planning on doing more testing today while I have some free time.

 

line level is supposed to be milliwats, not enough to burn resistors large enough to see. check for dc on the outputs you were using, and what the actual audio level is.

I'm sorry, I meant to say speaker level in my OP.

 

speaker level is much higher voltage than line level. line level is concidered to be less than 1 volt. what ever those resistors connect to is probably bad now too.

 

There's also a possibility that the input device and this amplifier do not share a common ground, and current flowed from one power supply "ground" to the other when you hooked them up. Since it worked fine for a while, this seems unlikely, but it's an easy problem to forget about so I though I would mention it.

 

owudnt a ground loop give a loud hum? it it was working ok for a while, and no loud hum, its not a ground loop.

 

Using my scope I was able to see distortion on op amp "U1" on all but pins 3 and 8 which are the non inverting input and Vcc+.

 

So I replaced the "noisy" op amp labeled U1 but there was no change to the signal. I'm not sure how to proceed.