Lightning Audio B250.1 very low output and distortion

[spooney]

A guy at work gave me this amp for making some repairs for him.Right away i noticed the power input terminals were trashed and the 12 volt leg was even pulled right out of the board.After replacing that the amp powered up fine and even played cleanly up to a certain level.After turning the volume up so far on my source unit I started to get some static like distortion that was in unison with the bass in the song i was playing.I ran a sine wave through the amp and was only able to get approximately 1.8 volts ac on the speaker outputs of the amp which is way low.I'm not getting any shorts on the output transistors so I'm thinking the problem lies somewhere in the preamp section.I was hoping maybe someone could tell me if that even sounds feasible. I know this amp is worth next to nothing even working but If I can fix it cheaply I will.If not I'll start scrapping parts out of it as I need the TIP35 and 36C's that are in the output section for another amp I have laying around.

[Perry Babin]

Did you confirm that you have both plus and minus rail voltage on the output transistors AND plus and minus regulated voltage (±15v) on the op-amps?

Have you tried moving all controls to make sure they're not between positions and not dirty/intermittant?

[spooney]

I did move all of the controls i have not however confirmed rail voltage.Can you refresh my memory and explain how to check the rails?

[Perry Babin]

Black lead on the transformer's secondary center tap.

To check the rail voltage, the red lead goes to the center leg of the output transistors. I would expect approximately ±35v. Confirm that you have voltage on all output transistors.

For the regulated supplies, the red lead will go to pins 8 and 4 of the op-amps (for 8 pin op-amps). You should confirm that you have voltage on all of the op-amps.

You should also check the resistance between the RCA shield and the secondary center tap. It should be ~0 ohms.

[spooney]

Resistance between secondary center tap and rca shield is at 0 ohms.One side of the amp the outputs transistors are at 43 volts and the other side of the amp the outputs are at -43 volts. On all 8 pin op amps I have either + or - 14.6 volts on pins 8 and 4. I'm assuming these are all dc voltage.

[Perry Babin]

If all of the supplies are OK, you could have a defective op-amp or the muting circuit may be engaged.

To find a defective op-amp, you'd need some way to trace the signal through the amp to see where it became distorted.

If you could find and remove the muting transistor, you would be able to determine if the audio section is defective or if the muting circuit was not releasing the clamp on the audio signal. I don't know anythig about these amps so I can't tell you precisely where to look for the muting transistors. They are typically found where the preamp signal (from the input/crossover/equalization circuit) goes to the power amp section. One terminal is typically connected to the secondary ground. The base of the transistor will be driven by a transistor or opto-coupler near the power supply.

[spooney]

is there any way to test voltages on the other pins of the op amps to determine if one of them is defective?I know it wouldn't find the distortion problem but possibly the volume issue?

[spooney]

I think i've found the transistors you are referring to Perry.I have a copy of the schematic.Do you think if you saw it you might be able to tell what parts were the muting transistors?

[Perry Babin]

Send it to me.
babin_perry@yahoo.com

Where did you get the schematic? Rockford told me that they were not available.

[spooney]

I e-mailed the support department at Lightning Audio and they sent it to me the next day.I'll send you a copy of it right now.Its in PDF form.