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.
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?
Have you tried moving all controls to make sure they're not between positions and not dirty/intermittant?
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?
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.
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.
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.
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.
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.
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?
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?
Send it to me.
babin_perry@yahoo.com
Where did you get the schematic? Rockford told me that they were not available.
babin_perry@yahoo.com
Where did you get the schematic? Rockford told me that they were not available.
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.
Cool thats the one I was thinking it was. Do I completely remove it from the board or just undo one of the legs?
I'd remove it completely. Unless the through-holes are really large, you may have problems with it intermittantly making contact which could make troubleshooting difficult.
OK, you'll have to trace the signal. The last time I asked, you said you didn't have a scope so you'll have to listen to the various points in the circuit to see where the distortion comes in.
A Radio Shack 'mini-amplifier' (RS #: 277-1008) makes a good signal tracer.
If you have an old home receiver with a tape monitor or auxiliary input, you can also use this as a signal tracer. The old ones with the switches that physically lock into place are less likely to be damaged by transients if you touch the probe to a part of the power supply circuit.
No matter what you use to trace the signal, you'll want to insert a capacitor in series between the probe and the amplifier. Use a 1-4.7uf non-polarized cap.
If you're willing to spend the money on the RS mini amp, that would be the best choice (for a signal tracer). You'll need to buy an input cable (anything with the correct connector on one end will work -- you'll need bare wire on the other end). I'd also recommend using fully enclosed headphones with it (it will make it easier to hear noise and distortion). If you have a set and need an adapter to fit the small jack on the mini amp, buy one of those also. Buy a mono to stereo adapter for the headphones so you'll have audio in both channels of the headphones.
A Radio Shack 'mini-amplifier' (RS #: 277-1008) makes a good signal tracer.
If you have an old home receiver with a tape monitor or auxiliary input, you can also use this as a signal tracer. The old ones with the switches that physically lock into place are less likely to be damaged by transients if you touch the probe to a part of the power supply circuit.
No matter what you use to trace the signal, you'll want to insert a capacitor in series between the probe and the amplifier. Use a 1-4.7uf non-polarized cap.
If you're willing to spend the money on the RS mini amp, that would be the best choice (for a signal tracer). You'll need to buy an input cable (anything with the correct connector on one end will work -- you'll need bare wire on the other end). I'd also recommend using fully enclosed headphones with it (it will make it easier to hear noise and distortion). If you have a set and need an adapter to fit the small jack on the mini amp, buy one of those also. Buy a mono to stereo adapter for the headphones so you'll have audio in both channels of the headphones.
You may be able to find a defective op-amp by simply measuring the DC voltage on the outputs of each op-amp (pins 1 and 7 for the 8 pin DIP chips). Most should be very near 0v DC (black meter lead on the secondary center tap).
If you find one that has significant DC, you would need to measure the input voltage (on the inverting and non-inverting inputs of the op-amp) to determine if the inputs are causing the DC or if the op-amp is defective.
If you find one that has significant DC, you would need to measure the input voltage (on the inverting and non-inverting inputs of the op-amp) to determine if the inputs are causing the DC or if the op-amp is defective.
u11,12,13 all have dc on at least one output if not both.There is also dc present on the inputs.The lowest measured dc on outputs was 5 volts and the high end was about 13 volts.
With your black meter lead on the secondary center tap, measure the DC voltage on each side of R599.
U11, U12 and half of U13 are DC coupled so if there's DC on 11, it will show up on the others.
U11, U12 and half of U13 are DC coupled so if there's DC on 11, it will show up on the others.
If you have the same voltage (~0v) on one end of R600 and you have both positive and negative regulated voltage on pins 8 and 4, it would seem that U11 is defective.
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