Hey guys I have been working on a power 1100a2 amp that someone got a hold of before me. After repairing all the damage they started I ended up replacing all the powersupply fets and all the audio outputs for both channels. I rebuilt the left channel driver circuit also. Only one channel burned out but both sets of powersupply fets and resistors were burned. I powered the amp on with the bias fully counterclockwise and the draws about 8 to 10 amps. The channel that had burned out had about 63v and the channel that had no problem was around 72v. The right side powersupply fets are starting to get warm also. I adjusted the bias on both channels with no effect. Any ideas fellas before I start digging through the driver circuits again?
Connect only ground and remote (no B+). That should allow you to look at the drive signals on the gate legs of the power supply FETs with no risk of damaging anything.
I will do that and see if I can locate any problem areas.. I did have a shorted temp sensor on the channel that blew.
Perry,
I went ahead and replaced both tl494c and the lm339m chip. I am beginning to think I may have a bad few on the audio side. My reasoning behind this that the powersupply fets that one channel were not getting hot until I replaced the audio fets.
I went ahead and replaced both tl494c and the lm339m chip. I am beginning to think I may have a bad few on the audio side. My reasoning behind this that the powersupply fets that one channel were not getting hot until I replaced the audio fets.
When the amp powers on it immediately pulls about 3 to 5 amps then 2 seconds go by and it starts drawing more current.
What FETs did you use as replacements?
What is the value of the gate resistors?
I'm not sure how to read the display. It appears that the scope trace wasn't aligned to the reference and it also appears that the amplitude is only a bit more than 5v? Is that correct?
What is the value of the gate resistors?
I'm not sure how to read the display. It appears that the scope trace wasn't aligned to the reference and it also appears that the amplitude is only a bit more than 5v? Is that correct?
I used a irf3205 with 100ohm gate resistor. As far as the scope goes the screen is acting g a little funny but you are correct the amplitude is 5v.
If you use 3205s, you need to reduce the value of the gate resistors. Generally, 51 ohms (used in some Rockford amps) or less will work. 47 ohm resistors are most commonly used. The lower value will square up the waveforms.
Okay Perry thanks I will change the gate resistors. Unfortunately i can not check the waveform again because my fluke 97 will not boot this morning. It's been limping along for a while now. I did find one bad output that I replaced. Maybe it was heated too long during install. Checking a few more...
There's a problem with these amps that can make it destroy outputs if there is even a minor problem. The problem is that it charges the huge bank of capacitors and then switches on the biasing for the outputs. If there is a problem, the outputs have to try to pass all of the stored energy. This generally causes the outputs to fail. If you short Q11 collector to emitter, it will force the biasing on full time so a limiting resistor can help protect the outputs. Without the jumper, the limiting resistor can only protect the power supply FETs.
Would you recommend the bias turned counterclockwise? I have notice that in some rockford amps starting with bias fully counterclockwise would cause excessive current draw.
Definitely. I've never seen a Rockford amp that having the bias pot fully CCW would cause excessive current draw due to the position of the bias pot. There may be some but I haven't seen any.
Well seemed to have found the problem. Of all the bad outputs I replaced I seemed to have had a one bad irf3205, irf9640, and irf640. I check them before and after.
I have only seen two do it, but once I adjusted the bias it was fine. Perry thanks for your help I am sure you have better things to do on your saturday. Oh by the way do you have a pic on how to hook the pot to the op amp on the bd series amp still?
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