I’ve got a Rockford 4600 that blew an output FET. I pulled the two FETs on that channel, and poking around I discovered that the main rail + caps are at +18V which is probably about right as the caps are 25v rated. But the - rail is at -27V. Any ideas where to look? Can’t find any bad components. I even replaced the TL494 and LM339M chips, no change. No 'scope to check waveforms yet, but I have one coming...
I was just measuring across the caps. I just re checked, and referenced to ground they are actually +27.5V on one rail and -18V on the other.
Although it's unlikely, it's possible that the positive rail cap is defective and allowing a lot of ripple to be on the rail and the meter is reading that.
Do you have another capacitor that you could parallel across that capacitor. It doesn't have to be identical. Anything 100uF or greater will likely be OK.
Do you have another capacitor that you could parallel across that capacitor. It doesn't have to be identical. Anything 100uF or greater will likely be OK.
I took out the rail caps and tested them.they are marked 2200uf and measure 1700uf. I temporarily replaced them with some 500-1000uf caps. The voltage is fluctuating a bit more but is essentially the same.
Why did you pull the outputs?
Do you have a solder bridge between any of the terminals of any of the output transistors?
Do you have a solder bridge between any of the terminals of any of the output transistors?
One FET was blown, so I pulled that pair and the driver to test. Now I can't remember why I pilled the remaining two pairs. One pair is still in.
(this is a four channel amp, there is one pair per channel) I have replacements for the one channel, do you think I should re install them all?
I checked all of the empty pads with a multimeter, no shorts.
I have been powering it up with a current limited power supply set to 10V, and the amp is drawing 0.5A, nothing getting hot. I tried stepping it up to 12V, and the rails went up to +34V (definitely too high, as the rail caps are 25V) and -22V, current stayed at 0.5A. The power LED is coming on through all of this when I apply voltage to the remote terminal.
(this is a four channel amp, there is one pair per channel) I have replacements for the one channel, do you think I should re install them all?
I checked all of the empty pads with a multimeter, no shorts.
I have been powering it up with a current limited power supply set to 10V, and the amp is drawing 0.5A, nothing getting hot. I tried stepping it up to 12V, and the rails went up to +34V (definitely too high, as the rail caps are 25V) and -22V, current stayed at 0.5A. The power LED is coming on through all of this when I apply voltage to the remote terminal.
Does the channel that still has FETs produce audio. If you have to check for audio, you need to clamp all heatsink mounted components to the heatsink.
I don't know that anyone has tried to measure this voltage and would not have done it myself. It's possible that voltage is leaking from other components and this is normal.
I don't know that anyone has tried to measure this voltage and would not have done it myself. It's possible that voltage is leaking from other components and this is normal.
There is little burps of audio coming about every two seconds. I tried putting the other fets back in, and they have the same output. In the channel that originally went out, the fets are heating up, so I pulled the driver transistor and that stopped the heating.
The biasing may be off in the channel that failed.
Do you see DC pulsing on any of the bridging speaker terminals?
Do you still have rails that are 27 and 18v?
Do you see DC pulsing on any of the bridging speaker terminals?
Do you still have rails that are 27 and 18v?
There is no DC at the speaker terminals. With all audio FETS in, the power supply measures +23V and -23V, at 12V input, but it is not stable, it fluctuates down a bit then back up in a pattern that correlates to the sound burps through the speakers that I mentioned before.
I'm powering it from a bench supply, limited to 1A. The amp display on the supply is fluctuating, mostly at 0.5 A, then it spikes in a 2 second rhythm to 1 A. I tried it with a battery fused at 5 A, and it powered up then blew the fuse in about 2 seconds.
OK, I re installed the driver transistor on channel 4 and that seemed to calm things down, voltage and amp draw are stable. Who knew pulling a component would create such trouble. Well I'm learning, and hopefully no harm done.
Turned the bias down in all channels. Channel 2 and 3 I am getting good audio. Channels 1 and 4 are working but badly distorted. I played with the bias a bit, it didn't seem to make any difference with the distortion. Channel 4 was the original one that had a bad FET.
Turned the bias down in all channels. Channel 2 and 3 I am getting good audio. Channels 1 and 4 are working but badly distorted. I played with the bias a bit, it didn't seem to make any difference with the distortion. Channel 4 was the original one that had a bad FET.