Having some trouble trying to fix this amp.
The amp is drawing excessive current when powered up. I've found if I remove one of the voltage regulators (MJE2955T) the amp seems to power up fine. I'm currently using a bulb on the 12V lead for current limiting. The resistor and diode associated with the regulator appear to be fine as they read the same voltage as the other side of the amplifier. This lead me to believe, although I'm not sure how accurate, that there was possibly a faulty op amp so I removed them yet there's still high current draw. I'm not sure if I'm even going in the right direction here. Any help would be greatly appreciated!
Shawn
The amp is drawing excessive current when powered up. I've found if I remove one of the voltage regulators (MJE2955T) the amp seems to power up fine. I'm currently using a bulb on the 12V lead for current limiting. The resistor and diode associated with the regulator appear to be fine as they read the same voltage as the other side of the amplifier. This lead me to believe, although I'm not sure how accurate, that there was possibly a faulty op amp so I removed them yet there's still high current draw. I'm not sure if I'm even going in the right direction here. Any help would be greatly appreciated!
Shawn
Hi Perry. Thanks for the response!
With the fuses removed there's still excessive current.
Here's the voltage on the pads with the 2955 removed
Base: -42.5
Collector: -42.5
Emitter: 8.9
These readings were taken with a 2.9ohm resistor inline with B+
I checked the 2955 using the technique in the tutorial and it looks like the transistor is fine.
Thanks for the help!
With the fuses removed there's still excessive current.
Here's the voltage on the pads with the 2955 removed
Base: -42.5
Collector: -42.5
Emitter: 8.9
These readings were taken with a 2.9ohm resistor inline with B+
I checked the 2955 using the technique in the tutorial and it looks like the transistor is fine.
Thanks for the help!
The Zener diode connected to the base of the 2955 is open.
If the Zener opened, the op-amps in the audio section of the amp have either failed or will fail. All should be replaced. These boards are very easily damaged. I'd suggest cutting the legs and removing them one by one (Tech Tips 6, item #8).
Does it draw excessive without the 2955?
If the Zener opened, the op-amps in the audio section of the amp have either failed or will fail. All should be replaced. These boards are very easily damaged. I'd suggest cutting the legs and removing them one by one (Tech Tips 6, item #8).
Does it draw excessive without the 2955?
Things are looking a little better for this amp!
I've replaced all of the op-amps (even used sockets
). I also replaced the zener diode for the voltage reg. and I'm having better results.
Three channels appear to work well with undistorted output.
One channel has distortion. This is one of two channels where I had replaced all of the output fets and drivers. I havn't pulled any of the new components just yet for individule testing, but I did notice that the bias pot has "no" effect for this bad channel. However, the pot by itself still tests fine......
Lastly, the amp draws approximately 1.5 amps at idle. I don't think this is excessive, but honestly I'm not sure if this is exceptable for this particular amp and if it needs further investigation?
Thanks
I've replaced all of the op-amps (even used sockets
). I also replaced the zener diode for the voltage reg. and I'm having better results. Three channels appear to work well with undistorted output.
One channel has distortion. This is one of two channels where I had replaced all of the output fets and drivers. I havn't pulled any of the new components just yet for individule testing, but I did notice that the bias pot has "no" effect for this bad channel. However, the pot by itself still tests fine......
Lastly, the amp draws approximately 1.5 amps at idle. I don't think this is excessive, but honestly I'm not sure if this is exceptable for this particular amp and if it needs further investigation?
Thanks
The over-current protection has been disabled so there's no way for it to bur those traces.
Check all of the following for the distorted channel.
Confirm that you have positive rail voltage on half of the outputs and negative rail voltage on the other half of the outputs.
Confirm that the resistors between the large driver transistors are within tolerance.
Confirm that there are no bad solder joints on the outputs.
Check all of the following for the distorted channel.
Confirm that you have positive rail voltage on half of the outputs and negative rail voltage on the other half of the outputs.
Confirm that the resistors between the large driver transistors are within tolerance.
Confirm that there are no bad solder joints on the outputs.
There is pos & neg rail voltage for all of the corresponding fets ~46v
The resistors for the drivers are fine
I've double checked all of the solder joints and everything seems to be fine.
It's definitely something to do with the biasing circuit... if I measure voltage at the gate for the fets, it doesn't change when I adjust the pot. The three other good channels will vary voltage at the fet's gate when adjusting the pot. I hope that made some sense anyway.....
Thanks
The resistors for the drivers are fine
I've double checked all of the solder joints and everything seems to be fine.
It's definitely something to do with the biasing circuit... if I measure voltage at the gate for the fets, it doesn't change when I adjust the pot. The three other good channels will vary voltage at the fet's gate when adjusting the pot. I hope that made some sense anyway.....
Thanks
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