I have an Orion cobalt 2100 that has a similar issue to problems that have been discussed here before. Problem is it doesn’t look like it was ever solved in the other threads.
This amp started taking out speakers then the magic smoke came out. Opened it to find a fried rail cap that also took out a trace. A couple of resistors desoldered themselves too.
I replaced all electrolytic capacitors. I tested the output fets and found one to be very leaky. I replaced all output fets with some equivalent parts that were recommended on a different thread. I also replaced all resistors that looked burnt in the middle.
The problem is the amp draws too much current at idle and heats up rapidly. The output fets get warm. I placed my meter in series with the source resistor and got about 60mA before the current limit on my power supply kicked in.
The driver transistors get really hot. I have already replaced them with no luck. They are marked with silver on top.
Any suggestions on what to check next?
Cheers
This amp started taking out speakers then the magic smoke came out. Opened it to find a fried rail cap that also took out a trace. A couple of resistors desoldered themselves too.
I replaced all electrolytic capacitors. I tested the output fets and found one to be very leaky. I replaced all output fets with some equivalent parts that were recommended on a different thread. I also replaced all resistors that looked burnt in the middle.
The problem is the amp draws too much current at idle and heats up rapidly. The output fets get warm. I placed my meter in series with the source resistor and got about 60mA before the current limit on my power supply kicked in.
The driver transistors get really hot. I have already replaced them with no luck. They are marked with silver on top.
Any suggestions on what to check next?
Cheers
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Did you try installing all but the leaky original output and see if there was a problem? If idle current is the issue, the threshold of the replacement transistors could be causing the problem.
Yeah, I don't think those are the best match on paper. I did one of mine a while back and selected a set that Fosgate used in some Punch amps because a) I had some leftover stock and b) They looked good on paper and ended up working well in the amp. But as usual, I didn't take any notes and just moved on so that's all the clue I can provide - Sorry. I just really don't recall. Perry probably knows them by heart. Maybe he can help there. The OEM N channel datasheet is attached.
Data sheet
Data sheet
Sorry it took a while to get back to the work bench. Installed the old output fets. Still the same issue with overheating small transistors. Do note. This problem is on both channels. Not just the one that had a leaky FET. Both sides had part getting so hot that they were melting solder and trying to or actually did, fall out.
Rails are at +31.3 and -31.3 v.
I still have not found a schematic online for this amp. The only one I have found if for the NT series.
What should I look for next?
Rails are at +31.3 and -31.3 v.
I still have not found a schematic online for this amp. The only one I have found if for the NT series.
What should I look for next?
What's the DC voltage across the gate and source for the FETs?
Can you see what they're using for biasing? The NT and other Orion amps use a diode string. I don't see that here but what they're using may be in the same location as it is in the other Orion amps (just off of the driver op-amp).
Can you see what they're using for biasing? The NT and other Orion amps use a diode string. I don't see that here but what they're using may be in the same location as it is in the other Orion amps (just off of the driver op-amp).
The diode pair are in series and come off pin 7 of the right input opamp. Shown to the left
The diodes read 1.216 as a pair .614 and .610 each.
The other set on that channel read .606 each.
The other side all measure within 10mV. As low as .605 and as high as .614
The diodes read 1.216 as a pair .614 and .610 each.
The other set on that channel read .606 each.
The other side all measure within 10mV. As low as .605 and as high as .614
If you bridge (connect jumper across) one of the diodes connected to the output of the op-amp, does the idle current increase or decrease?
By jumping out one diode, that channel stops overheating. Both the output transistors and the drive transistors. Tried this on both sides with the same result.
Completely bypassing the diode may be too much for the op-amp to compensate. Do you have a way to test for distortion? Scope, signal generator?
Yes. Just have to dig the scope out. No on the signal generator. A friend currently has borrowed that. I can play a tone from my iPod. I have several tone test tracks. Just have to get everything setup.
Sorry it has been a while. Life and other projects... The output over heating has been resolved by installing some correct value resistors. I also replaced all the 1n4148 diodes. I managed to get my hands on a "functional" amp. However there are some lingering issues. the driver transistors for the output transistors get hot. On the amp I am fixing, they get to about 170 and on the good amp, they are 150. I have ordered some load resistors to test the amp output. Once they show up I will test for distortion on the output.
The bias may still be too high. Compare the DCV across the source resistors in the good amp and in your amp.
both amps have .0005-.0007 across the source resistors no input or load connected. I got my resistors in. The good amp runs great. the bad amp has little output. even with the gain all the way up, it's basically giving unity. Blue is input, and yellow is output. The amp does seem nosier than the good amp. The op amps have +- 17v. The sub/mono switch appears to work correctly.
