One side of the board is puling 3.3 amps at idle (with power connected to only that side of the board). Also found a few of the PS Fets getting hot one side of the board. This would typically indicate a shorted driver, PS Fet or output transistor.
I pulled all of the drivers and they check good. I pulled the PS fets that where getting hot and they all checked good (replaced them anyway as seen in photo). Ohm checked the opposite side PS Fets and found and they all checked good.
I pulled all of the output transistors on the side same with the overheating PS Fets and they checked good.
I lifted the legs on the resistors for the output transistors on the other side and the amp is still pulling 3.3 amps at idle.
I pulled all of the caps and lifted one wire of the heat sensor and it amp is pulling 2.0 amps at idle (see attached photo for current state of amp pulling 2.0 amps). PS fets still getting hot (circled in photo). I could find no solder traces.
Would greatly appreciate any help on this one...
I pulled all of the drivers and they check good. I pulled the PS fets that where getting hot and they all checked good (replaced them anyway as seen in photo). Ohm checked the opposite side PS Fets and found and they all checked good.
I pulled all of the output transistors on the side same with the overheating PS Fets and they checked good.
I lifted the legs on the resistors for the output transistors on the other side and the amp is still pulling 3.3 amps at idle.
I pulled all of the caps and lifted one wire of the heat sensor and it amp is pulling 2.0 amps at idle (see attached photo for current state of amp pulling 2.0 amps). PS fets still getting hot (circled in photo). I could find no solder traces.
Would greatly appreciate any help on this one...
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You stated that the current dropped after doing TWO things? Which one caused the drop in current?
Without the thermal switch in the circuit, the amp shouldn't come on to pull current. Are you saying that the PS is operating after the thermal switch is disconnected?
Without the thermal switch in the circuit, the amp shouldn't come on to pull current. Are you saying that the PS is operating after the thermal switch is disconnected?
PS power devices are BJT 2N6488. If you lifted a lead on the heat sensor you have a shorted opto or remote transistor keeping it on.
I'd shotgun the actives in the supply for that one, all 11 2N6488, the MPSA06/A56's and the diodes and optos. I had to do it on an identical board HCCA 225 before. It will still be an economical repair even with ALL those parts.
Removing the caps dropped the voltage to 2.5 amps. Lifting one wire of the thermal switch dropped it to 2.0 amps.
No, the PS is not operating after the thermal switch is disconnected. However, my bench power supply is set at 12.5v DC and the DC voltage on the center legs of the PS Fet's is 8.13v.
Only the one 6488 circled in the attached photo is getting hot with the wire lifted on the thermal switch.
No, the PS is not operating after the thermal switch is disconnected. However, my bench power supply is set at 12.5v DC and the DC voltage on the center legs of the PS Fet's is 8.13v.
Only the one 6488 circled in the attached photo is getting hot with the wire lifted on the thermal switch.
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I had the same issue with that 225 I did. I did what I said above and it resolved the issue. I never checked to see what was actually causing the problem tho.
The non-494 amps won't start through a limiter. They just pull about 5 amps without starting.
You can measure the DC voltage across the emitter resistors to see if current is flowing through them. You don't need to desolder them.
The circled transistor isn't a PS transistor. It's the transistor that controls the rest of the transistors. It should not be passing current with the thermal removed. It may be leaky. If it's not, what's the DC voltage with the black on the emitter and the red on the base?
You can measure the DC voltage across the emitter resistors to see if current is flowing through them. You don't need to desolder them.
The circled transistor isn't a PS transistor. It's the transistor that controls the rest of the transistors. It should not be passing current with the thermal removed. It may be leaky. If it's not, what's the DC voltage with the black on the emitter and the red on the base?
I hadn't realized the non-494 amps won't start through a limiter. Thank you for that info.
The reason I desoldered the emitter resistors was to take the 6488 output transistors out of circuit.
I did realize the circled 6488 wasn't a PS transistor and likely was functioning as a regulator or controller. The DC voltage across its base and emitter is +0.320v.
The reason I desoldered the emitter resistors was to take the 6488 output transistors out of circuit.
I did realize the circled 6488 wasn't a PS transistor and likely was functioning as a regulator or controller. The DC voltage across its base and emitter is +0.320v.
If you remove it, does the current draw drop to nothing?
What's the B-E voltage without the transistor in the board?
What's the B-E voltage without the transistor in the board?
With the 6488 removed the amp draw drops to 0.
The B-E DC voltage without the transistor in the board is +0.780v.
The B-E DC voltage without the transistor in the board is +0.780v.
Something is driving it. It could be leaked electrolyte, a solder bridge, a defective component...
There was electrolyte on the board. A through cleaning got rid of it.
Does this amp need to be locked in the heatsink to power it up?
Does this amp need to be locked in the heatsink to power it up?