Hello to the diy community!
My friend gave me this old sub amp with words "It's in protection mode, it's yours". I'm trying to find whats wrong with it and right now it's far from instant success.
There is no schematics for this amp, so I'm just guessing what's what.
Power supply of this amp and it's protection circuity are build around TL494 chip. When in protection mode the TL494 doesn't even start, it's not generating sawtooth nor square waves, so no voltages to the amp.
What I've tried so far:
I desoldered a full bridge rectifier (two devices near a big *** relay) and with no load power supply started OK, no protection mode, 55v ac on the rectifier input. Square waves are little ugly (ringing) but they are present on all power fets.
I concluded that the problem is not in the power supply but in the output section of the amp. I checked output fets and they aren't shorted as far as i can say.
This amp has an output driving section I never saw anywhere else - there is one add-on pcb and there is just one unknown blobbed chip on it.
So now I'm kinda lost - are there any general method to find what in the output section doesn't let the power supply to start? Should I trace the protection circuity of the tl494 and go from there?
Any advice would be appreciated!
My friend gave me this old sub amp with words "It's in protection mode, it's yours". I'm trying to find whats wrong with it and right now it's far from instant success.
There is no schematics for this amp, so I'm just guessing what's what.
Power supply of this amp and it's protection circuity are build around TL494 chip. When in protection mode the TL494 doesn't even start, it's not generating sawtooth nor square waves, so no voltages to the amp.
What I've tried so far:
I desoldered a full bridge rectifier (two devices near a big *** relay) and with no load power supply started OK, no protection mode, 55v ac on the rectifier input. Square waves are little ugly (ringing) but they are present on all power fets.
I concluded that the problem is not in the power supply but in the output section of the amp. I checked output fets and they aren't shorted as far as i can say.
This amp has an output driving section I never saw anywhere else - there is one add-on pcb and there is just one unknown blobbed chip on it.
So now I'm kinda lost - are there any general method to find what in the output section doesn't let the power supply to start? Should I trace the protection circuity of the tl494 and go from there?
Any advice would be appreciated!
Attachments
I found nothing wrong on the main amp board so I went ahead and pulled the driver board.
D11 and D12 (H5 marking) testing as short both sides, so maybe something is shorting on this board, just need to figure either its those diodes or something else.
jacampb2 tracing and schematic will be super helpful here.
I wonder if it's safe to power on the amplifier with driver board plugged in but without the tiny driver chips board installed
D11 and D12 (H5 marking) testing as short both sides, so maybe something is shorting on this board, just need to figure either its those diodes or something else.
jacampb2 tracing and schematic will be super helpful here.
I wonder if it's safe to power on the amplifier with driver board plugged in but without the tiny driver chips board installed
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Reinstall the rectifiers and set the current limiter to about 5-10 amps.
Clamp all FETs and power it up. Does it try to power up?
Do you see a pulse of DC on the input side of the relay (assuming that the relay will not engage)?
Do you see rail-rail oscillation on the center legs of the output transistors?
Clamp all FETs and power it up. Does it try to power up?
Do you see a pulse of DC on the input side of the relay (assuming that the relay will not engage)?
Do you see rail-rail oscillation on the center legs of the output transistors?
Right now I have the whole driver board pulled from the amplifier,
If I reinstall it as-is then the amplifier tries to start up but goes into protection mode and shut down the tl494 chip, relay not clicking.
So even if I reinstall it back I will not be able to see output oscillation as there will be no power to the output section.
If I reinstall it as-is then the amplifier tries to start up but goes into protection mode and shut down the tl494 chip, relay not clicking.
So even if I reinstall it back I will not be able to see output oscillation as there will be no power to the output section.
Did just that, reinstalled the driver board, installed 4ohms on the output.
Powered with 12v, max current, but amp goes into protection mode.
I see a momentary voltage spike on one of the relay pins, relay doesn't click.
Also I see a voltage spike on the rectifier output.
There are no voltage spikes on the driver board voltage pins (+9v, -9v, +Vcc, -Vcc to their grounds)
While the driver board was pulled from the amp I poked all the components on it with dmm, except the blobbed opamps.
They are all ok, except R5 smd resistor - it should be 51Kohm but reads around 15Kohm
I'm still figuring out how to test those blobbed opamps.
Powered with 12v, max current, but amp goes into protection mode.
I see a momentary voltage spike on one of the relay pins, relay doesn't click.
Also I see a voltage spike on the rectifier output.
There are no voltage spikes on the driver board voltage pins (+9v, -9v, +Vcc, -Vcc to their grounds)
While the driver board was pulled from the amp I poked all the components on it with dmm, except the blobbed opamps.
They are all ok, except R5 smd resistor - it should be 51Kohm but reads around 15Kohm
I'm still figuring out how to test those blobbed opamps.
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