I have a Stetsom EX3000EQ that just stopped playing today. No FETs or outputs blew or shorted, the music simply stopped and the blue Power LED started to flicker and die out and I quickly pulled the power wires to prevent further damage until I got to check it out.
Unable to find anything shorted inside, just not getting +12v at various points inside the amp to allow it to power up. The +5v regulator is only showing 2v on the leg that should have +12v so there is a power interruption somewhere. I have a working one open alongside this one for reference but I just haven't been able to figure out where the interruption is occurring.
I swapped the 5V6 diode(D7) that is in the remote turn on circuit but that component is not defective as the one from the non-working amp, works in the working amp.
Unable to find anything shorted inside, just not getting +12v at various points inside the amp to allow it to power up. The +5v regulator is only showing 2v on the leg that should have +12v so there is a power interruption somewhere. I have a working one open alongside this one for reference but I just haven't been able to figure out where the interruption is occurring.
I swapped the 5V6 diode(D7) that is in the remote turn on circuit but that component is not defective as the one from the non-working amp, works in the working amp.
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Hopefully, someone with more information will help.
Have you pulled the board to see if you see any burned or discolored areas under the board?
Have you pulled the board to see if you see any burned or discolored areas under the board?
Yes, I have checked for any separations, burnt traces, discolored components or areas and cannot see anything. I am not familiar with how this amp operates and followed a few things from other Stetsom threads on here. As far as I understand, the 5v regulator powers up and then everything else from there but I just can't figure out where the +12v is being lost.
I will continue try to compare the working amp but hopefully someone else can throw up some things to check. Thanks.
I will continue try to compare the working amp but hopefully someone else can throw up some things to check. Thanks.
For the most part, the Brazilian amps operate off of a PIC that controls everything. That IC must have 5v to operate.
Follow the traces back from the 12v line feeding the 5v reg on the good amp. See where it's lost in the dead amp.
Follow the traces back from the 12v line feeding the 5v reg on the good amp. See where it's lost in the dead amp.
I am not finding where the 12v that feeds the 5v regulator is coming from... I am still tracing but also hoping someone familiar with Stetsom amplifiers can chime in. Everywhere I trace that circuit to ends with a voltage lower than what is at the regulator.
In the working amp this is also the case except at the other end of the 1UD diodes that are fed by the rectifier and have around 75v or so on the other side. But I assume that the 5v regulator would have to be working in order for the rectifier to be working...
In the working amp this is also the case except at the other end of the 1UD diodes that are fed by the rectifier and have around 75v or so on the other side. But I assume that the 5v regulator would have to be working in order for the rectifier to be working...
I expected that someone else would have helped. I have very little information on the Brazilian amps.
What's the part number on the 5v regulator?
What's the part number on the 5v regulator?
I just repaired a stetsom 10k and it also wasn’t powering up .
The problem I had was one of the windings was broken on the transformer underneath and I couldn’t find the break until I unsoldered the windings from the board .
This is very common in these amps for the transformer windings to break
Do you have input voltage to the regulator check that first if you don’t have input voltage to the regulator check the windings on the transformer for breaks
The problem I had was one of the windings was broken on the transformer underneath and I couldn’t find the break until I unsoldered the windings from the board .
This is very common in these amps for the transformer windings to break
Do you have input voltage to the regulator check that first if you don’t have input voltage to the regulator check the windings on the transformer for breaks
Checked the transformer windings and they appear to be ok. The one thing I have found is that the whole power supply circuit for the ICs reads 78ohms to ground in the non-working amp, while it reads 630ohms to ground in the working amp. My guess is that this strong ground presence in the VCC/VSS circuit is driving down the supply voltage, although I may be off base here. I haven't been able to figure out where the low reading is coming from in the non-working amp and am afraid I will end up destroying one(or both) amp(s) if I keep just removing components searching for the source. I only found one transistor located at Q9 which is marked AL(BCX53) that didn't read as a good PNP should. The collector of this transistor has the erroneous resistance reading but the pads of this location reads the same with the transistor removed. The voltage in pin of the 5v regulator is also in this circuit so there is not enough voltage to generate the 5v regulated output. Both 5v regulators read almost the same and they both work as I swapped them. I believe I need to find why the V IN/VCC/VSS circuit has such low resistance to ground.
Any ideas where to look next?
Any ideas where to look next?
follow the remote trace and see if there is a 4 pin ic connected to it. when these amp just die randomly thats normally the cause
Traced the remote signal and the only IC it goes to is an LM2903 8 pin IC located at U12. I swapped them and both ICs are good.
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I have two of these that work so I am trying to compare them to the non-working one and the only thing I keep coming back to is that the VCC circuit which powers up the ICs has a resistance of 80ohms to ground, while in the two working amps, the VCC circuit reads around 700 ohms to ground. I just feel like that strong ground presence on the VCC circuit is dragging down the power supply to the ICs but I just don't know how to find the problem.
The only last ditch solution I am coming up with is to lift as many VCC legs of the ICs as I can and apply +12v to the VCC circuit and look for the smoke...then order the replacement parts. There are three ICs which have legs that are too thin to lift so I might try separating the trace that feeds them but I would rather not damage the board any more than I already have so I might just risk damaging them.
Any other suggestions before I ram +12v from the + input into the VCC circuit? Would it be advisable to remove all of the PS FETs , Rectifier and Outputs before doing this? Anything else I should remove?
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I have two of these that work so I am trying to compare them to the non-working one and the only thing I keep coming back to is that the VCC circuit which powers up the ICs has a resistance of 80ohms to ground, while in the two working amps, the VCC circuit reads around 700 ohms to ground. I just feel like that strong ground presence on the VCC circuit is dragging down the power supply to the ICs but I just don't know how to find the problem.
The only last ditch solution I am coming up with is to lift as many VCC legs of the ICs as I can and apply +12v to the VCC circuit and look for the smoke...then order the replacement parts. There are three ICs which have legs that are too thin to lift so I might try separating the trace that feeds them but I would rather not damage the board any more than I already have so I might just risk damaging them.
Any other suggestions before I ram +12v from the + input into the VCC circuit? Would it be advisable to remove all of the PS FETs , Rectifier and Outputs before doing this? Anything else I should remove?
Have you removed the board from the heat sink and tried booting the amp up out of the heat sink? Have you tried flexing the board with a non conductive probe? Look for small pieces of wire from the speaker or power wire or other foreign objects that could be bridging some component. Are pins 1 or pin 7 of the lm2903 being driven high causing the chip to shut down? If so, then something outside the remote circuit is the problem. Check all of the passive components around the lm2903 for cracked solder connections or out of tolerance. Does that amp have a digital display on the bottom side of the board? If so there will be a small voltage regulator IC on the digital display board that sometimes fails and will cause this symptom. Do you have a temperature gun where you can read the temperature of components? If not, I have use some liquid flux dripped on components to see which one heats up quickly. You can use canned air turned upside down and sprayed on the board to frost components, then apply voltage and see what defrost quickly.
I got the amp to power up. As I suspected, there was a shorted capacitor at C42, and an open 1R0 resistor at R63. After replacing these items, both working and non-working amp's VCC circuits read very similar resistance to ground. The amp powers up with the 5v and 12v regulators both reading similar voltages to the 2ohm working one. The problem I have right now is that the rail voltage is reading somewhat low. This 1ohm is only reading +88v at idle, while the 2ohm reads +116v at idle. The board is marked +146v. I know this 1ohm amp had a higher rail voltage than +88v previously. What do I need to check to figure out this low rail voltage issue?
I tried a different rectifier with the exact same result. The amp powers up and plays clean audio. The amp powers up and the rectifier outputs +100v for a split second, and then it drops down to +88v. What components are responsible for the rectifier voltage level?
Try adjusting the voltage on your 12v supply to see if the rail voltage changes (likely abruptly).
My power supply does not have an adjustment. The power supply outputs 13.6v with the amp's rail voltage reaching +88v. I connected the amp to a car battery that was resting at 12.6v and the rail voltage reached only +86v.
In some of these amps, there is regulation but as far as I know, the pulse width isn't continuously variable. It comes in steps. You may be on the verge of switching between steps and depending on the pulse width, the voltage may change significantly. If this amp doesn't have this feature, my posts are irrelevant.
So how do I figure out whether it is regulated or not? Is there a certain IC that controls that, or maybe I have some wrong, or out of tolerance components on the board. The changes made are as follows:
The LM2903 was damaged during removal and was replaced with a LM293.
The capacitor that was shorted in the VCC circuit next to the driver IC was replaced with one that read 5.6uf using the capacitor tester of my Sonometer VC9808. The corresponding capacitor in the working amp read approximately 6uF with the same meter. I changed both in the amp under repair. This capacitor is between VCC circuit and Ground.
The 1R0 resistor in the IRS20957S VCC circuit was replaced with a good 1R0 resistor from a parts board.
All other parts in the amp are original but some capacitors, transistors and resistors were desoldered, removed and reinstalled while trying to troubleshoot the VCC circuit problem.
The driver ICs were replaced with new IRS20957S from Mouser as 1 was damaged during removal.
The LM2903 was damaged during removal and was replaced with a LM293.
The capacitor that was shorted in the VCC circuit next to the driver IC was replaced with one that read 5.6uf using the capacitor tester of my Sonometer VC9808. The corresponding capacitor in the working amp read approximately 6uF with the same meter. I changed both in the amp under repair. This capacitor is between VCC circuit and Ground.
The 1R0 resistor in the IRS20957S VCC circuit was replaced with a good 1R0 resistor from a parts board.
All other parts in the amp are original but some capacitors, transistors and resistors were desoldered, removed and reinstalled while trying to troubleshoot the VCC circuit problem.
The driver ICs were replaced with new IRS20957S from Mouser as 1 was damaged during removal.
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