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Old 16th May 2006, 09:44 PM   #1
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Default sg3525an PWM chip

Hey guys, i Have been trying to get in touch with Perry and havent had any luck so ill post it for everyone to have a gander.

I have an amp that is using the sg3525an pwm. i get good readings an all pins except pin 13. i get a 0.00v pin 15 is 12.45v.

the amp is in protect mode for over current. which means according to the manual;

RED LED: This indicator tells you that the unit has shutdown due to excessive current draw. This will occur if you have too low of speaker impedance, a shorted speaker, or a shorted speaker wire. When you have corrected the problem simply turn the unit off and then back on to clear this indicator.

i know that the guy that had it before me shorted the rca or seems to think so.

i have no speakers hooked to it nor a source input just sitting on the bench. and still get the protect.

i had one transistor read 164 ohms on one leg so i swapped it out. all the others read the same.

any suggestions?

also a side question. if i do a continuity test at either side of a resistor should i hear the beep? or does that tell me it is bad if i dont hear it?


thanks,
D
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Old 17th May 2006, 02:13 AM   #2
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I haven't received any email from you that I haven't answered.


What amp is it?

Did you go through the audio section checking for shorted output transistors, open emitter resistors, bad connections on the emitter resistors, defective driver transistors, shorted or open protection circuit transistors (typically connected across the emitter resistors for class AB amps).

It's unlikely that they cut power to pin 13 to shut it down. There is a dedicated connection for shutdown on that IC. Follow the circuit back to see what is feeding pin 13. It could be something as simple as an open resistor.

If they did indeed cut power to pin 13 to shut it down, you could try to force the IC on (by applying power to pin 13). You would need to remove the device that was previously delivering power to the pin to prevent damaging it.

Keep in mind that that removing the protection circuit can sometimes produce undesirable results (lots of things go up in smoke) so you must be very careful. An inline current limiter is a must. Even with that, some amps will self destruct when the biasing is switched on (when there is a problem and the protection circuits are disabled).

As a safety note... Wear safety glasses when you defeat any protection circuits. On large amps, a face shield is recommended.

Various meters will beep under different conditions. For most of my meters, a continuous beep is heard for low resistance values. For higher values, you may get something like you'd have for a diode or transistor junction. High value resistors will produce no beep.
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Old 17th May 2006, 02:49 PM   #3
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Default THANKS PERRY.

SOUNDS GOOD. I WILL DO A SEARCH ON YOUR SUGGESTIONS.

did you get he pictures i sent to you? i thought it was odd since it had been a week since the first one with no reply.
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Old 17th May 2006, 06:32 PM   #4
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I received nothing from you in the last week. Zip them up and send them again.
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Old 17th May 2006, 07:09 PM   #5
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Default thanks again.

Will do. thanks.
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Old 17th May 2006, 07:23 PM   #6
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Default here is a pic

Click the image to open in full size.

if you look at the pots to the right. the third green one from the top, there are 2 resistors that look burnt. let me know what you think.
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Old 17th May 2006, 09:46 PM   #7
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I don't see anything that looks burned. If there is a resistor that you think is burned, pull one leg of it and check it.

Although it's possible, it's unlikely that anything in the preamp section would cause the over-current protection to engage.
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Old 1st June 2006, 03:08 PM   #8
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Default good news

Perry i got 2 channels up and going last night. i may have an issue with the other 2 and need to track them down. it was a stuck transistor. i replaced it in kind nad low and behold it worked. i have another issue i wanted to run past you on it. if i do a continuity test on the line out rca's im getting continuity from the center pin to the ground pin. i pulled the rca from the board and it went away on the jack but stayed on the board. so my question is what would cause this and where do i look to try and track it down.
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Old 3rd June 2006, 01:41 AM   #9
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I don't know how you define 'continuity'. If you mean it's a dead short (~0 ohms), that's a problem. If it's hundreds of ohms or more, It may simply be drain resistors across the outputs.

Although they're not generally needed on a piece of equipment with a bipolar supply (both positive and negative supplies), the manufacturer may have included capacitors in the output line. The drain resistors would prevent the buildup of DC on the output. The DC could be from a slight DC offset on the output op-amp or DC from the device that it's driving.
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