PPI PC2400 not turning on - diyAudio
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Old 27th April 2010, 03:18 AM   #1
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Default PPI PC2400 not turning on

I have a PPI PC2400 that I had working at low levels. It originally came to me with blown PS FET's and a blown SG3525. It appears on this amp the PS FET's are driven directly from the SG3525, so it does not really surprise me the SG3525 went along with the PS FET's. I replaced both parts and all was good. I then went to power test it. I originally just used a MP3 player with WAV test tones. The drive level was not nearly high enough to get any useful power out even with the gain maxed. So I had to switch over to a function generator to get more voltage drive. When hooking this up my problems started. As soon as I hooked the ground of the function generator to the RCA inputs, the amp started oscillating (and hitting the 10A current limit I had set). The amp started making internal clicking noises, so I promptly turned it off.

I figured out the issue with the function generator, but after I got it running again it got to about 5W output, then the current just went straight down to 0. After this point I could not get the amp to turn back on at all. So I opened the amp up again to check the output and PS FET's. All of those are still fine. I powered it back on with just the +12V, remote, and ground hooked up trying to trace where power was and where power wasn't. I found that pin 15 of the SG3525 was only getting 1.8V. So I figured I had blown another 3525 during the oscillation. I removed the SG3525, looked at the pad without a part and it got the full 12V. I replaced the SG3525 again, but I still get 1.8V on pin 15. All the other pins sit at ~0V except pin 16 which sits at 0.5V. I figure Pin 16 is just trying to generate a 5V reference, but the input voltage is too small to get there.

So my next issue is trying to trace the power up to the SG3525. Does anyone have a schematic or diagram showing which transistors I need to check to get the correct power to the SG3525?
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Old 27th April 2010, 04:13 AM   #2
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That pin (on the 4400) is fed 12v via a 10 ohm resistor and a PNP transistor. In the 4400, the resistor is R104 and the transistor is Q33. I don't know if it will be the same in the 2400. If the resistor is out of tolerance or the transistor is defective, that would cause the symptoms you described.
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Old 27th April 2010, 10:19 PM   #3
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It does not look like the numbering is the same as the 4400. I have included 3 fairly large pictures of the internals if that helps you out. I am about to go out to check some parts, but I thought I would post this up first. Do you think a troubleshooting step of powering pin 15 of the SG3525 with +12V directly could help?

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Click the image to open in full size.

Click the image to open in full size.
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Old 28th April 2010, 01:50 AM   #4
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The transistor that feeds pin 15 of the 3525 is likely in that row of 5 transistors between the black resistor networks. Look for a direct connection between pin 15 and the 3rd leg of one of the MPSA56s.
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Old 28th April 2010, 03:54 AM   #5
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Actually it looks like it is Q17 (next to R56 in pictures 1 and 3) I pulled that transistor out and applied a low current limit 12V signal to the pad and the amp powered on. With the transistor out of the circuit, it still seemed to test fine though. I tried to reinstall, but still no luck. I hope nothing else is pulling it down, but I could not find anything else tied to that line except pin 13. That is another difference between the 4400 schematic. It appears pins 15 and 13 are tied directly together on this board unless I missed something.

Now to order more parts... I swear these little transistors are going to be the death of me. I already have the surface mount SOT-23 version of this transistor, but not the TO-92 like I need.
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Old 28th April 2010, 08:50 AM   #6
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With all parts in the amp, post the DC voltage on all pins of Q17 and the transistor next to it (Q18?) and the voltage measured directly across R56. Place the black meter probe on the amplifier's ground terminal. Place the red meter probe on the point where you need to measure the voltage.
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Old 28th April 2010, 03:03 PM   #7
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I will have to check the transistor next to Q17 later today. I did not find any continuity between any of the pins of Q17 and the one next to it. I did check Q17 in circuit yesterday, and here were the results. Certainly seems to point to Q17 being the issue.

Pin 1 (Emitter): 12.5V
Pin 2 (Base): 6.5V
Pin 3 (Collector): 1.8V

When Q17 was out, and no B+ cable hooked up, I measured R56. It measured 9.9 Ohm, so I assume that it is fine.

If you still think I should check the transistor next to Q17 I will. I did not get to play around with this amp a whole lot, I'm also trying to figure out a weird chassis vs earth ground issue on a kicker KX1200....
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Old 28th April 2010, 06:12 PM   #8
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If those voltages were taken off of Q17 when it was in the board and it's a PNP transistor (MPSA56?), it's defective.
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Old 30th April 2010, 04:15 AM   #9
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I replaced Q17, and the amp turned on fine with an idle current of 1.1A at 13V. The power was very clean on the output, but very inefficient. I could only get to 10W out with about 110W in (amp was drawing about 8.5A at 13V). I left it idle there for awhile, then some magic smoke started coming out. Amp was still outputting clean power when I shut it down. When I opened it back up, R56 was badly burnt. R56 is the 10 Ohm resistor attached between B+ and the emitter of the replaced Q17 transistor. Obvious answer is the SG3525 is drawing too much current or something else downstream is the problem. Even after the burning, the resistor still measured 11.5Ohm. I will obviously replace the 10 Ohm resistor, but is there anything else I should be on the lookout for? Voltages and waveforms from the SG3525 seem fine still. I would think if the SG3525 was blown again (or anything in the power supply or output FETS), the idle current would be much higher.
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Old 30th April 2010, 08:54 AM   #10
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You may have some sort of intermittent problem. If your meter has a min/max function, connect the meter across the resistor and reassemble the amp. If it does it again, the meter should tell you if the current through the resistor changed significantly.
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