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Old 14th May 2006, 06:07 PM   #1
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Default burnt resistors on PPI PCX-1250

Hi,

I have a precison power pcx-1250 amp, and it just recently stopped working. While connected to the sub, all of a sudden it started to just thump the speaker randomly and make the cone stand out of the basket, so I quickly disconnected it and opened it up.

I found two resistors that are completely burnt and I am unable to determine the value of them. They are R101 and R201 and are located right behind the RCA input connectors on the board. If anyone has access to this model amp and could retrieve the resistor values for me, that would be great. I looked through the 'show me your guts' thread and actually found a picture of this amp, but it wasn't hi-res enough to tell. However, I do notice that the resistors on that board(picture) are different from the ones that burned up on mine. Mine aren't blue like in that picture. Maybe that has something to do with why they burned in the first place. Do those blue resistors dissipate heat better?





Here are some pics of my install and amp:

Click the image to open in full size.


The Whole Board:
Click the image to open in full size.


Burnt Resistors:
Click the image to open in full size.




Some history about the amp:
I got it second hand from a trade I made on a forum. The person told me that it works perfect except for the low pass filter, which he tried to repair but was unsucessful. I just used an external crossover with the amp and I had no problems with it for over a year. I believe thhat this amp was refurbished because I can see a couple solder points that look like that were done by hand and are different from the majority. Upon opening it, I also found this where the xover switch is located:
Click the image to open in full size.



I'm sure that just replacing resistors isn't going to solve the real cause of the problem, right?
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Old 14th May 2006, 06:49 PM   #2
Netlist is offline Netlist  Belgium
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You might try to get the attention of TO-3, a member here who worked at PPI.

/Hugo
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Old 14th May 2006, 07:56 PM   #3
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Default Re: burnt resistors on PPI PCX-1250

Quote:
Originally posted by Netlist
You might try to get the attention of TO-3, a member here who worked at PPI.

/Hugo
Thanks for the props, Hugo.


Quote:
Originally posted by cutlass1991
Hi,

I have a precison power pcx-1250 amp, and it just recently stopped working. While connected to the sub, all of a sudden it started to just thump the speaker randomly and make the cone stand out of the basket, so I quickly disconnected it and opened it up.

I found two resistors that are completely burnt and I am unable to determine the value of them. They are R101 and R201 and are located right behind the RCA input connectors on the board. If anyone has access to this model amp and could retrieve the resistor values for me, that would be great. I looked through the 'show me your guts' thread and actually found a picture of this amp, but it wasn't hi-res enough to tell. However, I do notice that the resistors on that board are different from the ones that burned up on mine. Mine aren't blue like in that picture. Maybe that has something to do with why they burned in the first place. Do those blue resistors dissipate heat better?






Some history about the amp:
I got it second hand from a trade I made on a forum. The person told me that it works perfect except for the low pass filter, which he tried to repair but was unsucessful. I just used an external crossover with the amp and I had no problems with it for over a year. I believe thhat this amp was refurbished because I can see a couple solder points that look like that were done by hand and are different from the majority. Upon opening it, I also found this where the xover switch is located:



I'm sure that just replacing resistors isn't going to solve the real cause of the problem, right?
Cutlass, it looks like the 2 resistors that are burnt are the ground-coupling 10ohmers that 'isolate' the RCA(shield) ground to the audio ground. The fix is easy, just swap the resistors out with 10 ohm; 3watt parts. You won't like the cause.

Usually what happens is that the amplifier has a poor groung connection and the amp has to source its ground through the RCAs. As a result, all of the amplifies current tries to go through those resisotors but they cannot accomodate that much current. So, fix your ground and replace those two guys and you should be set. Let us know.
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Old 17th September 2006, 09:31 PM   #4
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Hope this isn't thread jacking but could this planet amp be the same ground problem? I can't read the values but on side is 0 ohm to RCA Ground and other side is 0ohm to Speaker - output.
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File Type: jpg planet audio amplifier.jpg (84.8 KB, 100 views)
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Old 17th September 2006, 10:09 PM   #5
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Quote:
Originally posted by confused256
Hope this isn't thread jacking but could this planet amp be the same ground problem? I can't read the values but on side is 0 ohm to RCA Ground and other side is 0ohm to Speaker - output.
Yeah. Those should be 10 ohms. I am suprised that they are that small(wattage). Replace them and make sure you check the ground connection to the amp.
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Old 18th September 2006, 12:02 AM   #6
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This input circuit is common to many amplifiers. For R101, they use values between 1K and 220 ohms. If you look closely, the parts in a parallel group on the board are identical. The designations have a difference of 150. If R251 has survived and you can read its value, that's what you need for R101.

Don't use a larger wattage resistor, if it burned, there's a reason. When it opened, it protected your head unit's shield ground. If you are concerned that it will further damage the board if it burns again, mount it ~1/2 inch above the board.

The attached image is part of a schematic for a different amp but it gives you the general scheme of things.
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File Type: jpg planet01.jpg (80.8 KB, 101 views)
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Old 18th September 2006, 12:28 AM   #7
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I retract what I said. Thanks, Perry. A schematic is worth 1000 words.
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Old 18th September 2006, 03:59 PM   #8
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I found a picture for a Planet 2200 and the value there looks like 220 ohm. One of the resistors burnt has one brown ring, that is all i can read, so judging on that I was confused cause brown = 1. But if i get it close I should be ok right?
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Old 19th September 2006, 01:31 AM   #9
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I can't remember seeing an amp that used a 100 ohm in that location so it's likely a 1k ohm resistor.


Did you try to measure the resistance with an ohm meter?


Before you reinstall it, check all of your speaker wiring. The secondary is likely isolated (secondary ground not directly connected to chassis ground). If that's the case and you have a speaker wire shorted to chassis ground, that could have caused this damage.
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Old 19th September 2006, 11:38 PM   #10
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Meter showed something like 7k. Ya the speakers wires might have toched ground they weren't take care of properly. It is going in a different ride that I will be installing all the power and connectors. Mostly I see people with bad grounds on amplifier and it kills the power supply inside.
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