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Old 9th August 2010, 04:52 PM   #1
chipper is offline chipper  United States
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Default Repairing Rockford fosgate punch 40X2 channel help

I wanted to try and repair a channel on a Rockford amp ive had sitting for months it works great but one side gives me 27 volts. Maybe some mosfets shorted out? Is it possible to buy some new mosfets and just slap em in and will function again? Or will the problem be present again? Could there be anything else short? I measured the big resistors and they read 0.01 is that good? Sorry for all the questions,,,still trying to learn. Thanks
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Old 9th August 2010, 07:20 PM   #2
FireII is offline FireII  United States
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Can you be more specific when you say "one side gives me 27 volts" ?
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Old 9th August 2010, 07:36 PM   #3
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Typically you would check the gate resistors and drive transistors prior to powering it up after fet replacement. Clamp things to the heatsink and fuse the B+ line to help preserve the new fets in case you missed something.
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Old 9th August 2010, 07:36 PM   #4
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Quote:
Originally Posted by FireII View Post
Can you be more specific when you say "one side gives me 27 volts" ?
I suppose it's 'rail voltage' I measured it with a dvm. Before that it was pushing the speaker out. It's the left channel that's down. What shout I look for on this??
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Old 9th August 2010, 07:42 PM   #5
FireII is offline FireII  United States
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Quote:
Originally Posted by chipper View Post
I suppose it's 'rail voltage' I measured it with a dvm. Before that it was pushing the speaker out. It's the left channel that's down. What shout I look for on this??

Set your meter to diode check, check the voltage drop between all three legs of your output transistors, meaning place one lead on leg 1 and the other on leg 2, then check 1 and 3, then check 2 and 3(especially). If you get anything near .0v then the transistor is toast. If you post a picture of the amp we can tell you which transistors are your out puts, or if Perry chimes in he can prolly tell you off the top of his head.
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Old 9th August 2010, 07:45 PM   #6
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Grab the 40_2 schematic from my site, it should be close to what you have.
https://sites.google.com/site/brandescm/home/projects

Outputs are labelled Q115, Q119, Q215, Q219
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Old 9th August 2010, 08:19 PM   #7
chipper is offline chipper  United States
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Quote:
Originally Posted by FireII View Post
Set your meter to diode check, check the voltage drop between all three legs of your output transistors, meaning place one lead on leg 1 and the other on leg 2, then check 1 and 3, then check 2 and 3(especially). If you get anything near .0v then the transistor is toast. If you post a picture of the amp we can tell you which transistors are your out puts, or if Perry chimes in he can prolly tell you off the top of his head.
Yes I already did that and I got on diode check (IRF9540) .004 according to my meter and it was making a beeping
noise indicating it was shorted and I got for (IRF540) legs 1&2 (gate to drain) .528. Gate to source i got .075.
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Old 9th August 2010, 08:29 PM   #8
chipper is offline chipper  United States
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Quote:
Originally Posted by brandes.cm View Post
Typically you would check the gate resistors and drive transistors prior to powering it up after fet replacement. Clamp things to the heatsink and fuse the B+ line to help preserve the new fets in case you missed something.
Can you tell me which ones are the driver transistors? Are they surface mount? I checked the small surface mount resistors and i got 11 ohms for R150 and 12 ohms for R152 they both have 100 marked in them. Are these 10 ohm or 100 resistors? Thanks for the help!
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Old 9th August 2010, 08:34 PM   #9
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I would replace all the output transistors for the left channel. Also check the large emitter resistors check if there is and discoloration on them.And also check the driver transistors for the left channel outputs.

If all check good replace the outputs and turn the bias pot fully counterclockwise.

Replace the IRF9540's and 540s's with the same part or switch to IRF540A

(A) as the suffix.

The 540N and 9540N's may give you problems as some of the rockford amps ive worked on didnt like the 540n's and 9540n's

Hope this helps
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Old 9th August 2010, 08:37 PM   #10
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They are surface mount transistors for the drivers They are usually marked 1a and 2a set your meter to diode check and test them the same way as you did the outputs. Make sure you pull the bad outputs from the board before u test them.

Also the resistors you tested marked 100 are 10 ohm resistors if you have 11 ohms on 1 and 12 ohms on the other they should be fine
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