Repairing a Infinity 7541a 4 Channel Amp

Discussion in 'General Electronics Chat' started by Sirenguy, Jun 17, 2008.

  1. Sirenguy

    Thread Starter New Member

    Jun 17, 2008
    Ok, this probably serves me right, buying a used amp on Ebay, but the original ad said one channel was not working. I bought it anyway, hooked it up, and all channels worked! My setup at the time was Infinity Kappa 3 Way 6x9's in the front doors (modified from coaxial to component- mounted mid/tweeters in the dash openings) and as of yesterday I removed the factory Infinity 6 3/4" drivers and installed Kappa 2 ways in the back doors. Here's where things got weird. As soon as I turned the stereo on, a Pioneer AVH-6500DVD, it started to squeal LOUDLY in all speakers once I turned it up beyond 7 on the volume scale. I turned it down, and that back up slightly, and it happened again. To worsen things, I see a plume of smoke rise from the front left tweeter/mid!
    Keep in mind, with the factory rears connected, it never did this. I checked impedances, or DC resistance mainly with a DMM. The factory drivers were 2 Ohm, and the new Infinitys read 3 Ohm. I disconnected the rear channel, and tried listening again. I turned on the engine, in case the amp was starved for voltage, and heard HORRIBLE engine noise at the same level as the radio's audio.
    I have since then disconnected the amp, and connected the speakers to the head units output, and surprisingly, they all still work! Still running a seperate powered sub from the preamp output.
    My amp obviously took a dump. I have no schematics, just a bench power supply, digital oscilloscope, a signal generator and a 5 1/4" Kicker as a test load. My idea is to ring out each channel, front to back, and see where things look odd. If anyone has suggestions, or even schematics, either would greatly be appreciated.

  2. bloguetronica

    AAC Fanatic!

    Apr 27, 2007
    You should never buy e-bay. He said that one channel was not working but I think he might have omitted the rest.

    When you buy e-bay, you are in serious risk of suffering credit card fraud. The site is know to be very vulnerable, and the webmasters simply don't care.
  3. Sirenguy

    Thread Starter New Member

    Jun 17, 2008
    Ok, I should clarify that I didn't post this thread for coaching on internet purchases. I'm looking for technical assistance. I originally bought this amp for troubleshooting purposes, and Infinity generally builds good stuff, so I figured why not buy a partially defective amp, fix it and save some money, as I have been doing for some time now. I'm simply using this forum as a technical resource to speed up the repair time, if possible. So if someone has some useful information, I would appreciate it.
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    Using speakers instead of resistors for testing a defective amp gets expensive right away. Never connect a speaker until proper amp function is established (especially a DC voltage across the load).

    With no schematics life can be interesting. If all channels amps are identical, you can always try to A - B test, assuming one channel works. The frustrating part is finding an IC that is bad, but not being able to obtain a replacement.

    Equipment manufacturers like to contract for custom IC's. The IC is very similar in function to standard ones, but the part number will not turn up any information, nor will it be available as a replacement part.

    Drawing up a schematic from the exposed circuit can be helpful. Expect the balance and fader control function to be a bit weird.
  5. Sirenguy

    Thread Starter New Member

    Jun 17, 2008
    I have some 3 Ohm, 10 W Dale resistors if need be, that can be used.

    There is a ton of filtering circuitry compared to other amps I have opened up before, so that would be timely, drawing up my own schematic. I can look up IC numbers to ID ins and outs, but intend to just follow the signal from start to finish. Can use the scope to see distortion.
  6. roddefig

    Active Member

    Apr 29, 2008
    Have you tried hunting on google for information? I was able to find a solution to an A/V receiver that was having issues after a fair amount of searching. Unfortunately, as beenthere mentioned, it involved a custom-made IC that I couldn't order so in the end I threw it out.

    Sorry I can't provide any more specific information, but good luck!
  7. SgtWookie


    Jul 17, 2007
    Here is a link to a downloadable service manual:
    Unfortunately, it isn't free - but it might just help you save your amp.

    That amp is 111W per channel. I'm afraid that your 10W resistors will be turned into cinders at even moderate power levels, and you've indicated the problem appears at level 7 on your stereo - what that relates to, I'm not sure. You should use resistors of around 200w rating or higher.
  8. Sirenguy

    Thread Starter New Member

    Jun 17, 2008
    Thanks SgtWookie! I'm ok with paying $10 for a's still way cheaper than a new amp. I actually have a programmable load bank in our lab I just remembered about, so I'm sure I can stick that on the output if the 10 Watt devices run hot. Or may just run up to Mendelson's in Dayton this weekend, and look for some big monsters.