First post, I'll try and make it short as I can.
I have an old JL Audio 300/2 that I bought with a JL 500/1 at a swap meet untested. I got them for $20ea and figured I'd find common issues and repairs/schematics online easily. Someone seemingly hooked the power to ground and ground to power on the 500/1 so I havent gotten to it yet.
The 300/2 powered up and the yellow 'low ohm light' lit up. I have a JL 250/1 that did this and replacing the output transistors fixed it-its still running fine today. So I tested them all while on the 300/2 board, and 3 of 8 seemed bad from what I understood about testing them (from Perry's basic amp repair page)
I replaced all 8 of these irf540's, cleanly, and cleaned/reapplied thermal paste, and even put a layer of kapton tape between the output transistors and the heatsink in case the old clear JL tape was misaligned or damaged.
The amp still powers up fine, but after 5 seconds the 'low ohm' light comes on. This happens whether or not I have any speakers or RCA inputs connected.
Can someone possibly tell me what other parts could go bad that would cause a low ohm light even with new output transistors(could be called mosfets or fets, not positive?) and hopefully find a part# to order from DigiKey or elsewhere?
With the backplate off, I can see that both sets of the green lights on the little daughter boards for each channel do light up and stay green, if that helps. Nothing looked smoked or sooted, nothing was loose, no bulging caps;nothing looks odd other than some sharpie marks on the board from what looks like a previous professional repair or from the factory.
Let me know if there's any other info that could help anyone help me, or any suggestions you might have for me.
thank you,
I have an old JL Audio 300/2 that I bought with a JL 500/1 at a swap meet untested. I got them for $20ea and figured I'd find common issues and repairs/schematics online easily. Someone seemingly hooked the power to ground and ground to power on the 500/1 so I havent gotten to it yet.
The 300/2 powered up and the yellow 'low ohm light' lit up. I have a JL 250/1 that did this and replacing the output transistors fixed it-its still running fine today. So I tested them all while on the 300/2 board, and 3 of 8 seemed bad from what I understood about testing them (from Perry's basic amp repair page)
I replaced all 8 of these irf540's, cleanly, and cleaned/reapplied thermal paste, and even put a layer of kapton tape between the output transistors and the heatsink in case the old clear JL tape was misaligned or damaged.
The amp still powers up fine, but after 5 seconds the 'low ohm' light comes on. This happens whether or not I have any speakers or RCA inputs connected.
Can someone possibly tell me what other parts could go bad that would cause a low ohm light even with new output transistors(could be called mosfets or fets, not positive?) and hopefully find a part# to order from DigiKey or elsewhere?
With the backplate off, I can see that both sets of the green lights on the little daughter boards for each channel do light up and stay green, if that helps. Nothing looked smoked or sooted, nothing was loose, no bulging caps;nothing looks odd other than some sharpie marks on the board from what looks like a previous professional repair or from the factory.
Let me know if there's any other info that could help anyone help me, or any suggestions you might have for me.
thank you,
The Q300 FET that is crossed in red is the only one that blew after I replaced all 8 of these transistors. The tiny tiny little “R309 and R308” resistors both measure 129 on my multimeter.
I know my soldering isnt perfect but I dont believe anything is touching where it shouldnt be from that. The amp powers up fine but goes into low ohm protect mode if I power it up with that Q300 FET installed. Can anyone help me find where to test or what to replace to get her going again?
Side note: I tested the FETs in circuit and figured I would replace them all because a few showed shorts when in circuit. Now only the Q300 does. I did not touch either of the yellow and blue POTs. Should I adjust these all the way down and reinstall a new set of 2 FETs in Q300 and 301? Could the BIAS adjustment blow the FETS on one channel? Sorry but I’m not sure what site to upload the images to in order tonpost a link to them.
I know my soldering isnt perfect but I dont believe anything is touching where it shouldnt be from that. The amp powers up fine but goes into low ohm protect mode if I power it up with that Q300 FET installed. Can anyone help me find where to test or what to replace to get her going again?
Side note: I tested the FETs in circuit and figured I would replace them all because a few showed shorts when in circuit. Now only the Q300 does. I did not touch either of the yellow and blue POTs. Should I adjust these all the way down and reinstall a new set of 2 FETs in Q300 and 301? Could the BIAS adjustment blow the FETS on one channel? Sorry but I’m not sure what site to upload the images to in order tonpost a link to them.
I did get them from Amazon so maybe it was a dud. I didnt have a load on the amp and only powered it up for roughly 5-10 seconds, they can really heat up and blow that quickly?
I will change out those FETs, turn the pots down and reuse all of the JL clear mica today, except a small 1” section that I’ll have to replace.
If it blows that again, what other components could cause one fet to blow? I’m not skilled enough to trace the circuit accurately enough to find anything wrong. Everything near the blown fet seems fine to me(which doesnt mean a ton)
Perry, i still only got the single email reply from you, nothing in my junk mail or anything
Thank you for responding
I will change out those FETs, turn the pots down and reuse all of the JL clear mica today, except a small 1” section that I’ll have to replace.
If it blows that again, what other components could cause one fet to blow? I’m not skilled enough to trace the circuit accurately enough to find anything wrong. Everything near the blown fet seems fine to me(which doesnt mean a ton)
Perry, i still only got the single email reply from you, nothing in my junk mail or anything
Thank you for responding
I have replied to all emails. What name am I looking for? People use forum names that have no relation to email addresses.
Were the FETs clamped to the heatsink when the amp was powered up?
If not and the bias was wrong, the FETs can fail in seconds.
Digikey and mouser are generally the most user friendly for parts suppliers in the US.
Were the FETs clamped to the heatsink when the amp was powered up?
If not and the bias was wrong, the FETs can fail in seconds.
Digikey and mouser are generally the most user friendly for parts suppliers in the US.
I hope setting the fets with a multimeter is possible and good enough to test if the repair worked or if there is something else wrong. If i just dial them back all the way down it should have the least chance of blowing again, even though it still wont be set correctly, right?
I emailed you from my work email first, but my personal is <email ID redacted>
It was odd because I got your email saying that you had already responded to my first email-but I never got your original response
I emailed you from my work email first, but my personal is <email ID redacted>
It was odd because I got your email saying that you had already responded to my first email-but I never got your original response
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