JL AUDIO 300/4 Amp Repair
Been attempting to repair two 300/4 JL Audio Amps. Kind of a hobby with little working experience.
I have two old school JL AUDIO 300/4 amps that i purchased dead, and after identifying dead output FETS, ordered parts and replaced each row.
Output and PS resistors checked out ok. All 4 output channel vertical boards have green LEDs lit.
low current bench test: 12V, w/1-3 amp current limit set. Both amps green light. But it appears a feedback loop constantly resets amp.
The wierd thing is that both amps are doing this reset loop. What are the odds that both amps perform the same way. Turn on, green light on, play distorted music for 1 sec, then silence. Wash, rinse, repeat.
What should I look for? I feel soooo close yet sooo far.
Any suggestions as to what might be next to study/observe would be greatly appreciated.
I do have a scope and can see a steady square wave on the PS rectifiers (when rectifiers are removed, when they are on board, amp does a reset loop after about 1 second of distorted sound)
Been attempting to repair two 300/4 JL Audio Amps. Kind of a hobby with little working experience.
I have two old school JL AUDIO 300/4 amps that i purchased dead, and after identifying dead output FETS, ordered parts and replaced each row.
Output and PS resistors checked out ok. All 4 output channel vertical boards have green LEDs lit.
low current bench test: 12V, w/1-3 amp current limit set. Both amps green light. But it appears a feedback loop constantly resets amp.
The wierd thing is that both amps are doing this reset loop. What are the odds that both amps perform the same way. Turn on, green light on, play distorted music for 1 sec, then silence. Wash, rinse, repeat.
What should I look for? I feel soooo close yet sooo far.
Any suggestions as to what might be next to study/observe would be greatly appreciated.
I do have a scope and can see a steady square wave on the PS rectifiers (when rectifiers are removed, when they are on board, amp does a reset loop after about 1 second of distorted sound)
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Thread moved to Car Audio.JL amps are very sensitive to current limiting/low voltage. The surge when the main supply tries to power up may be making the amp shut down.
It's also possible that you missed something in the output stage.
I'd clamp all FETs tightly to the heatsink (with new heatsink compound), use no current limiting and use a 10 or 15 amp fuse in the B+ line to see if the amp draws excessive current (for more than about a second).
It's also possible that you missed something in the output stage.
I'd clamp all FETs tightly to the heatsink (with new heatsink compound), use no current limiting and use a 10 or 15 amp fuse in the B+ line to see if the amp draws excessive current (for more than about a second).
Thanks Perry.
I did just that. assembled the amp, clamped FETs to the heatsink and connected a battery. attached an audio source, very low gain.
got an amp green light. all of the 4 output boards lit up (3 LEDs each)
put my scope on the leg of a rectifier...got a square wave, sound for one second, then flat line again. amp light still green, LEDs still on, but amp resets.
(I read a post of yours and tested the resistor next to the Q501? chip, the resistor tested within allowable range. Pin 2 on the 10pin had little voltage. The LEDs never flickered when push/pull of piggyback board...thought I was on the wrong track then.
Sadly I recently threw out an entire box of ribbon cables as I was clearing out the garage from my old PC assembly days. I will look a bit more tomorrow so I can flip the filter board over and study it.
I did just that. assembled the amp, clamped FETs to the heatsink and connected a battery. attached an audio source, very low gain.
got an amp green light. all of the 4 output boards lit up (3 LEDs each)
put my scope on the leg of a rectifier...got a square wave, sound for one second, then flat line again. amp light still green, LEDs still on, but amp resets.
(I read a post of yours and tested the resistor next to the Q501? chip, the resistor tested within allowable range. Pin 2 on the 10pin had little voltage. The LEDs never flickered when push/pull of piggyback board...thought I was on the wrong track then.
Sadly I recently threw out an entire box of ribbon cables as I was clearing out the garage from my old PC assembly days. I will look a bit more tomorrow so I can flip the filter board over and study it.
Ok...
Pulled all the output FETs...multimeter showed no shorts, but tester pointed at one IRF540 as unknown. for test purposes, swapped it with a known good IRF540.
All output resistor pairs showed 100ohm & 47ohm.
powered up on bench, 12V, 1amp current limited.
on scope, middle leg of rectifier pair showed v build up to +13 and -13 before amp appears to reset (flatline), then wave begins to raise again. Note: my PS can go as high as 5amp if need be.
Pulled all the output FETs...multimeter showed no shorts, but tester pointed at one IRF540 as unknown. for test purposes, swapped it with a known good IRF540.
All output resistor pairs showed 100ohm & 47ohm.
powered up on bench, 12V, 1amp current limited.
on scope, middle leg of rectifier pair showed v build up to +13 and -13 before amp appears to reset (flatline), then wave begins to raise again. Note: my PS can go as high as 5amp if need be.
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I don't think you're going to be able to use current limiting. With JL amps of the slash series, the fuse and being careful may be the best you can do.
New Info...
powered up amp using PS at 12V, 5A setting. no input signal. One Rail Cap was burning hot after 30secs. All other caps,fets,PS, coils, were almost stone cold...save for this single cap...
this cap is an obvious replacement which i had just done because when i pulled and tested the caps it was way off capacitance, so i replaced with a rubicon 2200uf, 50v of an admittedly different size but functionally equivalent as others.
all 3 other rail caps were basically room temperature. I am saying 'rail caps' because I believe that 2 caps smooth out the neg v, and other 2 smooth out the pos v. please correct me if wrong. 🙂
sooo that one cap is hot... i gather then that it is in series with the short somewhere down the line?
i have pulled all of the diodes in that area: D502 etc and they checked out with 0.4 v drop and 0 respectively. also pulled Q510 and tested on transistor tester, smt resistors match ohmage.
where should I be looking?
powered up amp using PS at 12V, 5A setting. no input signal. One Rail Cap was burning hot after 30secs. All other caps,fets,PS, coils, were almost stone cold...save for this single cap...
this cap is an obvious replacement which i had just done because when i pulled and tested the caps it was way off capacitance, so i replaced with a rubicon 2200uf, 50v of an admittedly different size but functionally equivalent as others.
all 3 other rail caps were basically room temperature. I am saying 'rail caps' because I believe that 2 caps smooth out the neg v, and other 2 smooth out the pos v. please correct me if wrong. 🙂
sooo that one cap is hot... i gather then that it is in series with the short somewhere down the line?
i have pulled all of the diodes in that area: D502 etc and they checked out with 0.4 v drop and 0 respectively. also pulled Q510 and tested on transistor tester, smt resistors match ohmage.
where should I be looking?
Attachments
When boards are designed, the silkscreen or pads typically indicate polarity. If you forget how a cap was installed, look at those indicators on other caps to see what's right.