Help repairing JL Audio 250/1

[Jimmy D.]

Looking for help either repairing this amp or sending to someone to repair. Amp powers up but low impedance light stays on all the time. Getting low level roaring, distorted output from woofer. Everything looks fine visually inside the amp - nothing burned or loose. The amp feels unusually hot at the heat sink next to Q404, Q405, Q406, and Q407 even after being powered up a couple minutes with little to no gain. The person who used the amp had it sandwiched tightly between his carpeted sub box and carpeted underside of his truck seat so I suspect heat was the culprit.

Ideas? I'm handy with a VOM and soldering iron.

Jim

[thaumaturge]

If output devices are numbered the same, look for those with different ohm readings.
Doc

[Perry Babin]

This link will tell you how to get the clamps off if you're having trouble removing them:
Jl audio 500/1 Transistor clamps

Q404-407 are the output transistors. Check them to see if any are shorted (as previously suggested). You should not read anything near 0 ohms between the legs of any individual transistor. Check Leg 1-2, 1-3 and 2-3.

Do not apply power while the clamps are off of the transistors.

If you haven't done so yet, read the basic amp repair page (link in sig line below).

[Jimmy D.]

Perry,

Thanks for the link for clip removal. I only measure low resistances on one transistor (all measured on board). Q405 read 28.3, 38.2, and 11.5 ohms between pins. All other output transistors measured either open or very high resistances between pins. Is Q405 bad? i don't see any part numbers. Is it also necessary to measure the power supply transistors?

Jim

[Jimmy D.]

Also when in diode setting all the pins on Q405 measure in the millivolt range as measured pin to pin (28,11,38). The other 3 outputs measure in volts as measured pin to pin.

Jim

[Perry Babin]

It appears that Q405 is defective.

Wipe some of the heatsink compound onto the faces of the transistors then wipe off as much as possible. Then you'll be able to read the numbers. They should be IRF540s.

Yes. Check the power supply FETs also.

[Jimmy D.]

Wow! Rubbing the face of the transistors was the ticket. I can see now that I have 4 ouput transistors, all IRF540 and 6 power supply transistors, all IRFZ44N. If I use the same method to check the IRFZ44N (in circuit) it appears that none of the IRFZ44Ns measure like the failed Q405. It looks like I should only have to replace Q405 but I read somewhere that you recommend replacing all of the outputs if you only replace one. On the right path?

Regards,
Jim

[Perry Babin]

At the very least, you need to replace the shorted one and the one working with it. It would be better to replace all 4. The cost and time is insignificant and will make it more likely that the amp will be reliable.

[Jimmy D.]

I was searching Octopart, Digikey, and Mouser for the IRF540 and see there are many variants on the model number. The four out of my amp have IRF540 with 615P just below the IRF540 label. Does that number mean anything significant? I saw an older post from you saying you used IRF540A in place of IRF540.

TIA,
Jim

[Perry Babin]

That was likely for a Rockford amp.

If the originals were IRF540s (no suffix), use an IRF540 or an IRF540PBF.