JL 300/4 help please!

[qsilvertwo]

Okay, so recently I bought a used JL 300/4 of a car audio classifieds forum. This would be my first amplifier and component speaker install. The seller said it worked great and everything but when I received it and hooked it up to power the low impedance light comes on after 5 seconds or so and stays on. This is with no rca's or speakers connected. Then after about a minute a high pitched buzzing/squeaking sound arises from somewhere near the power terminal end of the board. The amp is receiving adequate voltage, above 12v, according to my multimeter.

Needless to say I'm really bummed being stuck with this defective amp! I could possibly sell it as-is on ebay but I would still lose $40 from what I originally paid. So I decided I could manage researching the issue and possibly fixing a component myself even though I really have no experience with electronics.

At this point think I've gathered enough information to feel confident enough to start testing the transistors with a Craftsman DMM I have. I would just appreciate some confirmation and guidance if anyone would be willing. In the photo I've posted if you could tell me exactly what transistors are which, such as the channel they operate on, I'd like to know.

Here are my initial readings from the first six pairs of transistors plus the last single one to the left of the capacitors while still in the board:

-first number is from middle leg to left leg, second number is middle leg to right leg, and the last number is from left leg to the right leg.

q300 (OL,.441,OL)
q301 (.972,.441,.841)

q302 (.384,.451,.121)
q303 (.385,.451,.121)

q304 (OL,.443,OL)
q305 (.965,.439,.841)

q400 (.011,.007,.004)
q401 (.342,.438,.602)

q402 (.390,.451,.135)
q403 (.386,.451,.121)

q404 (OL,.441,OL)
q405 (.971,.443,.838)

u500 (.739)

...obviously q400, the left purple transistor is shorted, but are those others supposed to be an open loop? And should I test the four other pairs of transistors near the power supply? What about any resistors, perhaps?

Thank you ahead of time!



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[Perry Babin]

Q400 does appear to be shorted. There have been several other 300/4 threads. Do a search for them. They will cover many of the common problems.

[qsilvertwo]

So I snipped q400 out of the board and the low ohm light is gone and has never come back after several hours but I'm only using the front two channels anyway. The idle temp of the amp went way down too!

Looks like I should replace q400 and q401 as well to be sufficient, right?
Will any of these be fine...

IRF540: MAJOR BRANDS: ICs & Semiconductors

IRF540 "IR" Power MOSFET N-Channel 33A 100V

IRF540 28A, 100V, 0.077 Ohm, N-Channel Power MOSFET (IR)-The Electronic Goldmine

IRF540 - IRF540 MOSFET N Channel Transistor

?

[Perry Babin]

Any of those should work but I'd recommend only buying from authorized distributors. Digikey has them in stock:
Digi-Key - IRF540PBF-ND (Manufacturer - IRF540PBF)

[qsilvertwo]

I put the new transistors in yesterday and everything seems fine, there's no low ohm light and I get audio from all channels. Is it necessary to reset the bias? It hasn't been touched and is in almost exactly the same position as the other channels. But if so how do I do that with a multimeter? Thanks!

[Perry Babin]

There are bias test points on the board. If I'm not mistaken, the voltage across those points is generally set to 0.004v. Don't try it from the top, there's too great a risk of shorting to the bussbars.

You MUST have the heatsink in place and the transistors clamped to it when setting the bias.

[qsilvertwo]

Okay... I'm reading anywhere from 2.2mV to 3.1mV on one channel, 2.6mV to 3.4mV on another, 3.3mV to 4.1mV another, and then the right rear channel which is where I replaced the FETs there is nothing - 0.00mV. This is just idling with no audio signal/speakers.

Why is there no voltage? I do get audio out of it just touching the wires to the terminals and the pots are all in almost the same position at about 11 o'clock. What about the other channels should I increase them to closer to 4mV or let be?

[Perry Babin]

The voltage on the other channels is OK. Try turning the bias pot for the repaired channel slowly (clockwise). Does the voltage across the test points change?

Are all of the LEDs lit on the driver board for the repaired channel?

[qsilvertwo]

LEds are all lit up on all the driver boards. I turned the bias pot from where it was in the 11 o'clock position to 2 o'clock where it now reads 2.9mV idle. Is that fine? anything between 2 and 4mV right?

Oh, and why would it bottom out after replacing the FETs with the pot in the same position? What would happen next time if that same channel needs new FETs again? The pot can only go so much more to about the 4 or 4:30 position and be maxed. Should I have reduced the pot to the min. position before undergoing repairing the FETs or would that have made no difference?
Just curious anyways. I am just glad it's working properly now!

[Perry Babin]

I don't know what you mean by 'bottom out'.

Generally, the original bias setting won't cause excessive current flow with the new FETs. In a few amps, the bias pots are actually reversed (fully counter-clockwise produces maximum current draw). In at least one amp, the pots are reversed, left/right so one channel produces max current fully CW and the other produces max current fully CCW. That's why I generally recommend leaving them as-is until the amp is powered up.

When initially powering up any amp after repairing it, you should have all of the transistors tightly clamped to the heatsink and have some sort of current limiter in series with the B+ line. That helps protect the transistors if there is a fault causing excessive current flow.

2.9mv is OK.

If you have to replace outputs again and they come from a different batch of parts, you may have to set the pot to the 10 o'clock position. The threshold voltage of FETs varies quite a bit that that has a significant effect on the position of the bias pot.