Hello, recently I fixed up a Bell Model 6060 Amplifier and a 6070 Matching tuner with an original Bell MXP-1 Multiplex Adapter and in return for the work the gentleman gave me a Luxman Amplifier Model L-480 and $300 cash for the work. The only problem is (and he told me upfront about this) the Luxman amplifer doesn't work right, I plugged it in and turned it on and all I got was a flashing power indicator light and about a minute later smoke came pouring out of the cabinet with a pretty strong electrical burn smell that smelled like a burned resistor or something.

Anyways I took the amplifier apart and I noticed a couple of burn marks on the amplifier board (where the output transistors are located), and R419a and R419b and R420a and R420b were both pretty toasty looking and measured way out of spec (and I believe those resistors were what smoked inside the amplifier and were where the smoke was coming from in the amplifier).

From what I read online is that sometimes these old Luxman's were notorious for having their bias current drift way off value if they sit unused for a long period time (which was most certainly the case with this unit) and if so would the amplifier's bias current and idle current being off value cause the symptoms I'm experiencing with this Amplifier as far as having smoked
flame-proof resistors and having burn marks on the main amplifier board around where the idle current adjustment pots are at?

Thanks for your help in this matter.

Those resistors are in the driver stage, and the last four stages are direct-coupled, so no, this isn't a bias problem. If those resistors are smoking it means something is dead shorted, and if one of those transistors shorts, it takes the whole neighborhood with it usually, right up to the outputs. Those are NTE36/37 so they aren't expensive, so there's that. I'd bet you wind up replacing Q409-Q416. Maybe you'll get lucky and one of those electrolytics in the driver stage shorted and it's a quick easy fix.

There are fuses that should have protected the circuits. If they didn't, it's possible someone has been in there monkeying around where they shouldn't have.

It's a really nice amp.

Those resistors are in the driver stage, and the last four stages are direct-coupled, so no, this isn't a bias problem. If those resistors are smoking it means something is dead shorted, and if one of those transistors shorts, it takes the whole neighborhood with it usually, right up to the outputs. Those are NTE36/37 so they aren't expensive, so there's that. I'd bet you wind up replacing Q409-Q416. Maybe you'll get lucky and one of those electrolytics in the driver stage shorted and it's a quick easy fix.

There are fuses that should have protected the circuits. If they didn't, it's possible someone has been in there monkeying around where they shouldn't have.

It's a really nice amp.


OK, thanks for the tips for where to look next.

I was just looking around in the amplifier/output stage and the only thing I see is some burn marks around Q408a and Q408b, and then those toasty resistors I mentioned. I looked around to see if I could spot the fuses you were talking about that were intended for protecting the output transistors and I believe I found them and there seems to be 3 fuses and all three of them appear to be unblown, all three of them are rated at 1A.

There's also what apppears to be a 5 amp mains power fuse that is attached to the side of the housing for the capacitive coupling stage of the output stage and then ontop of that there are protection fuses for the speaker outputs and also the protection relay as well.

I looked all over the driver stage boards and the only capacitor I could see that looked like it may have failed was C504 on the riser board that comes up from the driver board that contains the protection relay and the circuit protection fuses, other than that all of the rest of the capacitors in the driver/output stage of the amplifier look to be in good shape yet, not bulging or shorted looking.

I have uploaded some pictures below showing the spots I'm talking about in the amp.

The first picture I uploaded is a picture of the driver stage of the amplifier showing the damaged resistors and the heat damaged on the board around the aforementioned transistors.

The second picture I uploaded is a picture of the driver stage's riser board that also contains all of the circuit protection features such as the protection relay and the circuit protection fuses.

The third picture is a close up view of the driver stage protection fuses you mentioned in your comment, note that the fuses are not blown.

The final picture is a close up shot of the previously mentioned C504 which appears to have failed, whereas the rest of the capacitors on the board and in the surrounding area appear unharmed.

Let me know what you think.