Luxman Amp Repair - Crackle in the Tweeter

[Super_BQ]

I've decided that my Luxman M-117 amp needs repair. I've noticed on certain passages or certain classica string insturment CDs I play, I keep hearing static/crackle/fuzz noises in 1 speaker. I swapped the speakers and the other speaker showed the same crackle.

So right now i've got the left channel board out as you can see in the photo. Without any schematic, how can I diagnose this puppy? I suspect that it's an input transistor that may be faulty. However, the board has like 10 different mini transistors on it.

Some assistance would be much appreciated

BQ

Link to photo is


http://www.geocities.com/lelamjade/MISC/

and then click on LUXMAN_M117_1.JPG

[Bose(o)]

I'd say you're on the right track with blaming a transistor on the noise. I know from experience, that transistors cause crackles, buzzes, etc. That's just my opinion though.

Oh, as for determining bad transistors, determine a reference voltage (diode check) or resistance and compare the values of one to the rest. The bad ones will show themselves to you.

[carlmart]

Originally posted by Super_BQ
I've decided that my Luxman M-117 amp needs repair. I've noticed on certain passages or certain classica string insturment CDs I play, I keep hearing static/crackle/fuzz noises in 1 speaker. I swapped the speakers and the other speaker showed the same crackle.

So right now i've got the left channel board out as you can see in the photo. Without any schematic, how can I diagnose this puppy? I suspect that it's an input transistor that may be faulty. However, the board has like 10 different mini transistors on it.

Without the schematics it's really hard to look for a fault of that kind.

Just for the looks I would try two areas: the trimpot, upgrading to a multi-turn one; and cleaning the output transistors legs, that at least on the photo look a bit dirty or the photo looks so. Clean the legs.

Before changing the trimpot take a note of the voltage across output emitter resistors and also the the approximate trimpot setting.

To look for a suspect stage, if you do not have a scope, feed a signal and listen to it after every stage. Follow the signal. You will need some kind of preamp for that and perhaps some headphone, and of course some shielded wire or point to hook it onto transistor or resistor legs. Be careful not to short anything when you do this.

Does the input have a pot? That might be suspect too.


Carlos

[Super_BQ]

Carlmart,

This amp is a straight 2 ch. power amp that is bridgeable. I've all the electrolytic caps on the PBC and they test OK.

I'm not sure if the trimpot idea would change anything as i'm pretty sure the pot on the PCB is used to set the bias.

Without going through a lot of hassles of testing each mini transistor on board, I really need to locate the schematic.

Other than the tweeter crackle, the amp sounds pretty good.

BQ

[djk]

The very first thing I would do is replace the bias pot, with a fixed resistor if possible.

PCN was a problem for many small signal transistor types. They measure fine, both in and out of circuit. Your time is worth more than the parts. Replace them all. Multi-junction bias diodes get noisey too (STV3, STV4, the dual diodes that Marantz used, etc).

The plating on the resistor leads may be failing. At about 15~20 years or so, copper-weld parts begin to have plating failures. The copper striker coat lets go of the steel lead and you have an open circuit. This can be very intermittent. Can happen with capacitors too.