Luxman M-120A has static and hiss in Left channel

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Hi everyone!
I'm helping a buddy figure out a problem with his Luxman M-120A amp. It has varied static and hiss in the left channel. Here's what I've done so far;

Cleaned all pots and switches with contact cleaner.

Checked and soldered any suspicious solder joints.

Removed the relay closest to top of unit, and cleaned all contacts. It was very corroded.

I've poked around inside the unit with a wood stick while it was powered on and none seems to change. The static gets louder and quieter at times, but the hiss is constant. Hopefully some of you has some has experience and advise on this amp/problem. Thanks
 
Have him hook up speakers to remote speaker posts and play in remote mode, it may be that that main relay is going bad. There is no loss in power or sound quality when played this way. Doesnt solve the problem, but its probably good for 10+ more years. Mine was.

I use it in mono, so I'm hooked to them big posts.
 
I hooked up the speakers to the remote speaker terminals. Still have the same static noise. I could pull and clean the other relay, I guess. But I'm feeling like it's not the relay, but a faulty cap or transistor in the left channel. Or even a dirty/bad adjustment pot. Any thoughts on any of this? Thanks again
 
I found a high heat area on the pre/control board.I replaced those 4 caps, & 2 resistors (c104 a&b, c106 a&b, R122 a&b) in that hot area, and the static cut to half. Then I removed and cleaned contacts of that 2nd relay, and the static is gone. Still has a very, very small hiss, but I've been listening to it for about an hour, and sounds good. There's a total of 3 relays in this unit. Anyone bought replacement relays for a Luxman M-120A ?
 

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background hiss sometimes is generated by small singnal transistors, at the preamp or at the power amp 1st stage. SOme transistors get noisy with the years. If you hear it's much louder than a normal hiss, try replacing the low noise transistors with newer parts, do only one channel to have a reference.
 
Still trying to figure out this faint static noise in this Luxman M-120A Left Channel. It's making the left channel slightly distorted. Currently I'm attempting to set idle bias voltage per the service manual. I'm getting -64 VDC. Manual says 70ma. I removed fuse F208, posiitve meter lead to pin 8, negative lead to pin 13. Am I missing something here?
 
Are you measuring voltage and looking for a milliamp (current) result? If so, change the meter setting.
 
Yes, That's what I was doing. My bad. Sorry about that. But when I set my meter to DC amps, I get no reading. Set it to AC amps, it bounces back and forth from 0.00 to 20ma. I've checked ma many times but I'm feeling confused
 
Try measuring DC Amps on other thing, like a 9v battery with a resistor.

BE CAREFUL, THE METER SET TO AMP METER IS A SHORT FOR THE CIRCUIT. ALWAYS SET IT BACK TO OHM READING AFTER USING IT. In a distraction moment you can connect the anmeter to some point to measure voltage causing a short. Don't ask me how I know.
 
Meter fuse is not blown. But I now have a more user friendly meter that I can check DC amperage. Ok, So, working from the service manual, Right channel DC offset is dead on at 0 VDC, Left channel bounces up and down. Can't get a steady reading. goes up to .20 millivolts and back to negative .20 millivolts.

Trying to adjust Left channel bias idle, turning VR201a does nothing. The reading I'm getting is -.27 MA DC. I pulled fuse F805 before hand, as instructed in the service manual. I've looked for bad solder joints, but have'nt found any. Any advise will be greatly appreciated.
 
Anyone care to give some thought on the idle bias adjustment problem I'm having? Thanks
 
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OK, downloaded the schematics file, I'm reading the luxman_m120a_schematics.pdf

To bias the outputs, you need a current flow through Q201 and R201 VR201 R202. A bad D201 will interrupt the current flow. Please check the voltage across D1, it shoud be around 2 Volts from R203 end to R201 end (ends connected to diode D201)

That will bias Q202 and Q203, so you should have more than 0.7V at Q202 and Q203 bases. That will bias Q204 Q205 bases turning those transistors ON, so they conduct, so a voltage drop should be present at R205. No voltage at those bases will turn off the transistors. No voltage across R205 means Q202 or 203 are not conducting.

Then, a current should flow across R206-207 showing Q204-205 are conducting. Please check voltage across R206-R207. You must have voltage across those resistors. That current flow will bias the output transistors with approx. 0.7V at the bases.

A current should flow across R212-R213, showing Q206 Q207 are conducting. That's the bias current you will adjust finally.

With this instructions, try to check each stage of the bias circuit.

Basically, you measure voltage across the emitter resistors at each stage, to see if the transistor is conducting. (Note that all the resistors I mentioned are connected to each transistor emitter)
 
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Thanks elnaldo. I reinstalled F805 fuse. The amp is now stuck in protect mode. Can I still take the voltage readings as you described above with it in protect mode?
 
I think so. But being in protection Mode means you have several volts at the output. I'd check/fix that first.

Please measure the voltage at the output emitter resistors against ground. It must be zero or at least less than 100mV. Check both channels to see which one is trigging the protection.

Don't let the amp powered for long time when protected. Just measure and power off. Or use a DBT to check for a short.
 
I checked the DC voltage at speaker terminals, and both channels are normal. Very close to zero. The output emitter resistors are the big white 10.5 watt, dual section resistors (eg: R209a)? Correct? I should check each outer leg of those?
 
Can't check at the speaker post because relay is not closed. Need to check at emitter resistor as previously suggested, any leg of resistor
will do just to confirm presence of Vdc on line.
 
Problem: varied static and hiss in the left channel.

To measure current (mA) the multimeter must be connected in series with the circuit.
This usually involves lifting a connector off a post and connecting the MM between
connector and post and completing the circuit. However most methods simply involve
measuring the voltage drop (mV) across a known resistor and calculating the current
from I=V/R.

Bias procedure for m120a calls for removing a fuse which breaks the circuit, then
connect the MM between the fuse holders. expect 70mA, will show as zero on amps
scale. Measure on dc mA scale.

dc offset bouncing between +20 to -20mV is not ideal (the bouncing).

Does the relay click 3-10 seconds after power ON, if not then there is excessive dc
on the line and you should fix that first.

elanaldo (post 16) gives some good steps on how to check the biasing.

VR201a alters Q201 Vbe, if there is no response to VR201a then there is an outside
chance Q201a has failed (suggest diode test), also outside chance D201a has failed.
These are quick GUESSES. Suggest follow elanaldo systematic measurements.
 
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