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

[TubeJunkie]

I checked those big wattage (10.5 watts) emitter resistors. Left channel was around .010 mv dc. Right channel was .004 mv dc.

Thanks mbz. I was following the M120a service manual: Removing fuse F805 and connecting my meter leads to jumpers 13 for negative lead, and 8 for positive lead. I never connected my meter leads to the fuse holder. Which may explain why I was not getting any movement on the trim pot?

 

[mbz]

I was following the M120a service manual: Removing fuse F805 and connecting my meter leads to jumpers 13 for negative lead, and 8 for positive lead. I never connected my meter leads to the fuse holder. Which may explain why I was not getting any movement on the trim pot?

I rechecked the service manual, you followed the correct procedure. I will do a quick recheck to make sure it makes sense.

I checked those big wattage (10.5 watts) emitter resistors. Left channel was around .010 mv dc. Right channel was .004 mv dc.

I assume this is the voltage drop across the resistor, ie, one probe on each end rather than measurement with one probe on chassis.
I also hope it was 10mV not 0.010mV and 0.004mV, my meter would not read so low.

 

[mbz]

Thanks mbz. I was following the M120a service manual: Removing fuse F805 and connecting my meter leads to jumpers 13 for negative lead, and 8 for positive lead. I never connected my meter leads to the fuse holder. Which may explain why I was not getting any movement on the trim pot?

I got a bit confused, normally thge check is on pins on the power amp board.
The procedure appears correct. Remove fuse, check pins on that fuse/power supply board.

 

[elnaldo]

Did you check voltages as I suggested, on the power amp circuit, with all fuses connected, to see if the transistors are conducting?

Every time I mentioned to measure a voltage "across" a resistor, I mean Red probe on one end and Black probe on the other end of the resistor.

Please check fuses are OK (measure continuity with your meter), then install all the fuses, and measure voltages at each bias stage as suggested.

If you doubt the power amp is getting power, check the +V and -V reaching the power amp section.

 

[TubeJunkie]

Checked output emitter resistors for high DC. The highest one was 4.5 mv. I was expecting to see one that was high, but didn't find it. I also checked voltage between R203 & R201a, it is 1.135vdc. Checked voltage across R206 & R207, there is none. As elnaldo stated previously, there should be voltage present across R206 & R207. So, what does this mean? Thanks

 

[TubeJunkie]

Back on this M-120a amp. I found a bad transistor. Q212a ( 2SC2240). I installed a new one, still stuck in protect mode.

 

[elnaldo]

If you don't have high voltage at the emitter resistors (against ground ), suspect the protection circuit itself.

But make sure the voltage is less than one volt at the emitter resistors, both channels .

 

[TubeJunkie]

I checked for high voltage on the output emitter resistors, didn't find any. I did find a broken trace on the front panel board, and a loose wire on the speaker switch. Fixed all that. Still stuck in protect mode. I'll continue to investigate the protection circuit and re-read all these previous posts. Maybe recheck those emitter resistor voltages, since I fixed the loose wire and circuit trace. Wish me luck!!!! lol

 

[elnaldo]

The relay is managed by the DC voltage sensor circuit and the "excessive current" circuit, (Q210, Q211, VR202 what I guess is there to set the threshold, check the service manual)

Check the schematic for the relay driver circuit at the bottom of the page. Q114 must be ON to close the circuit , biased by Q115. Without a deep understanding of the circuit, I suppose under DC at the output, Q118 is turned ON, turning ON Q116, shunting the voltage from Q115 base to ground. I think the same happens with Q116 under an over-current event, it will probably turn ON to shut down Q115, so the relay becomes OFF.

 

[TubeJunkie]

Thanks for all your advise. It is greatly appreciated. I checked Q114 thru Q118 with my meter set to diode check. All seem fine. Also D118 thru D120. All test fine. I pulled Q115 out of circuit for testing. Could not find any problems with those components. Is there a way I can test LY101 relay, without pulling it out of circuit? Thanks again!

 

[Chits]

I think this model has those double transistors in the input stage. They are a common cause for trouble. Replace with two ksa992 transistors with the emitters soldered together.

 

[TubeJunkie]

Hi Chits. Which transistors are you referring to? Thanks.

elnaldo what's your thoughts?

 

[elnaldo]

I think he referres to 2sa798 double transistors. If any in the unit, replace them all.

You can test the relay, you can apply the right Dc voltage to the terminals, with a current limiter resistor in series. The relay should "click".

Anyway I suspect a problem with the electronics, not the relay, but you can check it. You can check if the relay is receiving the right voltage at the coil.

We need to check voltages against schematic again .

Please remove the output transistors and measure voltage drop at the emitter resistors at the bias circuit. Also DC at the output emitter resistors. It must be near zero even without the output transistors.

Also, voltages at BCE at each transistor. As before, that will indicate where the problem is .

Some transistors can check OK on the meter but fail under the higher voltage in circuit.

 

[TubeJunkie]

I didn't find any double transistors (2SA798) in this unit. Please let me know if I've overlooked them.

elnaldo - I've removed all of the output transistors. See attached picture. All check ok, and very similar to each other in respect to they're like kind. So, I think they are all good.

When you say Check voltage drop, do you mean test between the emitter resistors legs? And when you say Check D.C., do you mean, negative meter lead to chassis ground? Please forgive my novice questions, I just want to have a clear understanding before I go probing blindly. Lol

Sorry for the slow reply, I've been down with back problems for several weeks. Thank you again for your help.

 

[elnaldo]

"Voltage drop" at a resistor means volt meter probes at each end of the resistor, to measure the voltage across the resistor itself. That shows you the current flowing at that resistor.

DC at the output emitter resistors means DC voltage against ground, Black probe to ground, Red probe at the junction of the emitter resistors or any end of the resistor, since they are very low resistance, it won't make a difference, we are checking if you have a high voltage or just some mV of DC offset.

Check you have the output transistors insulated from the heat sink.

 

[TubeJunkie]

Thanks elnaldo. Should I take those readings before I reinstall the output transistors?

 

[elnaldo]

I'd check before installing the output to see if it's safe to do it.

Use a DBT for the first time and then full power to take measurements.

 

[TubeJunkie]

Ok elnaldo , I used my Dim Bulb Tester to power on this amp without the output transistors installed. The light bulb dimmed quickly, so now I'll power it on with full AC and and take those readings that you suggested earlier.

 

[elnaldo]

Good. Check DC at the output rail (emitter resistors). If it's high, something is still wrong.

 

[TubeJunkie]

Hello again elnaldo ... I checked the voltage at the emitter resistors without the output transistors installed. All readings were .004 VDC. So, next I will reinstall the outputs and check again.