Need Help: Luxman M-02 Bias Setting

[Kecap Manis]

Hi All. I need help in bias setting for Luxman M-02. Unit is running but Left channel is rather hot while Right channel is only slightly warm with no input signal.

I'm a bit confused by the adjustment procedure described in Service Manual. There is a set of test pin as seen in PCB layout, left and right hand side of PCB, halfway up, consists of 3 test pin for Bias, Offset, and Earth. I trace the Offset pin to a point between R142 and R143, as confirmed by shematic. Also Bias pin to between R142 and Q122 emitter leg.

I took a reading with DMM, on Left channel, Bias pin to Earth is -33mV and Offset pin to Earth is -15mV. The Service Manual advise to set the bias voltage to approx 5mV.

For Right channel, bias pin to earth is -15mV and offset pin to earth is -8mV.

Could someone please explain this to me. Thanks.

I tried to attach the service manual but unfortunately the size is over allowable 1MB.

 

[Goldie99]

Hi - I'm not a Luxman expert, and I'm not sure if you're using the same manual (?), but I checked on the SM at hifiengine.com - following are the relevant parts:





https://www.hifiengine.com/manual_library/luxman/m-02.shtml

The adjustment procedures in the Hifiengine service manual don't actually refer to the test pins - but for DC-Offset, the measurements you've taken seem to be correct from the schematic (L = -15mV, R = -8mV). Both are fine, and well within the 0V +/- 50mV spec.

Your Bias measurements however are not correct - the voltage needed to determine bias is the voltage drop 'across' either R143a (L channel) or R143b (R channel), not the voltage from the bias test pins to earth. In this case, because R142a is essentially the same as R143a in terms of bias measurement, you should be able to just use the voltage difference between the bias test pin, and the dc-offset pin (= the voltage across R142a). The R channel would obviously be analogous.

'Indirectly', I think you've effectively measured them already - for the L channel, it should correspond to the difference between -33mV and -15mV = 18mV, and for the R channel, the difference between -15mV and -8mV = 7mV. Both look high, but the L channel very much so.

I'd be inclined to wait for a 2nd opinion, and then adjust them to the 4 - 5mV specified in the manual. I do wonder however why they're so high ? were they incorrectly adjusted ? or has it just happened over time, in which case I'd be more concerned about other potential faults being present ?

In any case, I'd suggest to be very cautious how you take measurements anywhere near the bias circuit, it's very easy to short something and cause no end of problems.... but hopefully you're using mini-grabber test leads, and turning the amp off before relocating them...

 

[Kecap Manis]

Hi Goldie99. Thanks for your clear explanation. Actually the amp has been in the attic for the last 15 years. The first time I took it out and checked, the bias pin to earth was like -60mV on L channel. When playing music the sound would gradually diminished and replaced by hum and buzzes, then after like several minutes the hum would gradually diminished and the music came back on.

The glue that was used to secure the lytics has turned bad and corroded nearby jumper wire. Some of the lytics in biasing circuit C117, C118, C119, C125 are of wrong value, larger than original spec, I believe these are from previous repair. So I pull them out, cleaned the PCB and put in mostly Pana FC lytics (value matching original spec), the original were mainly Rubycon blue and Elna. Now the bias value is as I explained in the first post, somehow drop by half. The only lytics I didn't replace is the main filter 2x15,000uF.

There's another set of caps that looks bad because of the glue, it is described as AC type caps in SM. I was told it is some kind of bipolar tantalum, or perhaps stacked film. It has Vx marking on the body, pic in attachment. They're bypass caps positioned right after 470uF lytics. Another set is at C102a/b but I don't know the purpose of C102.

Let me know if there're other things I should check. Thanks.

Hi - I'm not a Luxman expert, and I'm not sure if you're using the same manual (?), but I checked on the SM at hifiengine.com - following are the relevant parts:

View attachment 1003387

View attachment 1003388

https://www.hifiengine.com/manual_library/luxman/m-02.shtml

The adjustment procedures in the Hifiengine service manual don't actually refer to the test pins - but for DC-Offset, the measurements you've taken seem to be correct from the schematic (L = -15mV, R = -8mV). Both are fine, and well within the 0V +/- 50mV spec.

Your Bias measurements however are not correct - the voltage needed to determine bias is the voltage drop 'across' either R143a (L channel) or R143b (R channel), not the voltage from the bias test pins to earth. In this case, because R142a is essentially the same as R143a in terms of bias measurement, you should be able to just use the voltage difference between the bias test pin, and the dc-offset pin (= the voltage across R142a). The R channel would obviously be analogous.

'Indirectly', I think you've effectively measured them already - for the L channel, it should correspond to the difference between -33mV and -15mV = 18mV, and for the R channel, the difference between -15mV and -8mV = 7mV. Both look high, but the L channel very much so.

I'd be inclined to wait for a 2nd opinion, and then adjust them to the 4 - 5mV specified in the manual. I do wonder however why they're so high ? were they incorrectly adjusted ? or has it just happened over time, in which case I'd be more concerned about other potential faults being present ?

In any case, I'd suggest to be very cautious how you take measurements anywhere near the bias circuit, it's very easy to short something and cause no end of problems.... but hopefully you're using mini-grabber test leads, and turning the amp off before relocating them...

 

[CohibaJoe]

strikes again!!!!!

Remove this stuff and Replace the parts it touches.

 

[Goldie99]

Quite a few of the resistors on the schematic are labelled with a letter 'F' inside a circle. I can't see that mentioned in the key anywhere, but I would hazard a guess that it means 'fusible'. If so, they all need to be checked for value (there's quite a few !), some may well have gone high, with no visible change, and could be causing problems.

I'd also be watching for any signs of the heatsink(s) leaking....

Oh, and now I'm more awake, on the caps: C117, C118, C119, and C125 are nothing to do with bias, they all seem to be part of the protection circuits. Might be worth double checking you didn't misread the values or schematic if the values didn't agree ?

 

[Kecap Manis]

Hi Goldie. Do you know what 'fusible' means in this case? The SM specify 3 kind of resistors; Rd = carbon, Rm = flame proof, Rc = cement. The Rm and Rc doesn't explain much as modern resistor are covered in flame retardant material, also not clear whether they're wirewound or metal oxide type. Most of the larger axial resistors are green colour, like Vishay Draloric. Some are square white radial type, Noble make, R133 R136 R142 R143 if I recall correctly.

On this weekend, I plan to check R146a, R148a which is local for L channel, might be the culprit. One thing at a time, I also plan to change the bias trimpot if needed, I think the original trimpot is open type. I have Bourns sealed trimpot with suitable value.

This is a rather long project for me as I only have limited time on weekend.

Quite a few of the resistors on the schematic are labelled with a letter 'F' inside a circle. I can't see that mentioned in the key anywhere, but I would hazard a guess that it means 'fusible'. If so, they all need to be checked for value (there's quite a few !), some may well have gone high, with no visible change, and could be causing problems.

I'd also be watching for any signs of the heatsink(s) leaking....

 

[Goldie99]

I don't know for sure that they are 'fusibles', just from the locations, and the circled 'F', I suspect they may be - it could also just stand for 'flameproof' - so hopefully one of the Luxman guys will confirm.

If you Google fusible resistors, you'll see the difference - fusibles always fail 'open', and are flameproof, but they can be unstable and increase in value as they age.

 

[Kecap Manis]

I think the circled F is indeed fusible resistor as you pointed out, some kind of current limiting resistor, I just realized the amp doesn't have fuses on the DC side.

Regarding C117, C118, C119, and C125 in your earlier post. The PCB copper side shows sign that it has been worked on around these caps. The solder is different and since the PCB is covered in green lacquer, it's easy to spot if one part has been repaired, the lacquer turned brownish colour. I matched the caps value in schematic are identical to item in parts list so I'm quite sure they're not the original caps, their brand is also different. I know that some time value in schematic vs parts list vs the actual parts might be different. One example is C114, it is listed as 0.01uF in parts list but shown to be 0.1uF in schematic.

One more thing, RLY103 is not in place, it has been jumpered with wire. I guess this relay's purpose is to switch off RLY101 and RLY102 so cutting off connection to speakers in case of too high DC offset.

 

[Goldie99]

I think RLY103 is a protection relay isolating the headphones circuit in the event of a fault, analogous to the separate relays on spk's A & B. The DC-offset seems to be monitored by Q124 & Q125.

Sounds like you've got some checking & rebuilding to do.

 

[Kecap Manis]

I tried to measure the output emitter resistor R133, R136, R142, R143 but my DMM couldn't measure well below 1 Ohm. I replaced 2 of 47uF caps in ACC board which I believed is from previous repair, it has funny brand and with value much larger compared to SM. I measure all rail voltage as indicated in SM and they're okay. Then I carefully adjusted L channel bias to 8mV so matching R channel, I didn't replace the trimpot as the old one feels okay and responsive to adjustment. The Bourns trimpot I have is very small and wouldn't fit well.

Played some music and it sounds okay. Heat from L and R heatsink feels balanced.

Last thing I disconnect the CD player from the amp, then I turn both channel volume to max. There's an audible hum, since my DMM has Hz function, out of curiosity I measured the speaker output = 50Hz, measured main filter caps = 50Hz, measured other set of filter caps (C106, C107, C121, C122) in different section from the main filter caps = 0Hz. Does this indicate the main filter caps are bad? I've replaced all lytics except the main filter caps.

 

[llwhtt]

After the secondary transformer voltage, 50Hz, is rectified and filtered it is 100Hz ripple and if the filter capacitors were bad you should be reading 100Hz.

Craig

 

[Kecap Manis]

Hi Craig, your explanation makes perfect sense but I couldn't measure it more accurately since I don't have an oscilloscope.

One more thing that interest me with the M-02 schematic, C117 (+) is connected to C118 (+). I did double check the print on PCB is also in accordance with schematic. Any idea why they did it this way?

 

[Goldie99]

C117 & C118 are at the input of the DC detection part of the protection circuit. Installed as per the schematic they're essentially equivalent to a bipolar capacitor - so together they are able to handle +ve or -ve DC voltage biasing.

If you have for example a shorted output transistor, you don't know if the DC voltage on those caps (at the junction of R148a / R148b / R150) will be +ve or -ve, so they are arranged to handle both eventualities. A single bi-polar capacitor could also be used there.

 

[Kecap Manis]

Hi Alan. Thanks for your explanation.

The amp is working fine now apart from the slight hum problem. I'll get to that later when I find replacement for the main filter caps. So far has been unable to find one with suitable dimension, they're either too tall or unworkable lead spacing. I think I might end up strapping the new caps to chassis or drill holes for clamp fixing.

Cheers,
Heru