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Old 19th April 2015, 03:47 PM   #1
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Default Luxman M-4000 problems

Hi I am working on a Luxman M-4000 power amp that is giving me problems

I first had the amp given to me last fall & the problem was the right channel would come on distorted sometimes & other times it would not come on at all, so I replaces the relays, checked the bias & dc offset & all seemed well.

about 3 months later the distortion appears again in the right channel, no smoked parts or blown fuses at this point & the switches and pots are clean.

so after I get over the idiocy of them building the R & L preamp boards opposite each other (to prevent easy troubleshooting bu swapping R & L modules, I then discover the amp R & L channel amp modules are identical.

I then unbolted the amps from the unit noting they had been removed before as the bolt tips were somewhat rounded like from the use of vise grips or the wrong size wrench.

I then switched the amps to the opposite channel & the distortion moved to the left channel thus the right channel amp module has the problem.

I start by testing the output transistors (Toshiba 2SB554 & 2SD424) I found a couple has low hfe of like 23 & one I was getting some leakage with my meter in one direction, so I replaced all 6 however that did not solve the distortion problem.

I then moved to the amp board PB-1019, the resistors & caps tested within range so I started with (Q402) 2SC1745 & (Q403 ) 2SA855, as I did not have these parts on hand I swapped them from the working left channel amp

at that point the left channel was still working fine with the Q402 & Q402 from the bad right channel amp but now the right channel amp was dead, no smoke no new blown up parts yet just no sound so I triple checked my solder for bridged traces and all looked good so I figured perhaps I heat killed the 2SC1745 removing it & re soldering it.

I had a hard time trying to source a cross for the 2SC1745 but I found a cross to 2SC1844 NTE199 which now appears to have been wrong as after power up about 30 seconds after the relays clicked in I heard a sizzle noise in the left channel & the right amp smoked up a 100 ohm 2w resistor both the Q402 & Q403 as 5 out of 6 of the power mosfets & perhaps the 2SC738 & 2SA483

so now that I am into it this deep I might as well redo the whole board the only problem I have is a proper substitute for the 2SC1745 & the 2SA855 as well one medium diode D401 which shows a double diode in the same direction sign on the board & schematic it says its an SK SV-02 but nte crosses to both a zener diode & a rectifier diode & i am leaning twards the rectifier one as no zener symbol is on the board or schematics ?

if someone who has experience with these amps & knows about these three parts could point me in the right direction it would be much appreciated as these are a little before by time, I am a retired 80's & 90's technology tech.

thanks in advance, Roger

here is the service manual
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Old 20th April 2015, 05:43 AM   #2
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First of all: nte suggested crosses are always wrong. This is why the amp died. Never use their parts for any reason. STV 02 diode strings (two diodes in series) are obsolete, impossible to find, so be bery careful not to destroy them.
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Old 20th April 2015, 09:53 AM   #3
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thanks that explains the 1.0 voltage drop rather than .5 so the diode is working fine,

now I just need a confirmed cross for the 2SC1745 & the 2SA855 pair that will not blow the amp up lol

what I used was the nte199 & the nte234 & thus I should have checked further as nte has no cross for the 2SC1745 & the nte 199 & 234 are slightly different in their values.

the only info I have found for a cross for the 2SC1745 is (2SC1775A & 2SA872A) or (2SC2240 & A970) both of which are different & without the original 2SC1745 data sheet it is hard to source the best sub
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Old 20th April 2015, 10:31 AM   #4
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2SC1745 Datasheet & Equivalent - 2SC1745 Transistor DataSheet PDF. Parameters and Characteristics. Electronic Component Catalog
Support for Flying Mole, Allen & Heath, Valve amps, older designs & 2CVs.
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Old 20th April 2015, 10:52 AM   #5
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My substitute manual shows 2SC2634(Matsushita)and 2SC1740LN(ROHM).

Gajanan Phadte

Complement of 2SC2634 is 2SA1127

Last edited by gmphadte; 20th April 2015 at 11:00 AM. Reason: added content
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Old 20th April 2015, 02:53 PM   #6
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Try ksa992 and ksc1845. I haven't checked your transistors but these are usually a great replacement for almost any old to92 transistor.
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Old 20th April 2015, 03:40 PM   #7
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thanks I will check into it, BTW the 2SC1745 & 2SA855 complement are TO-39 style if it matters ?

the 2SC2634 is 2SA1127 are hard to get (France & Hong kong) lol

the ksa992 and ksc1845 I can get both here in Ontario Canada
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Old 20th April 2015, 06:44 PM   #8
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Upon closer inspection, I think ksa992 and ksc1845 will be perfect. These are highly useful when restoring old amplifiers - great transistors to have on hand, so get 1000 each.
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