View Full Version : Luxman L-85V works for 5 minutes


compact1060
07-13-2009, 11:43 AM
Hello,

I bought a not working Luxman L-85V for the fun of making it work again.
The seller said when he bought it, it worked for 5 minutes then it stopped.
It had no blown fuses and when I turned it on with a light bulb in series the lamp dimmed.

When I opened it up I saw someone had been working in it before. The outputs (2sc1079 and 2sa679) were replaced with 2sd551 and 2sb681 on the left side and 2sd551 and 2sb554 on the right side. The ones on the right side were blown together with 3 other transistors on the 1037 board:2sb537, 2sc1940 and 2sc1941. (According to the service manual the 2sc1941 should be a 2sc1940 also.) 2 resistors were burned and open; 82ohm and 4,7 ohm.

I replaced all 4 outputs with on semi mj15024/mj15025 and all the other parts with right ones according to the service manual. I also measured all resistors on this board and recapped the entire amp.

I turned it on with a light bulb, adjusted bias and DC and started listening to Willie Nelsons " the maker ". It sounded good for 5 minutes then the music stopped and I saw a bit of smoke. The same parts are burned again.

The only thing I didn't replace on the board are the small flat caps, the little blue ones, and some sort of mini diodes (D01,D02). I don't know what to replace these with, could they cause such damage ? On the PB1038 board the resistor next to the relay should be 2watts and has been replaced by 5watts.

It's becoming difficult for me cause I don't see a way to test the amp without
burning the same parts again.

Any help is more then welcome.


http://www.audiokarma.org/forums/attachment.php?attachmentid=159322&stc=1&d=1247499675

http://www.audiokarma.org/forums/attachment.php?attachmentid=159323&stc=1&d=1247499729

http://www.audiokarma.org/forums/attachment.php?attachmentid=159324&stc=1&d=1247499763

dspear99ca
07-13-2009, 12:15 PM
You say you set the bias and DC balance... do you have a schematic and/or service manual? And you say that the same parts fried... all of them? Outputs, small-signal TO-92's, and resistors?

Without seeing a schematic, it's hard to guess. Varistors that monitor heatsink temp to adjust bias would be a guess, or the bias transistor itself. Post schematic if you've got one.

compact1060
07-13-2009, 12:33 PM
hello, thanks for helping,

It's a PDF, I'll try to get it to Jpeg.

compact1060
07-13-2009, 01:05 PM
http://audiokarma.org/forums/images/attach/pdf.gif

compact1060
07-13-2009, 02:02 PM
I forgot to say:

The 2sc1940's stayed ok this time.
Fried components (only on the right channel )are:

the output transistors
2sb537
82ohm resistor
4,7 ohm resistor

dspear99ca
07-13-2009, 02:17 PM
The 2sc1940's stayed ok this time.

Fried components (only on the right channel )are:

the output transistors
2sb537
82ohm resistor
4,7 ohm resistor

What part numbers are they on the schematic.

compact1060
07-13-2009, 02:35 PM
the outputs: Q11 and Q12
2sb537: Q10
82ohm resistor: R21

the 4,7ohm resistor is not on the schematic and it
doesn't have a number on the board.
It's the burned one on the PB1037 picture
on the right side above the bias pot.

dspear99ca
07-13-2009, 03:43 PM
5 minutes seems to be about how long it might take for heat to destroy an overdriven component. I'd have a real good look at Q607 & Q608. If the bias was way too high, the first then to fail might be the Q610 driver. If Q610 fails short-circuit you get full rail voltage to the base of the output Q612 and, via a pair if 82 ohm resistors, to Q611. POOF! The 4.7 ohm resistor is probably off the base of Q610, yes?

compact1060
07-13-2009, 06:45 PM
I measured Q607 and Q608 again and both are ok. They both have 185 hFE and i'm measuring 4 directions infinite, 2 directions diode.

The 4.7 ohm resistor goes to the base of Q609 and this one (2sd382)turns out to be fried also. I'm pretty sure it was ok before I turned the amp on cause i desoldered both for testing and put the one from the good side in the bad channel and vice versa.

dspear99ca
07-13-2009, 06:57 PM
It's not that common (thank god) but transistors DO experience heat-related failures, i.e. work fine until they warm up. If the bias goes way high, and it's kinda' sounding like that may be happening, you'll lose multiple transistors as you are experiencing. As it's different transistors that are failing each time, or at least not all the same ones, you must look for what's common to them, and that's the bias transistor. It's also possible that a transistor checks out fine with a tester, but real-world conditions cause it to fail.

compact1060
07-13-2009, 07:17 PM
I heard about that phenomenon, was allready fearing something like that. I think I'll certainly replace the bias transistor (2sc734) with a new one.

It still doesn't feel safe to turn the amp on with all the new transistors installed though. I interchanged the bias transistor before i turned the amp on. I think if the one coming from the bad channel was bad it should have done damage to the good channel and it didn't.

msskks
07-13-2009, 09:21 PM
Just a thought, but could the bias/offset pots be flaky?

Also, next time leave it plugged into the bulb.

compact1060
07-13-2009, 10:22 PM
Hello,

Sounds like a good idea to leave it plugged into the bulb, didn't know you could do this while playing music. The lamp will start burning the moment it begins to go wrong preventing further damage, is that right ?

I had been measuring bias for an hour and it was stable. After that i played music through headphones for a long time without a problem, it's only when speakers are connected that it crashes.

Would it be possible to measure bias while playing music trough speakers and with the bulb in series ? I never meausured bias while an amp plays music, don't know what kind of value would be too high in such circumstances.

When I adjusted bias I didn't have the service manual yet, and attached wires to the emitter resistors to measure. I posted the manual a few posts back. What would they mean with " CUT OUT RED " ?

thanks for helping everybody.

ryuuoh
07-13-2009, 10:52 PM
Looks like you need to lift the two red wires to set the bias.

hamrules
07-13-2009, 11:56 PM
the outputs: Q11 and Q12
2sb537: Q10
82ohm resistor: R21

the 4,7ohm resistor is not on the schematic and it
doesn't have a number on the board.
It's the burned one on the PB1037 picture
on the right side above the bias pot.


Looks like that 4.7 is showing in the drawing connecting to C513 & Q509 but it is not numbered.

compact1060
07-14-2009, 09:31 AM
You're right I overlooked it, it's just not in the parts list.

msskks
07-14-2009, 07:19 PM
Hello,

Sounds like a good idea to leave it plugged into the bulb, didn't know you could do this while playing music. The lamp will start burning the moment it begins to go wrong preventing further damage, is that right ?

I had been measuring bias for an hour and it was stable. After that i played music through headphones for a long time without a problem, it's only when speakers are connected that it crashes.

Would it be possible to measure bias while playing music trough speakers and with the bulb in series ? I never meausured bias while an amp plays music, don't know what kind of value would be too high in such circumstances.

When I adjusted bias I didn't have the service manual yet, and attached wires to the emitter resistors to measure. I posted the manual a few posts back. What would they mean with " CUT OUT RED " ?

thanks for helping everybody.

The bulb should come on at a dim level when you turn the amp on. Maybe bright initially tapering down to dim, depending on the amp idle power draw. The bulb will flicker with your music, up to a point, getting brighter with volume. You will have limited power and then the amp will clip if you try to drive it harder. The bulb limits maximum input power to (insert bulb wattage here) and no more. That's the purpose- if there is a fault, the bulb limits current to (hopefully) prevent component damage. But it gives you a chance to run the amp into a load reasonably safely. It sounds like the problem only shows itself into a load. Don't use your high-dollar speakers on this amp until you get the bugs ironed out!

You can only measure bias w/out a load connected and no signal source. It also sounds like the bias may not be set correctly.

Mike

compact1060
07-14-2009, 08:19 PM
I'm gonna try this, sounds like the safest option. I'll recheck bias according to the manual.

I ordered the parts today so I'll have to wait a bit.

wolfe
08-29-2009, 08:47 PM
Dear Sir, i hope you have resolved your problem.
i recently have my Luxman L85v amp volume pot replaced by an unknown brand. The replacement job was not well done and the volume pot is not functioning properly.there is no info on this replaced pot
i like this amp and would like to get the properly spec of the volume pot so that i can look for a replacement from Alps or a good used replacement if possible.
If you are still owning this amp, would you be able to provide me the spec of this volume pot ? Or can you advise me where can i look for a proper replacement.
Thank you and beg your pardon for joining your forum with this irrelevant issue.

Best regards,
Wong

Blue Shadow
08-30-2009, 10:25 AM
Wong,

The schematic provided in an earlier post shows the volume control as VR3a b. It has the resistance of this control listed in the diagram. It is probably an audio taper pot.















it is 200kΩ

megawatt
08-30-2009, 10:50 AM
I would suspect the diodes. Especially D601/D602. This are directly assocaited to bias control.

also replace any zener diode. These break down with use over time.

resonator
11-10-2009, 09:33 AM
-reviving an old thread

i'm fixing the same amp here. in my case i've replaced 2sa1079/679 with mj15015/16 and 2sb537/d382 with mje15030/31. they work fine. however i have problems with the 2sc1940.

do you guys know of any modern day alternatives for the 2sc1940? apparently they're out of production already. they come in a weird sp-3 package which is propriety of nec.