Luxman B-12 Blown Outputs...Bonus Nudies!

[Mchaz]

Well, I was enjoying some King Crimson (see avatar) at less than responsible levels, and my left channel took a dump. As I was rushing over to power down the system I took note that the lights on the corresponding mono block were dead. Just a fuse, I thought!

Well, the replacement fuse blew within the first few seconds after power-on. Upon checking the outputs I found one of three 2SD665's (NPN) with E-C & E-B shorts. Also, one of three 2SB645's (PNP) has an E-C short and E-B open. Is it common to blow just one pair of transistors in an amp with paralleled outputs like this? Are there common causes for outputs to blow like they did? I attached schematics in case anyone wants to take a peek.

I still need to poke around the driver section and check devices, but, thankfully, nothing looks toasty. Some searching revealed that MJ21195G and MJ21196G are good On-Semi replacements for the outputs, and the prices aren't bad at Mouser.

As promised, nudies!

 

[for_p1]

It is possible that drivers gone too. Generally I suspect secondary breakdown in transistors due too like speaker impedance. This amp does not seem to have current limit protection.

 

[CohibaJoe]

I am :drool: over the nudies....

This is also calling out for a Recap.....

 

[Vint Age]

Is it common to blow just one pair of transistors in an amp with paralleled outputs like this?

Too much lark's tongue in aspic will do it every time. Love that album! :yes:

I'd say that there's fair odds that the other output transistors took some serious stress when that happened and if it were mine I'd likely change them all ...

 

[Mchaz]

It is possible that drivers gone too. Generally I suspect secondary breakdown in transistors due too like speaker impedance. This amp does not seem to have current limit protection.

Hmm, interesting you say that. I do know these B-12's were paired with Infinity RS 2.5's from purchase until I acquired them a few months back. The system wasn't used much by the previous owner for the last 10 or so years, but I know he thoroughly enjoyed it in his younger days. These amps have probably seen some very low impedance spikes from those Watkins woofers, so I could be seeing the result of collateral damage over the years.

The speakers being powered during the blowout were ADS L-1590 II's. They are an 8Ω nominal speaker, but I recall reading some lore that they are "amp killers". I never had problems pushing them with my old Adcom GFA-545II, though, and I even had the clipping lights blinking on a couple of occasions.

I am :drool: over the nudies....

This is also calling out for a Recap.....

Yup, it's extremely well built, so I wanted to share. :yes: Practically everything is metal, and it's all tied together with so...many...screws. I only spot a grand total of 12 electrolytic caps (excluding the main filters), so you can bet those are getting replaced. I hope the filters have life left in 'em because they are an oddball size: 15,000uF, 80v, 3" diameter x 2.5" height. The only potential replacements I could find were some made by Cornell Dubilier which were not in stock anywhere, and look to be $100-150 a piece (that adds up with 2 ea. in two amps).

 

[Mchaz]

Too much lark's tongue in aspic will do it every time. Love that album! :yes:

I'd say that there's fair odds that the other output transistors took some serious stress when that happened and if it were mine I'd likely change them all ...

That's the plan. :yes: I wish I had some better test equipment to really look at the health of the other outputs. They could potentially come in handy later down the road if they are still good.

This is the first piece of gear I've fried during use. I feel the outputs had a good final ride ("Eeeaaaa-sy Mon-ehhhhhh!").

 

[Mchaz]

We have confirmed driver death! Q119, a 2SA968, has a C-E short. It's 2SC2238 compliment tests okay, though the HFE seems a little low around 80 (tested with a cheap multimeter). Likewise, Q106 & Q107 (both 2SD668A's) test okay with HFE of 93 and 109...at the low end of their range. It's obvious the drivers need replaced, however should I leave the 2SD668's as is or swap 'em out for good measure?

I'm looking at swapping the drivers for Toshiba 2SA1837/2SC4973. These have been okayed by EW for 2SA968/2SC2238 driver replacements in older threads, and are still readily in stock at Mouser along with the MJ21195G/MJ21196G outputs.

 

[Baron K Rool]

For complementary pairs, replace the whole pair, if using equivalents. Changing half is only okay if you have exact replacements.

 

[cdfac]

looks like that amp already had the DMC op amp replaced by one of those custom jobbies.

 

[Blue Shadow]

As with many Luxman power units, there appears to be some heat discoloration/damage. Worthwhile to do some dissipation engineering for that unit. It is said the LSS from Lux is some of their best stuff.

 

[Mchaz]

I'm back! I'm finally ready to tackle this amp after pushing the project to the sidelines for months.

UPS dropped off a lovely box of caps, transistors, etc from Mouser on Wednesday, and I set aside a few hours today to get some work done! Here's the accomplishments:

• All of the 'lytics were swapped for new Nichicon HE series 105° caps save for a couple of small Panasonic FC series.
• Bias and DC offset pots were replaced with Vishay 19 turn trimmers to save headaches later during the adjustment phase.
• Each output board had a 0.047uf Rifa PME271M metalized paper X2 rated cap (C202/C302) tying the output transistor collectors to ground. The Rifa caps developed cracks all through their casing, so I swapped them for polypropylene film caps from EPCOS. I hope this change in cap type won't degrade the sound!
• Fairchild KSC3503DSTU transistors were installed in place of the original 2SD668A pre drivers that were looking rather charred.
• The toasted 2SA968/2SC2238 driver pair were swapped for Toshiba 2SA1837/2SC4973.
• All new semiconductors got a coat of fresh Arctic Silver Ceramique thermal compound before mounting up to the heatsink.

Here's a hastily snapped pic for your viewing pleasure:

I hope to get it all buttoned up by the end of the weekend, so wish me luck!

 

[rasenthiran]

Hi Machz,

Good luck..recently I purchased the B-12 power amplifier and created a thread few days ago,stereo power amplifier and I am also quite worried about the outputs to. It has the original outputs now. What do you think that caused outputs shorted??

 

[Mchaz]

Hi Machz,

Good luck..recently I purchased the B-12 power amplifier and created a thread few days ago,stereo power amplifier and I am also quite worried about the outputs to. It has the original outputs now. What do you think that caused outputs shorted??

My best guess is that this amp was driven hard for years. The original owner bought the Luxman gear new along with Infinity RS 2.5's. So, these amps have driven low impedance dips their entire life on account of the Watkins dual voice coil woofers in those speakers.

Enjoy your new amps! I wouldn't fret too much about the outputs unless you are pairing the amps with difficult to drive speakers. If they blow...c'est la vie...they can always be replaced. Sure, they'll be missing the Luxman logo, but high quality modern subs should perform fine. I plan to AB my monoblocks afterward to compare the fully original amp with this one that I am working on. Honestly, I doubt I'll be able to detect much, if any, difference.

 

[rasenthiran]

Thanks Mechz, Lucky we can still get replacement for this power amplifier. That was a good news,parts is one of the important thing to consider when using vintage gears.