Luxman LV 105U

madtech

Well-Known Member
Luxman-2.jpg

Have a Luxman LV 105 U on my work bench that is dead as a hammer. The power switch is good, all the windings in the primary measure fine but the leg with the internal transformer fuse is open. Anyone tear one of these apart before to go after this fuse? This things in great shape and it would suck to toss it. tks!
 

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Bypass it. Those thermal fuses are putrid little bast#rds that fail for no reason when they get old, often just from turn-on current.

You could dig into the windings, make a big mess and fight to install another one, only to totally balls-up the entire transformer.

It's only in certain markets they were mandated, and that requirement seems to have been dropped- I rarely see modern gear with primary thermal fuses anymore.

Bring the primary into pin 1 on J1003. From the pic, there appears to be a primary M205 line fuse in any case (on the power switch PCB)
 
Great, thanks for the input! I'll give that a shot.There is another line fuse on the power switch PCB.
 
And we have lift off! Primary into pin 1 on J1003 to bypass the transformer fuse. Thanks to restorer-john for the advice!

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I can see someone has been in it before, the mosfets are labelled in marker pen on the heatsink. Make sure they are Toshiba originals (J115/K405) in that amp- anything else is a no-no.

Monitor the transformer temperature over several hours to make sure it isn't getting hot.

Also, if it's the multivoltage version, confirm correct voltage setting on the selector switch.
 
Yes, I could tell from the soldering and marker that someone has been in here before me. The transistors are marked J200 and K1529 (not marked Toshiba). The selector voltage is correct and the transformer (in my mind) is running too hot. I can only leave my hand on it for 20 sec or so. Too hot?
 
Definitely too hot. The thermal fuse tripped for a reason in this case.

You can disconnect all the secondaries and then power up the transformer unloaded- if it still runs hot, the transformer is toast.
 
The J200 and K1529 are not the same as the J115/K405.

It really annoys me that some techs are too lazy to source the originals. The amplifier is a complicated design, and fragile in some ways but worth the effort to bring back if the transformer isn't toast.
 
Thanks for all of your help! I really do hate working on kit that someone else has been messing with. I'll unload the transformer and check to see if original Toshibas are available.
 
With the transformer secondaries all unplugged and the amp plugged in and powered up, the transformer is almost stone cold.
Time to find some output transistors..... :)
 
It's time to take stock of the situation.

Confirm the secondary voltages on the transformer are all good (unloaded). You want to make sure the secondaries are not damaged from the current and heat that blew the thermal fuse.

Pull out the replacement mosfets. They have quite different characteristics to the originals and will not be going back in that amp. :) You will be rebuilding the amplifier with original parts where possible and if you run into trouble with the mosfets, let me know. They are not cheap but available ex-Japan.

The tube section in that amp is the second differential stage for the power amp, and the sockets get very touchy and there will be dry joints all around the tube bases. Be very careful with repairing and rebuilding that part- it is very easy to break the plastic mounts (ask me how I know that).

You can get the amplifier working in both channels right up to the final power stage by disconnecting CB7301&7302 and having the J115/K405s out of the amp, without risking more mosfets and without cooking your transformer. Once that is done, you can rebuild the power stage from scratch if necessary and DBT it with a functional front end without losing the expensive Toshiba FETs.

You may have one channel oscillating or both, or one pair of P/N FETs are hard on or partially on. Regardless, they have to go.
 
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Actually the left and right channels sound fine as I took the risk and tried speakers. It took about 15 min for transformer to start generating too much heat, then I shut it all down. I'll check all the secondaries and start looking for some original output FETs. Looks like there's a lot of re-soldering going to take place too. Thanks for all of your help, it definitely has saved me a lot time and effort!
 
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Any source for the secondary transformer unloaded voltage values? I'm not seeing anything in the service manual.
 
Found a working donor for ref secondary, no load, voltages (thanks Dan). All center taps were half the voltages and resistance so for future reference for anyone working on the L105U: S15 - .9 ohms, 16.25 v; S12- 50 ohms, 205V; S13 - 12.9 ohms, 50V;
S14 - .2 ohms 8.44v; S1 - 1 ohm, 89V.
The working donor's values were almost the same but S13 was 3 ohms more but the voltage was the same. Transformer is good! Donor amp had the smaller power supply caps replaced. Might be time to get a recap out of the way.
 
Bypass it. Those thermal fuses are putrid little bast#rds that fail for no reason when they get old, often just from turn-on current.

You could dig into the windings, make a big mess and fight to install another one, only to totally balls-up the entire transformer.

It's only in certain markets they were mandated, and that requirement seems to have been dropped- I rarely see modern gear with primary thermal fuses anymore.

Bring the primary into pin 1 on J1003. From the pic, there appears to be a primary M205 line fuse in any case (on the power switch PCB)

Old discussion but wanted to thank you because this worked for me too.
 
Zombie Thread revival…

I have an LV-105 (NOT a “U”) US 120VAC that went dark on the main board.

Small xformer still functions, lights up tubes and front panel LEDs.

Main board not to much.

Original owner. I’m not a tech. Have kept it running over the last 38 years cleaning all contacts/switches/dials, replaced tubes, finding/re-flowing cold solder connections as they crop up.

Will the by-pass fuse on the main transformer work the same on mine as the “U” variant?

Thanks in advance,

RD
 
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