Luxman LV-105u replacing outputs help!

[rinsbag]

I need some info regarding replacing the output transistors. I realize finding original replacements is not going to happen. I have purchased a pair of IRFP9240 and IRFP240 after reading some threads about this amp.

I need to know if I need to modify anything else inside to get these to work. Also should I replace both channels with the new parts. I believe that one channels original outputs are still working. So do I need to replace both sides?

I did try and install these new parts on the dead channel. Everthing went OK I thought. But as I turned it on and try to adjust the Bias I am getting 0.0v on one side and the other seems fine I can set it to the 65 mv the service manual asks for.

I have done many tests to see where I am getting voltage. Or for short resistors. I found a couple that were out of tolerance and replaed them accordingly. Voltage is making it to the main board. On the side with the original outputs. My measurements are not to far off the required values. The other side gets voltage but nothing across the emitter resistors. I cannot find the short. Please help

I am not looking to send this out to be fixed by a tech. I want to do it myself. I do have some knowlege in electronics but I am not a tech or professional. Please anyone that can help me I greatly appreciate it.

Do I need to look at the driver board? Or main board? or other mods to get these outputs to work.

The amp's tubes both light up. I really like this amp. Please reply.

[Gigapod]

Hi rinsbag,
Since nobody replied, here is my (limited) knowledge of the issue:
1) I have bought an LV-105u recently, left output stage is burned out. I haven't had much time to look into it and repair the damage.
2) The Luxman LV-105u is a great amp, however the output stage MOSFET transistors are known to burn quite easily if driven too hard. IMHO they are operated too near their SOA.
3) Original transistors are 2SK405/2SJ115, which can be found but you really have to look hard on the net.
4) AFAIK nearest replacements are 2SK1529/2SJ200, and 2SK1530/2SJ201.
5) I have read about people using 2SK1058/2SJ162.
6) AFAIK IRFP9240 and IRFP240 are not really recommended.

Here is an extract from the schematics showing the output stage.

I am preparing a small quantity order for 2SK1529/2SJ200 devices, if you need two pairs or more just PM me.

[jacco vermeulen]

Gigapod,

you do not accept PM's !

[Gigapod]

Hi Jacco,
Well, I checked the Enable Private Messaging option in my control panel and it is set to "yes". Do I have to do anything else to enable PMs?

[rinsbag]

Hey Sorry its been a while since I was working on this amp. But seeing it just sitting there sucks. I now know a bit more about this but still could use some assistance.

Gigapod. Is there any thing you can tell me about using the formentioned? I have been testing for the correct voltages and some seem really close to the specs in the schematic. How close do they have to be?

Please reply. anybody. Is it a power supply thing?

[Lord Winter]

Hi guys !

I've given a few thoughts on this matter, these are my suggestions:

- the original output devices are similar to standard hexfets.

- the most important difference is the amount of gate voltage needed
to "open" the device, also called as Vgs(th) or Vgs(off).

- 2sk405/2sj115 Vgs(th)= 0.8V to 2.8V

- IRFP240/9240 Vgs(th)= 2V to 4V

- pinout is the same.

- just drop in the IRFP's and try to adjust the quiescent current to nominal value (100 mA ?)

- if you cannot set the needed amount, try to replace R7301/7302 with
4.7kOhm resistor, this will extend the range of Vgs setting.

I hope you dont fry anything !
Good luck !

[ilimzn]

K1529 and J200 can be used as replacements with no further modification. K1530 and J201 will also work fine and offer even better robustness. Although the input capacitance of these parts is somewhat higher, the higher gm makes them about the same to drive as the originals. I have tried both these options and given the higher Idmax of the K1530/J201, would use them as the preferred option.

From the IRF cam, the ideal pair to use would be an IRFP340 and IRFP9240, but an IRFP240 will work as well. The bias generator will have to be modified due to (much) higher treshold voltages. Unlike the K/J parts, with a treshold between 1 and 1.5V, the IRFP parts will be closer to 4V each for the 100mA standing current. The original trimmer will not have enough range. R7301/7302 have to be replaced by higher values, try 5.6k.

I have not tried the K1058/J162 combo, and would not do it based on the lower maximum ID of these parts. This might work in the smaller LV103, though. The problem here is that the pinout is different, the source resistors have to be removed and the bias arrangement modified to a single series pot (all other parts must be removed/shorted as necessary).

[rinsbag]

Thanks for the replies everyone. I will let you all know what happens. I really enjoyed this when it worked.

[evette]

are your output transistors mosfet or bipolars?
if they are mosfets than they must be 2sj115,,and 2sk405 types. you can still get these devices.

[iaguiler]

Hello! I was hoping someone could inform me on how to take the measurement of 65 milliamps using an ammeter. I have no idea how to take the reading. Also, what is the purpose of this adjustment? Thanks!