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Luxman 5L15 Bias adjustment

esteboune

esteboune

BleuBlancRouge
Hello everyone,

I recently acquired a Luxman 5L15, a great piece!

I manage the adjust the DC offset to 0V (before= 0.5VDC on the right channel :dunno:.

However, I am struggling with the Bias adjustment.

I do not know the target value and most importantly the 2 measuring points.

Does anyone have this information (No service manual available for this model) ?
The schematics are enclosed

thanks in advance!




upload_2020-9-23_22-12-7.jpeg
 

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Found this on the Luxman.io group site.

Robert Karl Stonjek 11/26/19 #10466

OK, I figured out the BIAS and other adjustments. This is a tentative list as I haven't actually done them as yet:
BIAS RT701 (Blue)
On Power amp board
Test across 0.22 Ohm 5W output resistors
7mV +/- 1mV for 32mV (27~36mV)
40mA max
Tested cold

DC Offset RT601 (Red Horizontal, all others vertical trim pots)
Under amps near DML 01 (Q601)
DC Offset Phono Preamp RT101 (Red) (I think this is what that adjustment does...more when I've tried it)
Left side of main board behind DML 02 (Q101)
Meter level adjust High sensitivity RT 401 (White)
Meter level adjust Low Sensitivity RT 402 (White)
Do high sensitivity first
Cluster of four white trim pots right side of main board.

Thread continues with successful presentation of the amp with needles working together.
 
I see you asked on the .io group. Looks like you got a copy of what I posted from Robert. Hope this is info you can use.
 
thanks a lot Blue Shadow for your messages.

indeed, i create a message on the .io group.

I think I manage to adjust the DC offset, and the Bias.

let's have a look!
1. Dc offset for the phone Preamp, I did not touch it.




2. Bias adjustment
On Power amp board
Test across 0.22 Ohm 5W output resistors
7mV +/- 1mV for 32mA (27~36mA)
40mA max
Tested cold


3. VU meter level adjustment, I did not touch it !
Meter level adjust High sensitivity RT 401 (White)
Meter level adjust Low Sensitivity RT 402 (White)
Do high sensitivity first



4. DC offset



Overall boards view
 
Results:

1. Bias Adjustment:



1.1: Left channel:

7mV :bigok:



1.2: Right channel:

7mV :bigok:


2. DC offset:

2.1: Right channel:

0VDC :bigok:


2.2: Left channel:

1mVDC :bigok:

 
Excellent results, all in about 36 hours. Took me about 15 years to get my Lux tube amps converted and operational. But most of that delay was waiting for the Internet. Glad to see Robert still on .io and helping out.

I see your amp has replacement DMLs in it. The ones Lux used to repair the amps that had early failures. These are supposed to be just fine.

Thanks for posting how to pics to make this thread helpful for the possibility I can finally add one of these to my collection or for others. Only LRS piece I have is the 5F70.

Enjoy your amp.
 
That is a fact... I'll give you a shout when I get a break in the action over here.

John
 
@esteboune

Thank you so much for posting this!

Is there a chance that we could lose access to these photos on this and your other excellent threads if that web host (if that’s the right name) changes things up in the future?

I just saved all the pics for the 5L15 adjustments just in case. Thanks again!
 
I've shared this vid already somewhere but I'm leaving it here because this is the best 5L15 thread on AK.

Thank you to the video creator's son who had to type in the subtitles. I thought closed caption was automatically done by google magic but there is nothing for Polish to English just yet.

 
Hi All, I'm refurbishing my 5L15 these days.
I've managed to get the bias and the DE offset as per the specs described above.
One question, how to set the DC offset for the preamp. I've located TPa and TPb close to the DML, I guess that these pins are where to hook up a DMM.
Any idea?
Many thanks!
Phil
 
Thanks Binkman, but the point here is that there's no SM so far for this amp, these pots are not the ones for the w/meters (w/meters pots identified elsewhere).
The preamp (I guess) pots are the ones shown hereunder (post #4)


IMG-5028.jpg
 
Hi, I've tried to connect my DMM between TPa and ground and it looks like it's how it works...
Adjusting the pot, I can set the voltage close to 0V. I'll replace the pot by a multi turn one to make it easier....
It someone could confirm or give another explanation, welcome!
 
Philsound said:
Hi, I've tried to connect my DMM between TPa and ground and it looks like it's how it works...
Adjusting the pot, I can set the voltage close to 0V.
Click to expand...
I saw TP a an b... , wonder what the other pin on top pic is? but if it works w/your test. How much off before you attempted adj.?
 
Around -150mV and moving when adjusting, going positive and negative, very sensitive, that's why I will install a multi turn pot.
I found the same method in another Luxman SM, same TPa,b , and decided to try.
The last ones are the 4 pots for the VM...but this requires to know the input level in mV to calibrate the VM
 
No info on the 5L15 out there but the 5C50 preamp has a similar offset adjustment and it appears the method used is the same as for that preamp.
 
Last edited:
Thanks for this... I just picked up a 5L15 and im going through a few checks and this is immensely helpful.

I have some other 5L15 questions I plan on asking in a separate thread, or in DMs to get started. Keep it up!
 
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