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Luxman L3 38VDC On Speaker Output?
Luxman L3 38VDC On Speaker Output?
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Old 11th September 2016, 08:56 PM   #1
klh2000 is offline klh2000  United States
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Question Luxman L3 38VDC On Speaker Output?

Hello, A friend asked me to look at his luxman l3, because the left channel went out. He said "I was just finishing an album and then the speaker went bzzz - pop, Then there was no output on the left channel." So, I told him that I would look at it. The right channel was perfectly fine, but the left channel seems to have all kinds of problems. First, the left speaker output had 38 volts on it, so I opened it up and measure the voltages at the all of their transistors. Every single one has 38 VDC at every pin? I i took them out and all of them tested fine. I just can't figure it out.

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Old 11th September 2016, 09:27 PM   #2
vzaichenko is offline vzaichenko  Russian Federation
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Luxman L3 38VDC On Speaker Output?
Hi, first thing to measure - the rails.
The voltages over C409 and over C413. What are they?
Please don't connect the speakers until you fix the amplifier (just in case).
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Old 11th September 2016, 10:05 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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There is no speaker protection so the bzz pop was the speaker burning out or the fuse preparing to blow due to DC on it, so yes, DO NOT connect another speaker to that output until you have repaired the fault.
All the transistors cannot have 38v on them! Check them properly. It is one of the simplest designs ever produced, designed by RCA in the late 60s when 40636 transistors (2N3055/MJ2955 were popular).
Check the -ve rail fuse first.
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Last edited by JonSnell Electronic; 11th September 2016 at 10:14 PM. Reason: More info
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Old 11th September 2016, 11:04 PM   #4
klh2000 is offline klh2000  United States
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Quote:
Originally Posted by vzaichenko View Post
Hi, first thing to measure - the rails.
The voltages over C409 and over C413. What are they?
Please don't connect the speakers until you fix the amplifier (just in case).
Quote:
Originally Posted by JonSnell Electronic View Post
There is no speaker protection so the bzz pop was the speaker burning out or the fuse preparing to blow due to DC on it, so yes, DO NOT connect another speaker to that output until you have repaired the fault.
All the transistors cannot have 38v on them! Check them properly. It is one of the simplest designs ever produced, designed by RCA in the late 60s when 40636 transistors (2N3055/MJ2955 were popular).
Check the -ve rail fuse first.


Okay, The voltages on c409 and c13 are 38.9 and 39.3 respectively.

All of the fuses are fine, and all of the power supply rails are at the correct voltages. The transistors do have 38v on them. I put the negative probe to ground and every pin is a 38v on all transistors.
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Old 11th September 2016, 11:06 PM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by vzaichenko View Post
Please don't connect the speakers until you fix the amplifier (just in case).
Its possible the speaker has blown as well so check its resistance before using it again or you might blow up the fixed amp.
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Old 11th September 2016, 11:14 PM   #6
klh2000 is offline klh2000  United States
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Originally Posted by nigelwright7557 View Post
Its possible the speaker has blown as well so check its resistance before using it again or you might blow up the fixed amp.

I don't know about the speakers, my friend didn't say anything about them. I have just been using an "expendable" speaker for now.
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Old 13th September 2016, 12:44 PM   #7
irext is offline irext  Australia
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I suggest that your expendable speaker may now be expended. You will need to do some basic tests with the power off first. Check for shorted output transistors with an ohm meter. Particularly Q407 (since you said the output was pegged at +38V). Check from Collector to Emitter. If shorted it will be close to zero ohms and it's associated driver (Q405) will also likely be shorted. To fully test a transistor you need to test it out of circuit but if it's shorted it's obvious, even in circuit. Check all the obvious things first. such as supply rails, fuses etc. For instance, if the negative supply rail is gone the output will go to the positive rail unloaded (no speaker connected). Once you get to the point of being ready to power up remove the + and - supply fuses and temporarily put in their place 47ohm 1/2w sacrificial resistors. Power up without speaker connected. If everything is in balance and the DC on the output is minimal and can be adjusted to zero the resistors can be removed and the fuses put back. If there is still a fault then usually one or both of the resistors will go up in smoke and you know there is more work to be done.
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Old 13th September 2016, 12:52 PM   #8
Mooly is offline Mooly  United Kingdom
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Luxman L3 38VDC On Speaker Output?
Quote:
Originally Posted by klh2000 View Post
Okay, The voltages on c409 and c13 are 38.9 and 39.3 respectively.

All of the fuses are fine, and all of the power supply rails are at the correct voltages. The transistors do have 38v on them. I put the negative probe to ground and every pin is a 38v on all transistors.
Measure again. Do all the pins have PLUS 38 VOLTS which is what you are saying, or do some have PLUS 38 and some MINUS 38.
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Old 13th September 2016, 03:41 PM   #9
east electronics is offline east electronics  Greece
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check the obvious if output transistors are shorted
Notice though that statistics have a lot to do with drivers 2235-965 presenting both real failure and "mechanical" failure IE these will measure perfect outside the circuit and fail if powered inside the circuit
Finding replacements for those is not an easy task
Kind regards
Sakis
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Last edited by east electronics; 13th September 2016 at 03:53 PM.
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Old 22nd September 2016, 08:27 PM   #10
klh2000 is offline klh2000  United States
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Quote:
Originally Posted by east electronics View Post
check the obvious if output transistors are shorted
Notice though that statistics have a lot to do with drivers 2235-965 presenting both real failure and "mechanical" failure IE these will measure perfect outside the circuit and fail if powered inside the circuit
Finding replacements for those is not an easy task
Kind regards
Sakis

Ok, all of the output transistors are now replaced, along with the driver transistors. Now when i go to power the unit on, the line fuse blows.
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