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.
Service Manual: http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Luxman-L-3-Service-Manual.pdf
Service Manual: http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Luxman-L-3-Service-Manual.pdf
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.
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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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.
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.
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.
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.
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
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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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.
I used everyone's advice and nothing has worked. One of the output transistors was bad, so i changed them and the drivers.
Forgot to mention, what if the bulb tester is blinking ( dim-bright-dim-bright-dim... etc). it is also blinking along with the power light.
Progress...
Okay, I have made it further. I was an idiot and forgot to put Q404ab in, replacing these fixed the bulb tester blinking problem. The right channel is working perfectly.
The left channel still has issues:
-Vbe of Q402a is 9v
-Vbe of Q404a is only 0.32v, which seems kind of low
-Vbe of Q407a and Q408a is 0v
Okay, I have made it further. I was an idiot and forgot to put Q404ab in, replacing these fixed the bulb tester blinking problem. The right channel is working perfectly.
The left channel still has issues:
-Vbe of Q402a is 9v
-Vbe of Q404a is only 0.32v, which seems kind of low
-Vbe of Q407a and Q408a is 0v
Vbe normally refers to the voltage across the base emitter junction and would normally be in the 0.7 volt region for all devices in a working amplifer. 9 volts is not an impossibility but it would only apply to a transistor under a reverse bias condition (i.e. a fault condition).
How does the polarity of the 9 volts relate to the device type (NPN or PNP) ?
How does the polarity of the 9 volts relate to the device type (NPN or PNP) ?
The 9 volts is base positive on a PNP transistor.
I have used all modern equivalent parts.
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