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Luxman L-80v protection relay *Assistance needed*

G

Guydee

New Member
Hey everyone,

I have a lovely old Luxman L-80v that was working beautifully one day before running into issues.

I was playing a vinyl record and noticed that the music stopped. The relay had engaged and gone into 'protection mode'. I powered it off, then turned it back on. Music would play (although not very nicely) and I noticed that the speaker/woofer cones were protruding outwards (as if there was a constant positive signal).

I didn't force the issue, but I did power it on/off a few times trying to diagnose the problem.
After a few power cycles, the relay stopped engaging entirely. I have read through the existing threads on Audio Karma which have been very helpful, but I cannot seem to isolate the problem part so far. I have replaced the output transistors, and have tried looking into troubleshooting Q101 - Q110, but my knowledge base is limited.

I am very keen to learn though, so please share your wisdom with me (all the details you may take for granted).

Please note: at the time the issue arose, I was using a 4 pole double throw switch for 2 amplifiers into 1 pair of speakers. The switch is well built with no shorts and a 'break before make' design. As far as I know, there was no shorting or arcing within the switch. The other amp is fine and I have been using it subsequently).

Thanks a lot team.

Guy - Sydney, Australia
 
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Protection usually happens for a reason like having any excessive DC at the outputs. I would check all output, driver, and pre driver transistors along with any emitter resistors.

I have never ever thought it was a good idea to ever have two amps/receivers connected to one pair of speakers no matter the safety factor involved with the switch box itself.

Been doing this some 40 odd years and have never done this. I would choose the inconvenience of just physically switching the speaking wires. I have seen too much damage around here where people have used these switch boxes with this kind of configuration.
 
2 amps hooked up to the same speakers? never heard of that before.
unless you mean a Bi-amp configuration through a electronic cross over.

since the amp made a big cone excursion id say it has DC on the output. strange that the protection didnt prevent this. might mean it also has some issues with the protection itself
 
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Concensus is that you have a dc voltage issue on the spkr line.
Next step would be to confirm this and identify the faulty channel
Measure dc voltage at left channel power amp pin 102, black probe
connected to chassis/black speaker post. Then repeat for right
channel pin 102, note sign +/-. expect mV.
 
Hey team,

Thanks so much for the speedy replies.

I agree Cademan, despite the best thought out safety switches, I don't think i'll be AB testing speakers with a toggle switch anymore. If you have any tips on identifying the output, driver, and pre driver transistors or emitter resistors and/or what values to expect/where to find those values it would be much appreciated.
I had also suspected excessive DC voltage so I'll walk you through what I've learnt so far through other forums.

I have measured the voltage at the point marked in pink on the image below (R113) (as per Sonavor's advice "The DC offset was a different matter. Since the protection relay wouldn't engage I couldn't measure across the speaker outputs so I measured the voltage at the node between R113 and R125 to ground on each channel" on this thread http://audiokarma.org/forums/index.php?threads/luxman-l80v-project.431309/).

29o3o7r.jpg


I hope I have measured at the right spot (the node between R113 and R125).
The left channel showed ~160mV and the right channel showed a huge 44v. Those figures are with the DC offset pots turned all the way down.

I would love to verify these numbers using your method mbz, but I'm not quite sure which probe you are referring to. Based on the image below (also supplied by Sonavor) do you mean P102?

1s1ge1.jpg
1s1ge1.jpg


Thanks again for your patience with me guys. Your knowledge will be passed on!
 
Yeah, me being lazy again. 44Vdc on the right channel indicates a failed c-e? short. You will need to diode test some
transistors, find the short then look at other possible failures. Will try and get a list of candidates, outputs obviously.
 
Diode test Q107,8,9,110, in circuit, amp powered off. markthefixer has posted a nice graphic on transistor diode testing if you need it.
looking for a c-e short.
 
Hey all, sorry to revive this one after so long.

I got the amp up and running after replacing Q107, however after a few hours of happy listening, I've got the same issue. The amp has fallen into protection mode once again, so I need to keep digging to find what is causing Q107 to fail.

I recall someone mentioning a specific resistor which may be the culprit of causing Q107 to fail, but after searching all of the threads for hours, I can't seem to locate that comment.

If anyone could point me in the right direction, that would be great. I feel like I'm close to having this amp back up and running for good.
 
What did you replace Q107 with? Where did you get the replacement.
Resistors R118 (22ohms) and maybe R121 (100 0.5W) but probably R118.
With Q107 damaged you should check Q109.
 
Guydee said:
Thanks for jumping back in and helping out. I replaced it with exactly what came out, NEC B536 (and respective D381) from ebay
Click to expand...
Possibly fakes, should try to find modern day equivalents from a reliable source.

melb.
 
That transistor substitution thread gives,
2SB536 <- KSA940
2SD381 <- KSC2073

Little busy at the moment, maybe someone can confirm suitability of above for the L-80V
 
Hey all,

Just picking this one up after finding another wave of inspiration. I have replaced q107, q108, q109 and q110 with equivalents purchased from RS Components here in Sydney. I have also replaced r121 and r122 to eliminate further failure. A moment of success, the protection relay engaged and I could measure approx 260 mV across one channel before 2x 5amp fuses went. The fuses run to p702 and p703, I believe to the transformer. I also heard some crackling in r124 and possibly r123 before the fuse went.

I replaced the fuses and can’t even power up with out a bright flash and immediate blown fuses.

Looks like I’ve fried my new replacement output transistors q109 and q110. I’ll put another order in I guess.

Once I have my replacement parts I don’t want to fry the whole lot again. If anyone has any advice as where else to look for gremlins, your advice is much appreciated.

Keen to get this machine up and running.

Thanks in advance, Guy
 
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G'day mate;
I don't have a diagram of a 220v dim bulb tester (australian ac plugs and outlet) but you need one to test before replacing those output transistors. it will detect a short in the power supply without blowing fuses.
This is the U.S. wiring. note hot and neutral.
upload_2019-5-14_8-12-1.png
 
Binkman said:
G'day mate;
I don't have a diagram of a 220v dim bulb tester (australian ac plugs and outlet) but you need one to test before replacing those output transistors. it will detect a short in the power supply without blowing fuses.
This is the U.S. wiring. note hot and neutral.
View attachment 1508098
Click to expand...
Good call. I thought that might be a good idea. Can you advise on what wattage bulb would be a good start? I need to do a bit of reading about how/when to use dim bulb testers, but when you say “before I replace the output transistors”, do you mean I should try find a fault in the transformer before installing the fresh transistors?
 
100 watt bulb. the dbt limits both voltage and current. If he bulb stays bright and does not dim then this indicates a short usually in the power supply section. General usage is to test any untested SS amp or receiver. e.g. you also use it when you do replace those transistors. will prevent more damage.
 
Another good tip for these older luxman's is take magnified look at pcb's solder side for aged solder joints especially the driver transistors may appear less shiney, but overall you need to take good look at joints.
 
Hey all, thanks for your replies.

I’ve installed all four new output transistors and fired up the amp through a dim bulb tester.

The light globe starts bright, dims, but only slightly and appears to slowly get brighter. I suspect, as stated above by Binkman that I may have a short somewhere along the power supply section. Any assistance in where to look, and what to look for is much appreciated.

Thanks all.
 
Guydee said:
Hey all, thanks for your replies.

I’ve installed all four new output transistors and fired up the amp through a dim bulb tester.

The light globe starts bright, dims, but only slightly and appears to slowly get brighter. I suspect, as stated above by Binkman that I may have a short somewhere along the power supply section. Any assistance in where to look, and what to look for is much appreciated.

Thanks all.
Click to expand...
Sorry for the spam - the amp is back up and running! Adjusted trim pots and the relay engaged. Playing like a champ now. I’m the happiest person in the country right now.

Thanks all.
 
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Well that was a most excellent change of fortune between 5:33 and 6:14 this morning (AK Time). I guess the adjustments were way off causing the amp to stay protected then the adjustments got the amp to its happy point...making you pleased with the work you did. Enjoy.
 
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