Luxman r-115 volume troubles

[salperta]

Hi AK! Been a longtime reader, new poster.

I recently picked up a Luxman R-115. Off the bat I noticed some problems with a scratchy volume knob and balance, so I deoxited all the pots. Balance knob works well now, but the volume knob is still acting strangely (so I think...). When the volume is all the way down (with balance center), there is still very faint volume coming out of the left speaker, and when volume is all the way up, there is no sound coming out of the left speaker. And meanwhile, when the volume is low, there is no sound from the right speaker, and as I turn it up, it is consistently weaker than the left speaker, until the left speaker cuts out near the top of the volume range. It's almost as if the volume knob is beginning to raise the volume of the left speaker before the right. How does that happen!? Is it actually the volume potentiometer, or could it be something else? Tried searching on the forums, but didn't find any solutions to this particular conundrum.

Not super knowledgeable about these things, so I thought I'd appeal to the masters at AK.

Thanks to everyone for all your help in the past, and in the future.

 

[Binkman]

Welcome to A.K.!
pg. 13 of the sm show a rotary motorized vol. pot. I guess you figured it out. deoxit5 take some time to dry but I doubt it's the V. pot. We've had at least half dozen posts in this series recently that all needed service but no Vol. issues. We don't know your skills but these older lux's are prone to aged solder esp. in the amp board w/ driver and output transistors w/aged solder. But you need to pull mag. glass out and inspect all of em on the pre too as solder joints are pretty small. To me it sounds a floating imbalance. the amp is trying to compensate and lagging. indicates cap work.
That's where I'd start along with check rail voltages basic stuff. It's well worth doing right. All lux owners are very happy.

Better book the sm if you don't have it as it's not listed in hifiengine lib. page but there in their search bar. https://www.hifiengine.com/manual_library/luxman/r-115.shtml

 

[century tek]

The control itself could certainly be bad but check the obvious first like the deoxit you just did, plus look for any cold solder joints from the control mounted to the circuit board.

 

[salperta]

Thanks for your replies, Binkman and century tek!

So I took out the rotary motorized volume pot to see what I could see, and indeed, I did see some trouble. I opened it up and found that one set of wipers was bent back out of shape, and the carbon film (am I right in calling it that? excuse my technical mistakes!) had an actual chip taken out of it, as though something had gotten stuck in there, bent the wipers, and chipped the carbon film. I have no idea how that would happen. Anyhow, I bent the wipers back, and tried to position them so that neither one "wiped" over the dinged area. I plugged it in and some of the anomalies were gone--the right channel loss at low volumes, the left channel loss at high volumes, and the faint left channel sound at zero volume were all cleared up. However, I've still got the imbalance, where the left speaker is consistently louder than the right speaker.

Here's what I've tried:
1) switching the cables, switching the speakers, and switching from A to B--no change, volume consistently lower on right channel.
2) tried it with both cd input and FM--no change.
3) tried it with cd straight--no change.
4) Tried clicking various buttons to see if any would cause a breakup, but didn't notice anything.

I'm pretty good at fine motor skills, but I have very little electrical knowledge, so any help in narrowing down where to start looking would be really helpful! I actually resoldered the volume pot back to the circuit board it is attached to, so hopefully those solders are not the problem.

Binkman--forgive my ignorance, but can you point me to an easy tutorial on checking rail voltages? I have a multimeter, but I'm afraid that's about it for measuring equipment.

Thanks again to all, I very much appreciate the help. That left channel sounds really good, hope I can get them both sounding as good!

S

 

[Binkman]

skip the rail v check.
I want you to study "idle adjustments" in the ser. man. first you want to check left and right main boards. and compare to the spec, noted 6mv+/-2mv. follow the warm up period directions etc. assure vol. knob is off.
see pdf pg 6 and refer to pg. 16 to identify test points & svr adjustment pots. if your meter reads neg just reverse the probes.

WARNING.. YOU CANNOT SHORT THESE OUT WITH YOUR PROBES. ( or any components leads for that matter) Wrap some elec. tape to allow only the very tips to show. be better if you had mini grabber test cables. if adjustment is necess. then you can adjust in very gradually as the pots are sensitive. then apply the probes to check. Clockwise is positive on the pots. DO NOT ADJUST BEYOND THE 8mv ! or crank the pots positive as you'll blow outputs. Nor make any adjustments if in spec. just leave alone.

report back on findings.

 

[salperta]

Hi All,
OK, I did the tests, thank you for the guide on that one, Binkman!

On the right main board:
When I measure between post 2 and 3 I'm reading 5.4mV.

On the left main board:
When I measure between post 1 and 2, I'm reading 4.6mV.

That seems in spec, so I didn't want to make any adjustments, unless you think that .8mV is making any difference?

Thanks again, looking out for more advice.

Saul

 

[Binkman]

Only prob. with lux. serv. manuals is there are no voltages denoted to reference only component values. (in addition all b&w scans)
A little history of this unit would help.. orig. owner or any idea of how many hours on it? It is 19yrs. old
How much of an imbalance L/R are you talking about on bal. knob, a lot? small fraction? Assure the knob is pos. on the shaft correctly? You could hit the bal. pots again w/deoxit5.. assure you move the pots back and forth bout 20 times.
-----
We had a recent lux owner with similar imbalance that turned out to be a couple of transistors on one of the main boards with 'aged' solder joints. I mentioned that already. What happens over time is the transistors heating and cooling causes the solder to age. On page 16 identifies the main boards transistors. I'd fast reflow all of them and wires soldered to the boards at least inspect those. Don't take cold or aged solder joints lightly. Cause for many a prob. easily addressed.

see how works out post back.

 

[salperta]

The imbalance is pretty extreme - very noticeable. I'll try the balance pot again with deoxit, definitely worth a shot!

In terms of history, I've really got no clue. Picked it up for a song off Craigslist since I've heard great things about the Luxmans. Guy I bought it from was a young man who wasn't sure what the make/model was at first and had to check, so I figured it probably wasn't his personal gear. Really dusty when I got it and a nice aroma of old cigarettes . I'll shoot him a message though -maybe he'll have some insight.
I'm going out of town today through the next week, but when I get back I'll reflow transistor joints. I'll post back here.

Thank you again for your guidance and patience. I'll get back to the forum next week.

S

 

[salperta]

Welp, I took out the main boards, reflowed the solder on the transistors and... blew the main fuse. I had it on FM for about 15 seconds, wasn't getting any sound in either speaker, and then the fuse blew.
Not sure what I did wrong. Would a bad solder joint cause this? Or is it possible I crossed some solder joints?
I'll take the boards out and take a close look. Not sure what to do next except try to see if I screwed up anything obvious.
Saul

 

[Binkman]

Would a bad solder joint cause this? Or is it possible I crossed some solder joints? 'll take the boards out and take a close look.

But you need to pull mag. glass out and inspect all of em

I don't don't write this info because I like to read my posts. a bridged solder joint is a short.. if you spent too much time on soldering you could have damaged a joint. reflowing a solder joint should be fast. You should always inspect your work and make notes of what you've done. maybe you did something before you took the week off?
-------
I should have asked if you had a dim bulb tester and if you had checked transistors. but now you'll have to build a dbt and check the transistors where you did work or more generally the main amp board? I'd also check the diodes if you haven't. that's a fast easy power off check.. we all do it just to eliminate that possibility from the get go. However I assumed that you were getting signal to the speakers in the OP.

 

[salperta]

Hi Binkman,

I did my best with a mag glass to observe, but I'm pretty new to all of this so it's certainly possible I overlooked something. I didn't do any work inside the receiver before I left, so probably something I did in the last couple of days. I tried to be quick with my reflowing--the larger transistors took a little longer to heat, but as soon as the solder reflowed, I removed the heat. I do know which joints I reflowed, so it shouldn't be too hard to recheck my work. First I'll put together a dbt--pretty sure I have all those parts lying around. Am I correct that for the dbt, since the R-115 outputs 70 watts, I should test using a 140 watt bulb? I'll check the transistors and diodes as well. Before I can run the dbt I'll have to get some new fuses though!

Does the fact that it was that main, external fuse that blew suggest anything about where the problem is?

And finally, yes, I was getting signal to the speakers originally, so something happened in the meantime, d'oh!

Thanks so much. Always glad to be learning, frustrating as making mistakes are. Very grateful for the assistance of the experts here.

Saul

 

[Binkman]

Am I correct that for the dbt, since the R-115 outputs 70 watts, I should test using a 140 watt bulb?

No,, 100w is avg. as unit wattage consumption not speaker watts. A big big amp could use a 200w, small amp 75.. not really a calc. rule of thumb.

Does the fact that it was that main, external fuse that blew suggest anything about where the problem is?

Yes somewhere you worked on.. .. usual damage is transistor.. but follow the pcb traces when you find the issue to inspect related in the chain. i.e. driver transistors to output. also bias transistor circuit.

 

[salperta]

Well, I finally got some new fuses, and built the DBT. When I turn on the receiver, the first relay clicks, and as soon as the relay clicks, the light goes on, then it goes off, and the relay clicks, and the light goes on, etc. etc. etc. This happens quickly and repeatedly, maybe two or three times a second. I'm going to take out the boards today and start looking around for the short, but does this information give any clue as to where the short may be located?
Thanks so much as always.
Saul

 

[salperta]

Listening at the receiver while this is happening, I'm pretty sure it's the speaker protection relay, I think on the Speaker Terminal PC Board. Not really sure if that gives any clue-probably just means it's shorting somewhere, yeah?

 

[Binkman]

Test with dbt and relay clicks on and off?.. yes a short. Your first hunch. As I said usually transistor related.. check outputs down to drivers and related eg. diode check fast and easy.. look at solder joints aged. you got some testing to do. Power supply is where it all starts and common to replace the mains caps for starters to assure a clean source. Seems like the protect circuit on the speaker relay is working but don't use good speakers for testing.. nor power up till you resolve it. we'll be here. just test before buying stuff.
bink

 

[salperta]

Hi Binkman et al,
So I did my first bit of testing, and was getting some weird results, and I don't really know what I'm looking at so I thought I'd see if someone could help me interpret!

I tested the large Toshiba transistors, 2SC3280s and 2SA1301s. The two boards differed drastically. I think something is going on, but don't know what.

My results are attached. If it had to take a wild guess, something seems to be seriously up with some of the Left Board transistors...but I have no idea whether it has to do with the transistors themselves or something else. (btw, in my document, when I write "BC" it means, positive multimeter lead on base, negative lead on collector, etc.)

I've also attached the data sheet for the two types of transistors in case that helps.

Thanks so much for any help figuring out what I'm looking at here. I truly appreciate it.

S

 

[Binkman]

This should help.. assuming you at least disconnected/ unsoldered the output leads? you should read similar readings though you may see a slight meter diff. but should see OL.. the meter for good.

DIY sticky about transistor testing basics.
https://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/
copy courtesy from @markthefixer
Here are the results of multimeter diode tests on TWO GOOD bipolar transistors example, a 2sa725 PNP transistor and a 2sc945 NPN transistor:

first the PNP:
black lead to base, red lead to collector, result = 675
black lead to base, red lead to emitter, result = 688
red lead to base, black lead to collector, result = OL = No conduction
red lead to base, black lead to emitter, result = OL = No conduction
black lead to emitter, red lead to collector, result = OL = No conduction
red lead to emitter, black lead to collector, result = OL = No conduction

next the NPN:
black lead to base, red lead to collector, result = OL = No conduction
black lead to base, red lead to emitter, result = OL = No conduction
red lead to base, black lead to collector, result = 677
red lead to base, black lead to emitter, result = 680
black lead to emitter, red lead to collector, result = OL = No conduction
red lead to emitter, black lead to collector, result = OL = No conduction

Here is a graphic:

 

[salperta]

Aha! You're a lifesaver Binkman. I had read and reread that sticky, but nothing was matching with my readings, which was why I was confused... but it was because I was reading them IN CIRCUIT! I removed them all. All except two tested what I think is good, with the results being between 450 and 550 when they registered (all with the appropriate pairings), and OL on all other pairs. The two that tested bad were both from the left board, one 2SC3280 which read:

black lead to base, red lead to collector, result = 030
black lead to base, red lead to emitter, result = 030
red lead to base, black lead to collector, result = 030
red lead to base, black lead to emitter, result = 030
black lead to emitter, red lead to collector, result = 000
red lead to emitter, black lead to collector, result = 000

and one 2SA1301 which read:

black lead to base, red lead to collector, result = 081
black lead to base, red lead to emitter, result = 081
red lead to base, black lead to collector, result = 081
red lead to base, black lead to emitter, result = 081
black lead to emitter, red lead to collector, result = 000
red lead to emitter, black lead to collector, result = 000

SO, that's one problem, I'm guessing, which I'm assuming will need replacing. Before ordering anything, I assume I should test some other parts so I can order anything I need at one time.

Is there an easy way to determine what to check?

You mentioned: "check outputs down to drivers and related eg. diode check fast and easy," but what with my poor knowledge, I'm not entirely sure what exactly to do. Sorry for my ignorance. Should I test all other transistors on the main boards? How about the other boards? And how do I figure out which diodes to test, and do I need to remove them as well before testing?

Apologies for all the questions, so grateful to be learning.
S

 

[Binkman]

Basic focus is on the two L/R output boards as common for damage as you found. I forgot to mention that the transistors on the heat sink? only needed to have the leads lifted off off the pcb so you need to apply heat sink paste if completely removed. Diode check is in circuit. OL one way, flip leads to get a reading. If no diode check on meter just use resistance scale. OL both ways = open bad.
- After replacing parts then give the dbt a test w/no inputs, speakers and vol off. You'll have to recheck the bias at full power, if the dbt checked out good. Don't know how far you want to take the project. Get up and going considering your skill level perhaps best at that point.

 

[salperta]

OK, so I finally got some time to do some work on this thing. I replaced the two shorted transistors, and it passed the DBT. I'm happy to say I seem to have undid any damage I did in the first place, but unfortunately I've made no progress toward fixing the problem. As suggested in another thread, I tried connecting the preouts on the luxman to the CD in on a known working amplifier and outputting that second amp to speakers, and still I got the sound differential -- weaker right channel.

On a whim, I removed the volume pot again and took some readings. The two "paths" (I'm not sure what you call them, sorry) for the left and right channels are reading different max ohmage. One reads 142kOhms, and the other reads 164kOhms. The pot is rated at 150kOhms. Do you think this difference in 20kOhms could be causing the volume differential? I've been tapping around inside the amp with a chopstick to see if jostling any loose connections inside the amplifier makes any noticeable sound difference, but it does not.

Is it possible I'm onto something with the volume pot? And if so, anyone know where I can get a replacement that will work? I know they no longer make the original alpine pot. It's a dual 150kOhm pot.

Thanks as always, looking forward to hearing any opinions.

S