Luxman r1500 restoration

Audi0

Super Member
I just picked up a Luxman r1500 for a project. The preious owner said distortion at high volumes (past 12 oclock)and caps were spewing fluid down the boards. I'm just guessing but is it just a standard cap job to get it running good again? I've never had to clean cap juice before what' the best way to clean it? I don't have time to look at till the weekend. Just looking for some thoughts.
 
I usually use rubbing alcohol and a brush to clean off the cap juice, followed by compressed air to make sure everything gets blown off well. Let it dry for an hour or two before plugging it in, but if your working on it that's probably not an issue anyways haha.

Replacing the caps is not a guaranteed fix for your problem, but it is a great place to start since it sounds like they are questionable on your unit. Weak capacitors in the power supply can lead to distortion at higher volumes, so I say go for it. Let us know what you find!
 
Will post a picture tomorrow of what I'm dealing with. Looks untouched under the hood except somebody crammed cardboard against a few boards. Bad solder joints I suspect. Also I don't know why the previous owner was screaming the caps were leaky as I don't see much if any. For sure there is bad caps as there is a visable dome at the top of quite a few. The only thing that scares me with restoring it is alot of the diodes look like germanium ones.
 
I was doing a general caps order and it just occurred to me that the service manual does not have the procedure to calibrate the power stage. How would I go about doing that?

20171217_005722.jpg 20171217_005636.jpg
 
Many times the black Nippon Chemicon caps in old Luxman are pretty good. The main amps are prone to failure, or loud hickups because of deteriorating transistors, but if it behaves correctly the "12 o'clock" thing MIGHT be overloading the amp only. Most amps are pretty loud at the 12 oçlock position so whether it is bad or not, first listen for yourself before making the same conclusion as previous owner.

In the beginning of the service manual they write 50 milliamps a s bias but this is rather high. This would be some 25 millivolts over one of the output transistor emitter resistors.
If the voltage is over + and - 50 volts the 50 milliamps bias would already result into 5 Watt dissipation each side. So the amp will be running warm on the back side even idling only.
Myself, I made sure my SQ507x and L309V went not over 40 mA whether hot or cold. One can carefully heat up the amp back side using a heater or hairdryer, but take care not overheat anything. A dummy load or having no neighbours and cranking it up is better for heating....A temperature probe comes in handy, or assume if you can not press your finger against without pain it is 55 degrees celcius or more.
IMO for distortion there is nothing wrong when bias is just 20 milliamps or so, maybe even less.

Personally, I suggest determine whether the amp is at fault, at all. BTW yours looks pretty clean, a lot better than this picture of the detriorated amplifier cards of my L309V before refurbishment.

309-2.jpg

The R1500 is a real nice receiver, mine is still on the pile of unrestored stuff (not much time for it)
 
Last edited:
Fired it up and measured the dc. It's 350mv a channel which I assume is causing the distortion?

There is 2 set screws on the boards which one does the dc offset?
 
Last edited:
Can anybody chime in what would be the ideal route to fix the dc problem? I think the differential pair of transistors have no modern equivalent ? I did adjust the bias to 160mv but still to high.
 
OK, maybe a little late attempt to bring some clarity in the LUXMAN R-1500 biasing (idle current setting)

The service manual is a bit cryptic in this regard, saying the idle current should be set to 50mA without saying more specific WHERE it is measured.

Knowing LUXMAN gear of the same time period, I would guess that the bias current is measured on the POSITIVE rail of the power supply to the power amp section. At least, this is how it is done on basically all pre-1980 LUXMAN gear. (sometimes you need to cut a wire to be able to connect an ampmeter, like for the L-85V for example)

To accomplish that measurement you will need to remove one fuse on the POSITIVE rail and connect a milliamperemeter across the fuse terminals. There are four 2A fuses located on the upper side of the chassis right next to the power transformer, two with red wires connected and two with blue wires.

Remove one "positive" fuse, connect the milliampmeter instead, set the total current to 50mA with VR802 and adjust the DC offset with VR801 for the selected channel, repeat adjustments if needed. Put back the fuse. Do the same for the other channel.

This way is a little too cooky to my taste, but as no other reasonable means are given anyway, this is our best bid.

When the total current of 50mA is set according to the above one may wish to measure a voltage drop across emitter resistors R825 or R826 (whichever is more accessible), to see what is the actual power transistor idle current. Just for indication, in my R-1500 I've measured about 11mV voltage drop across R825 which gives roughly 21mA of bias current. Quite reasonable, I would say.

Take care and good luck to all of you!

Regards,
alejon
 
–– this thing runs like a dream and shines up nicely : )

volume control doesn't make the sound louder so much as bigger and wider.

i changed almost ALL the transistors – which solved the bulk of any problems.
–––> i could have used the original output transistors – but i burned one by accident, had to replace with a whole set.

I measured the frequency response of my (restored) amp, and it started rolling off around 300KHz for some reason.
 

Attachments

  • DSC09227.JPG
    DSC09227.JPG
    58.5 KB · Views: 37
  • DSC09215.JPG
    DSC09215.JPG
    42.3 KB · Views: 35
I measured the frequency response of my (restored) amp, and it started rolling off around 300KHz for some reason.

That must be the metric number as 300 Thousand Hz is I believe well beyond what this amp should do. But it sure does look good in the pictures.

I wonder if @Audi0 has gotten his receiver working.
 
That must be the metric number as 300 Thousand Hz is I believe well beyond what this amp should do. But it sure does look good in the pictures.


I know right?!
you'd expect something like the specs:
35Khz i thought.

I ran a sinwave oscillator into the amp's aux,
and attached a scope to the speaker outputs.
my oscillator doesn't have μ-second gradations if that's what you meant by metric,
and i had a frequency counter to check input also.

i did replace a few components in the amp
 

Attachments

  • DSC09309.jpg
    DSC09309.jpg
    76 KB · Views: 14
No, µ isn't metric. I used metric because that 300kHz is way too high for a unit that age. No one was doing it. I think 30k is very believable. Of course, newer transistors will go a lot higher and maybe this is part of why your number is so high.
 
yeah, new transistors... more modern... did my best to measure and match them...

re: fq
i was also taken by how gradually the frequency response rolled off.
like, you might quote a number at which there's no noticeable attenuation,
100k for instance,
then at 350k you still have a very faithful signal, all be it with some attenuation,
but with no noticeable distortion.
 
Back
Top Bottom