Samson Rubicon R8A - repair

[gnd]

Hi all.
I just bought two used Samson Rubicon R8a monitors, and one monitor has a failure.

One monitor is all fine, but on other it is like 3 kHz would be boosted by a large amount. I measured freq response with mic and it is exactly 12dB boost at 3kHz. It starts at cca 1200Hz and it ends at cca 5k. Also when listening to just noise coming out of monitor, it sounds like 3kHz would be boosted by EQ with sharp Q.
Boos is independent of level. Drivers are mechanically fine. It seems to me that failure is in crossover.
I plan to take out the amp board and have a look.

Would anyone happen to have schematics for these monitors? It would be really helpful. Thank you.

 

[gnd]

More info. Checking the manual, crossover is at exactly 3kHz, and it is 4 pole (24dB/Oct) filter.
Probably this would mean that one part of crossover is shifted, and therefore 12dB peak?

I plan to open it, disconnect speakers, and measure crossover response to see which half of it is off. What would you suspect? Capacitors?

 

[gnd]

Did some freq measurements today. Measured at output terminals that connect to speakers, outputs of amplifiers.

https://dl.dropboxusercontent.com/u/11010077/rubicon%20005%20all%20copy.jpg

https://dl.dropboxusercontent.com/u/11010077/rubicon%20006%20all%20copy.jpg

https://dl.dropboxusercontent.com/u/11010077/rubicon%20007%20all%20copy.jpg

Back of monitor shows following crossover response:
https://dl.dropboxusercontent.com/u/11010077/crossover%20manual.JPG

It seems both monitors have resonance present arround 3kHz, just one is much worse. Aging components?

Will measure both also at crossover output, to see if it is crossover (preamp) problem or output amp problem.

Here is entire backplate of monitor, with all data:
https://dl.dropboxusercontent.com/u/11010077/backplate.JPG

 

[gnd]

Any ideas about what could be the problem?

And, yes, if anybody can share schematis of these monitors, it would be really helpful. 

 

[gnd]

After measuring preamp (crossover) outputs, curves are the same as at amp outputs. So, as suspected, problem is with crossover.

On below graph green is from the good one, and red is from the bad one. But I suspect both are faulty, just one is much more.

https://dl.dropboxusercontent.com/u/11010077/rubicon%20009%20preamp%202.bmp

Any service experts arround? What do you suspect? How do you recommend that I proceed?

 

[gnd]

I got original schematics from Samson support. R8a is identical to R6a.
If anybody else needs it, here it is:

https://www.dropbox.com/s/c77cg2crk2b3ycp/Samson%20Rubicon%20R6a%20R8a.zip?dl=0

Looking at schematics, if anybody has any ideas why the 3kHz peak described above, please let me know.

 

[audiomixer]

how about you should not have bodged with the correction filters, specially the one located at 2.8kHz (around U6a, first page, with the note REV1.73.

you might put a 10k pot instead of R39 / R40 and test result at different positions. to me it looks like your x-over is fine, the filters are deranged...

- Michael

 

[PRR]

Bad solder joint on RX2 RX3 RX4 RX5 RX6 RX7

It should be possible to deduce "which one" with deep understanding of this filter topology. It is a 4-section low-pass feeding the woof, and then a high-pass is extracted by blending the input and different parts of the four low-pass stages. However I'm sure a bright light, good flux, and a hot iron will find/fix it quicker than heavy brain-work.

 

[gnd]

I've melted solder on all points arround crossover. Problem in NOT solved.

I just cannot figure out where the problem would be. I can measure it, but cannot pinpoint it.

So now I've measured level difference between good and bad channel, at 2800Hz, where the peak is most prominent.
Below are results entered in schematics:
https://www.dropbox.com/sc/rksmib6em3375ta/Vx8VtZrW-E

So, just before crossover level @2800Hz is identical. Then within crossover the bad channel increases level, 4,8dB in first stage (HP), and 9,8dB for LP.

How do I find which resistor is wrong? Which one would you say is faulty?

 

[gnd]

Also I have remeasured all resistances within crossover, at IC legs, and both boards show the same resistances. So it seems problem are not resistors or bad solder joint of resistors. Am I concluding this properly?

So if not resistors, there is not much left, it could be capacitors, or ICs.
How do I test this? Any suggestions?

 

[audiomixer]

were exactly did you inject the signal? did you omit the input section all together and inject directly on RX1 or through the regular input? the HF chain exhibits the same issue, albeit damped by the HP filter so less visible (your rubicon%20009%20preamp%202.bmp). best would be to disconnect RX1 and send your test signal from there.

just wondering ...

- Michael

 

[gnd]

Ok, I found it.
It was capacitor in the last stage of crossover, cx5 in schematics.
I did simulation of circuit in EWB, and changing cx5 from 10n to 5n did in direction of what I was measuring.
With simulation I got exact measured values for levels @2800Hz with cx value of 5.63nF. I desoldered this cap, measured it, and it measured 5,6nF.
So, thats it.

Label on cap says 10n (2A103J), but it measures 5.6n only.

Replaced it with 10n polyester, and all is fine now.

The broken capacitor is mylar poliester, and looks like this: http://www.uxcell.com/100v-10000pf-polyester-film-2a103j-capacitor-100-pcs-p-126666.html
Could I replace it with normal polyester like this one? http://www.rshelectronics.co.uk/category.php?id_category=30

 

[lietuvis]

Hi is it still posible to obtain a schematic for samson rubicon r8a or r6a
Thanks in advance
Aldas

 

[gnd]

Hi is it still posible to obtain a schematic for samson rubicon r8a or r6a
Thanks in advance
Aldas

Here is link to schematics for Rubicon R8a and R6a (schematics is the same for both models)
https://www.dropbox.com/s/c77cg2crk2b3ycp/Samson%20Rubicon%20R6a%20R8a.zip?dl=0

 

[lietuvis]

Thank You GND