Velodyne SPL 1000 II distortion

Hello to the group.
I have inherited TWO of these subwoofers that have seen very little use. Unfortunately, they both have severely distorted output. They were driven by a McIntosh MX135 preamp with very long (30 foot) runs of small gauge shielded phono cable thus probably very low input signal strength with a lot of noise. Both failed simultaneously and have exactly the same symptoms. Power on just fine, drivers (subwoofer speakers) are intact, do not rub, and resistance of voice coils are the same. Both were plugged into surge protectors and no known lightning, etc. I have a feeling that the chronic distortion blew something in the sub preamp, but without a schematic to be found anywhere, this amp board is very difficult to troubleshoot. I have a shop complete with all manner of test gear including signal generators, digital Tektronix scope, multimeters, rf voltmeters, etc. I am pretty handy having been a ham for 45 years. One of my hobbies is restoring Collins radio gear. Any suggestions, oh gurus, would be beneficial. If I can fix these things myself, I can avoid about $250 apiece for repairs plus shipping of the amps . I have a feeling that the problem will be identical in both units. Thanks in advance for the assistance.
 
I can't imagine such a scenario damaging the sub woofer amplifier. Such a long run could give rise to hum pick up and might cause your pre-amp to oscillate if it is not tolerant of a lot of capacitance on its outputs.

I would sweep with a sine wave generator and make sure that there is actually a problem with the sub woofer. (I would start with very low amplitude, say 10mV at around 100Hz.)

More likely they were already broken when you got them.

What is the voice coil resistance?
 
Thanks, Guys. Same distortion with line level and speaker level. Bjorno, the bad capacitor idea is worth a try. Will order some Mouser with a higher voltage rating and recap. First thing that you do with an old receiver. The coupling caps go bad. Collins used a paper capacitor years ago called a "Black Beauty". They were used for interstage coupling and were notorious for failing. What do I have to lose? Even if I have to send it off, that has been ruled out. Will advise. THANKS AGAIN!
TY
 
Orthodoct,

Some caps on the pre-amp go bad with age, causing (among other things) what you are describing which eventually leads to a "dead amp".

This is exactly what I experienced with several (20+) SPL Series I and Series II amps.
I recommend replacing all the pre-amp electrolytic caps.

As for the schematics, Velodyne will send them upon request (via email or phone) provided that the model you have is not a current one.

I hope this helps - good luck with your project.

Cheers!
 
Hi dudaindc,

I have a SPL 1200 about 10 years old which works in principle, but the bass power diminishes over the time - do you have a schematics where I can fix the problem or would you recommend to change all caps?
thanks in advance
habadudl

Habadudl,
Please see schematics attached.
have fun!
 

Attachments

  • velodyne_spl-10_control-pcb_sch.pdf
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Hi dudaindc,

thank you very much for the schematics; may I ask some questions in addition to the schematics?
1. is the preamp part of the SPL-10 the same as for the SPL-1200?
2. C1 and C2 at line in are pretty low in value - is it purposely to cut off low frequencies for the main amp (line out)?
3. does the main switching power amp connected to CN1 not have any caps?
4. maybe there is another cap (C96) in area D10 to renew which seems to forward the signal to the power amp?
Thank you for your competent help again!
habadudl