Cerwin Vega SUB-150 Loud Hum - diyAudio
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Old 1st April 2013, 06:52 PM   #1
EricB2 is offline EricB2  Afghanistan
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Default Cerwin Vega SUB-150 Loud Hum

Hi... I am new to this forum and hope someone here can help before I have pulled my last hair out. For reference, I am an electronics nooob with little prior experience with circuits. I am fairly intelligent and can follow basic instructions. My only diagnostic device is a DMM (and some resistors/caps of various sizes).

My problem:

I have a Cerwin Vega SUB-150 with a fairly loud hum (even when no input is connected and the inputs are shorted) but still pounds the bass when in use. The amp is composed of an active crossover and an amplifier linked with a 6 wire plug. the schematic is in the next post below.

what I know so far:

*The DC voltage at the speaker terminals is around 35mv (35.0 using lowest DC setting on my DMM) so I do not think that is the problem..
*I replaced the two largest capacitors (80v 6800uf) on the amp board as there was evidence that they had leaked and it is now much louder, but still hums.
*The power LED on the active crossover has the red/green LED colors reversed (ie. it now lights up red when in use, and lights green in standby and is also lights green when completely powered off).
*Of the 6 wires that connect the crossover to the amp: When the #6 white audio lead is disconnected between the active crossover and the amplifier, the hum completely disappears.
*I grounded the negative side of a comparable remote cap (no resistor, straight to gound) and used a wire from the positive side of this remote capactior to touch the negative side of C7. It totally resolved the hum, but audio quality was destroyed and speaker clipped at volume. I replaced this cap but the hum is unchanged. The resistor to this cap R26 is fine.
*The U1 voltage regulator (12v regulator on amp that is used to trigger the crossover) is almost instantly so hot that you cant touch it when powered up. Output voltage and input voltage are identical so it appears to be shorted. I removed U1 and applied 12v to the output pin area using a 12.26vdc wall transformer. It powered on and the hum was unchanged.
*The power cord is 2 prong, but adding an earth ground does nothing. using various outlets around the house does nothing. My audio filtering surge protector for my home theater does nothing.

I have 2 hairs left and would appreciate any/all advice. Again, I am a noob so be gentle with me. I have a strong desire to understand and fix this. It is not the end of the world if I wind up smoking it as I am doing it as much for the knowledge and experience as anything else. Working on this amp is more fun for me than killing time watching TV. Thanks!

Last edited by EricB2; 2nd April 2013 at 05:25 PM. Reason: Calrified Testing to Date
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Old 1st April 2013, 07:28 PM   #2
EricB2 is offline EricB2  Afghanistan
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Default Schematics

Schematics attached... The capacitor that resolved the hum is C7... to say I added another in series is actually incorrect. I grounded the negative side of a comparable remote cap (no resistor, straight to gound) and used a wire from the positive side of this remote capactior to touch the negative side of C7. It totally resolved the hum, but audio quality was destroyed and speaker clipped at volume. I replaced this cap but the hum is unchanged.

I also removed R26 and tested it. It is perfect at 47k. R43 is the frequency knob. If I touch the bare metal post with the plastic knob removed, the hum gets louder. I hope this info helps.
Attached Files
File Type: pdf sub150preampschm.pdf (478.4 KB, 72 views)
File Type: pdf sub150ampschm.pdf (419.0 KB, 77 views)
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Old 1st April 2013, 11:42 PM   #3
Waddy is offline Waddy  United States
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Eric,

I have spent just a few minutes reading your description of the problem and looking at the schematics. My guess is that there is a short circuit on the +12 voltage regulator. The clipping is probably because your +12 rail is at 0V (or at least much lower than +12). The regulator I understood to be hot to the touch is U1 on the sub150ampschm.pdf file. It looks to me like it is just a standard 7812 regulator. Is there any way you can replace this regulator? Also, I would make sure there are no other short circuits over the capacitors decoupling this regulator. One thing to watch out for is if you got the capacitors (the electrolytic power supply caps) reinstalled the right way. If they are backwards, I think they may act like a short circuit for a while, then explode.

I hope this helps!
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Old 2nd April 2013, 01:34 AM   #4
EricB2 is offline EricB2  Afghanistan
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Thanks Waddy! You are correct U1... gets smoking hot while U2 is just a little warm to the touch. U1 voltage readings for input pin 1 and output pin 3 are identical at 15.6vdc. I removed U1 and it prevents the amp from switching on whatsoever. I am considering applying 12vdc (from a basic plug in the wall transformer with 12.26vdc output) to the output and see if this changes anything.

There is no clipping except when I eleminate the hum by applying a grounded capacitor to the + side of C7 on the crossover schematic.

New caps are installed correctly which greatly improved the performance. this is a mid 90's amp.
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Old 2nd April 2013, 01:49 AM   #5
EricB2 is offline EricB2  Afghanistan
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Also: I paralell tested every cap on the crossover PCB, but not all of them on the amplifier PCB as it is attached to the heatsink and the cap wires are not easily accessible. If U1 is bad, perhaps it is because I live in a very lightning prone area. I recently purchased the amp and have no history otherwise.
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Old 2nd April 2013, 02:07 AM   #6
EricB2 is offline EricB2  Afghanistan
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Update: I powered it up using the 12vdc wall transformer applied to output pin3 on U1. Same loud hum. and the voltage at input pin 1 on U1 is actually 15.26vdc, not 15.6.

as for checking for shorts on the decoupling caps (which I assume to be C3, C5, C9, and C10), can I detect a shorted capacitor by just measuring resistance across the capacitor? What would an acceptable resisitance be for the following capacitors? I know that they seem to drift when measuring resistance.
C3 and C5: 75v 470uf
C9 and C10: 16v 100uf
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Old 2nd April 2013, 02:34 AM   #7
EricB2 is offline EricB2  Afghanistan
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The resistance values of the caps are all over the board. sometimes they stop at a specific resistance, then i can test them again and they rise to near infinity.

Bearing in mind that the voltage regulator U1 is removed, here is a snapshot or my first readings:
C3: rises to infinity with leads matched (red to +, black to -) 40.3k ohms with leads reversed
C5: initially stopped at 39.2k with leads reversed and 92.8k with leads matched, but rose to infinity with leads matched next time i tested it
C9: rises to infinity either way
C10: first time it slowly rose to around 20.6K with leads either way. Retested and rose to infinity with leads matched.

I hope that all of my posts mean something to someone. Thanks!
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Old 2nd April 2013, 07:21 PM   #8
EricB2 is offline EricB2  Afghanistan
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I bought a 7812. Will install it in a few hours when I get home.

Any advice on voltages I should check or places I should check for stray voltage that may be the source of my hum? When the amp is powered on with the inputs shorted, the speaker pulls back in its basket noticeably... not too much. but its noticeable. when I power it off, there is a small pop and the speaker moves back out once the capacitors have drained.

Also, I have been told that the 2 color, 2 pin LED lights work by basically being 2 LEDs in one... current goes one way and it is red... current is reversed and it turns green. If this is true, then the power in the amp may be reversed somehow as the colors are reversed. any thoughts on this would be much appreciated.
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Old 6th April 2013, 08:01 PM   #9
EricB2 is offline EricB2  Afghanistan
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Thanks for the overwhelming responses.

Please delete my account. thanks.
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Old 17th January 2014, 04:08 AM   #10
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Hello forum,
Can someone please tell me what the final solution was to this problem? I am in the exact same and would love some assistance. Thank you very much, Chris
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