KLH subwoofer repair

[AzGuy]

Hi all. I need some help repairing a powered KLH subwoofer model: ASW80-80. The back has high-level inputs from amp: L&R and out to speakers L&R, OFF-Auto-On switch, volume 0-10, low-level inputs-L&R via RCA jacks, Phase switch-Rev & Norm, and Crossover Freq. 50-150 Hz.
There is nothing connected to the amp. It has a constant hum that I can't get rid of. I tried the ground loop fix-didn't help. I've inspected the circuit boards for burned components, hot spots on the board, and stressed electrolytic caps. I've seen nothing yet. I'll continue inspecting for cracked solder joints or open components. I have test equipment-an O'scope and meters, but without a schematic, I have no idea what signals/voltages I should or shouldn't be reading.
Does anyone have experience with this or a schematic or where I can obtain a schematic? It's an older subwoofer and not worth taking somewhere to be looked at. Besides, I like to fix stuff so this is a challenge. Thanks.

[phoenixdigital]

Looks like you and I have the same issue but different subs (see my other post about Dail Subwoofer). I'm also doing for the challenge and learning purpose too

https://www.eevblog.com/forum/repair/dali-subwoofer-repair-and-learning-excercise/

The benefit is I have an older service manual with a circuit diagram. That said my model is newer and I have just been tracing through the PCB and there are a few differences here and there but the core design seems to be the same.

Without a circuit diagram it'll definitely be harder.

I'm no expert but I'd start by checking the diodes with a diode check on your multimeter. It kinda drew me to a possible problem with a bridge rectifier which I've ordered a replacement for.

I'd also scan your circuit for opamps. One of them will be introducing the hum somewhere. When my oscilliscope arrives I'm going to

• Measure the V+ and V- rails of all the opamps (I have 5) which I'm assuming should be a stable DC signal. If it's fluctuating then that'll likely be introducing a fluctuation on the output.
• Measure the outputs of the opamps - If I can find where the hum is being introduced that should narrow things down.

Attached is an example of the opamps on my board.

Good luck. I'll be keeping my eye on your thread.

[phoenixdigital]

Oh and any zener diodes near a bridge rectifier could give a hint to the voltage that should be on that rail. Alternatively you could measure the AC input to the bridge rectifier. Also put the input/output of the bridge rectifier on the oscilliscope.

[AzGuy]

Thanks phoenixdigital for the input. This is a great starting point.
I'll take some voltage readings and scope some points around the opamps and see if I can find the source of the hum.
This will be an in-my-spare-time project, but I'll post what I find and hopefully some pics for the next person.
I emailed KLH yesterday. I'll see if they wind up being any help.
Thanks again.

[AzGuy]

My subwoofer panel has 2 circuit boards: a power supply and a control board. When taking voltage readings, I noticed one of the two 4700 microFarad 35Volt electrolytic caps at the bridge circuit was slightly bulging at the top. With electrolytic caps, this tells me it is in the process of failing. One of the caps measured within tolerance and the bulging cap wasn't even close. I ordered some caps to replace these and I'll hope that this will take care of the hum. I'll replace all the other electrolytic caps also. Time and heat are not usually their friend.

[stephr1]

I have one of those subs that I picked up from Frys so many years ago. Sits around collecting dust   Last time I had it running, tho, I don't remember any hum problems. Altho, the auto-turn on always seemed a bit sketchy.

More to your point, tho....

I also have a Klipsch KSW-150 (150W, 10") sub. I've had that sub for 20+(?) years now. Some years ago I ran into a "skritching" noise that made me wonder if the subwoofer voice coil was falling apart. After careful diagnostics, I determined it was a solder joint on/near one of the op amps that had gone cold. Fixed that and then a year or 2 later the sub developed a hum. 1st I thought it was coming thru the LFE channel. Not so. Soldered the P/S filter caps. While I had the amp out, I also resoldered all the other connections of components in the main P/S section.  The sub has been doing great ever since.

Moral of the story....if it's AC/60 Hz hum, that usually means something in the P/S. The only other possibility is a cold solder joint on the ground of another component. However, if that were the case, the issues may be different, or worse, than AC hum.

Since you have a bulging cap, that would be my 1st suspect. Since you're going to be in there replacing the cap, I would also suggest that you go thru the rest of the supply section and resolder everything, just to be sure.

BTW - Just because a cap measures the "right" capacitance, the ESR (Effective Series Resistance) might be too high to continue any further effective filtering.

Don't know if it helps but I attached the schematics for a KLH ASW10-120 Power Amp and P/S section. May have serious differences from the ASW80-80 but it may be similar enough in the P/S section to provide some help.

Good luck.....

[AzGuy]

Thanks a bunch, stephr1. Any input/information is helpful. Yes, the schematics are probably slightly different, but around the power supply section, there are a lot of similarities. Your filter caps at the bridge are bigger, but the components in the area are the same as mine.
I have the filter caps, for the bridge circuit, ordered and I should get them Saturday, the 17th. I'm anxious to see if it takes care of the hum.
I'll post what I find after replacing the caps.

[stephr1]

Glad it helped. BTW - You can order caps with higher voltages (and capacitance) to provide some margin of protection (and additional filtering), but definitely not lower.

The difference in caps is prob'ly due to the difference in the amps. The ASW10-120 is 120 watts and the ASW80-80 is 80 watts (ASW10 is most likely a 10" sub and ASW80 is 8"). To support the higher power the voltage needs to be higher (simplified, P=V*I where I= V/R and, since R is a constant 6-8 ohms, voltage has to go up) but capacitance can be less (not always, but maybe in this case with added filtering).

Also check for possible cracks in the PC traces (in the circuit trace lines and also from pad to line). Cracks in this area may not be visible at all

As a tangent...I also have a 20+ year old Sony Rcvr. Abt 10 yrs of ownership, it started going into protect mode. That turned out to be solder joint issues on a few of the power output transistors (prob'ly heat-related). Resoldered all of them and never had a problem after. Very nice sounding rcvr. Unfortunately, it was a few years before HDMI so great for sound but not video. May buy an HDMI-to-5.1 audio decoder and see if it can be usable again. Couple it with an RPI/OSMC HTPC and make it a self contained home theater system 

[AzGuy]

I just replaced the two 4700 µF filter caps at the bridge circuit-No Hum!
I measured the two caps I took out. The positive side cap measured within tolerance and the negative side cap, well kinda wasn't there- it measured .047 nF. I also noticed it was leaking on the bottom of the cap.
Before I reconnect the amp, I'll replace the rest of the electrolytic caps and reflow solder joints. I don't want to do this again for a while.
Thanks for all the input and help.