Subwoofer Failing (1 Viewer)

[Joe S]

My Dahlquist PDQ1500 was making a slight humming sound the other day (when turned off). I unplugged it before leaving for work and forgot about it for a couple days. When I plugged it back in, it started making lots of noise, with no signal applied. I unplugged it and managed to plug it in without the noise starting (if I wait a few seconds, it will stop).

Anyway, when I turn on my receiver and apply a signal it might sound OK, but when it hits a loud passage it makes a popping sound.

I'll probably take it apart this weekend and probe around, but the best way to fix it would be to replace the amplifier. I'll have to take measurements to see what size I need.

I've heard about BASH amplifiers, sold thru Parts Express. Anyone got any experience or recommendations? I'd like to get it operating again and a better amp would likely improve the sound.

 

[Joe S]

Took a quick look at the circuit board- I think one of the power supply filter capacitors went bad. I've got to find two 4700 uF, 50 V caps to replace the cheap Chinese caps they used. I read on the web that these type caps are likely to go bad.

 

[Joe S]

OK, for anyone following this...

Received some replacement caps (got two 6800 uF, 63 VDC to replace the 4700 uF, 50 VDC caps) and soldered them in. I connected the amp to the speaker and plugged it in- no unexpected noises. Connected the LFE cable and it seems to work fine. I turned everything off and put some silicone rubber around the base of the capacitors on the PC board to provide a little support and keep them from vibrating. I'll let it cure overnight and put everything back together tomorrow. Then, I'll give it a good stress test with one of the bass-heavy DVDs of my choice.

I found that there's an amplifier module on the PC Board, an S53AMI, that was used in JBL and other subwoofers. If you search for it on Google, it seems that there were lots of problems with these. Fortunately mine has lasted 5 years+ and was not damaged by the bad power supply capacitor.

Now that I've mentioned it, it'll probably blow next week!

EDIT: Everything back together and working fine. This is a poor design amp (cheap), if you look at the schematics in a JBL service manual that can be found online. As an engineer, I feel compelled to improve on it, but it's not worth the effort. Maybe someday I'll spring for an SVS.

 

[fountain_fiend]

Joe, thanks for posting your adventures 6.5 years ago, they have been helpful to me. I have a bit of electronics experience and have jumped into fixing my PDQ-1500. I had the same symptoms as you, and one of my 4700uF caps is just slightly domed on the top, whereas the other is a bit concave, so I'm planning to replace both. What did yours look like?

If you are still around, why did you go to the 6800uF/63VDC caps? Do I want an audio specific cap?

Here is my digikey search so far:
http://www.digikey.com/product-search/en?FV=fff40002%2Cfff80009%2C3400d1%2Cfc01eb%2C1140050%2C3f0001a%2C3f0002a&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=2&ptm=0&fid=0&pageSize=25

Also found this eBay option for a few more bucks: http://www.ebay.com/itm/2pcs-Nichicon-KW-6800uF-63V-audio-capacitors-HiFi-HiEND-/261242486256?_trksid=p2054897.l4275

What would you recommend?

Thanks so much if you are able to help!

 

[Joe S]

Joe, thanks for posting your adventures 6.5 years ago, they have been helpful to me. I have a bit of electronics experience and have jumped into fixing my PDQ-1500. I had the same symptoms as you, and one of my 4700uF caps is just slightly domed on the top, whereas the other is a bit concave, so I'm planning to replace both. What did yours look like?

If you are still around, why did you go to the 6800uF/63VDC caps? Do I want an audio specific cap?

Here is my digikey search so far:
http://www.digikey.com/product-search/en?FV=fff40002%2Cfff80009%2C3400d1%2Cfc01eb%2C1140050%2C3f0001a%2C3f0002a&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=2&ptm=0&fid=0&pageSize=25

Also found this eBay option for a few more bucks: http://www.ebay.com/itm/2pcs-Nichicon-KW-6800uF-63V-audio-capacitors-HiFi-HiEND-/261242486256?_trksid=p2054897.l4275

What would you recommend?

Thanks so much if you are able to help!

Hello fountain_fiend,

I think the bad cap I had was slightly swollen (domed at the top). I went with 6800uF to give a little more filtering. The higher voltage rating is better since they originally built these things with cheap components. I think the circuit operated at about 40 volts. The original caps were rated at 50 volts. The higher voltage rating just gives them a little more safety zone in case the voltage goes higher because of some other malfunction, etc.

I soldered the new ones in and used some silicone glue to keep them from flopping around. I'm not sure about audio specific caps, but the Nichicons or Panasonics should do fine. I would probably get the Nichicons you found on eBay.

My PDQ-1500 worked for a couple more years after my repair, then developed another problem with distorted sound. Those type of problems can take a long time to troubleshoot and if it was the amplifier output module (S53 AMI- google it), you can't get replacements, so I ended up buying an Epik Empire sub, which is a definite improvement. I still have my PDQ-1500, may get around to buying a new amp for it someday.

Good Luck with your PDQ!