Subwoofer amp: Want to repair or override "lazy" standby/power sensor

[2222]

Possible it could be green green (brown or orange) gold. Even what I perceive as the gold might not be. (It *seems* to be shiny.) Really hard to definitively identify.

[JustMeHere]

Try a subwoofer specific RCA cable.  I've had this issue before and I eventually found a non-sub cable that would trigger the woofer, but I recommend a real sub cable.

Also, I would not! power the subwoofer with a power jack from the back of your amp.  When the woofer hits, it will be drawing power away from your amp's actual amplifier.

I try not to question actual experience too much, but what possible reason could there be for an audio cable terminated in RCA to 'not work'?  I suppose you could check it with an ohmmeter to make sure it has low resistance and no leakage, but I've never heard of a 'subwoofer specific cable' outside of the lunatic fringe that is high-end audio.

As far as power, this sub isn't very powerful so an amp or two at the mains plug is all you'll see. That shouldn't drop the mains voltage at the amplifier very much.  That could be tested as well.

I'm going to suggest that the OP figure out a way to totally disable the standby feature and leave everything else the way it is.

Better shielding against 60 Hz noise.

[themadhippy]

Better shielding against 60 Hz noise

Is  there a version available for us in 50Hz land?

[JustMeHere]

Better shielding against 60 Hz noise

Is  there a version available for us in 50Hz land?

Yeah. Same thing should work for 50Hz.  Nothing expensive.

https://www.monoprice.com/product?p_id=2681

[themadhippy]

https://www.monoprice.com/product?p_id=2681

Fancy, a bit of tv coax , some shiny phonos and a dab of snake oil,bet there not even 75 ohm phonos.

[JustMeHere]

https://www.monoprice.com/product?p_id=2681

Fancy, a bit of tv coax , some shiny phonos and a dab of snake oil,bet there not even 75 ohm phonos.

Perhaps, but I had a subwoofer doing the same thing with cheap RCA cables.  Tired a couple different ones.  Tried it with a shielded cable and it worked.  It must of had the right snake oil.

[tooki]

https://www.monoprice.com/product?p_id=2681

Fancy, a bit of tv coax , some shiny phonos and a dab of snake oil,bet there not even 75 ohm phonos.

Perhaps, but I had a subwoofer doing the same thing with cheap RCA cables.  Tired a couple different ones.  Tried it with a shielded cable and it worked.  It must of had the right snake oil.

Shielding isn’t snake oil.

[CaptDon]

My recent experience with an amplifier similar to yours with the discolored resistors showed that those resistors were a voltage dropping network each one going to a 15 volt zener diode to create + and - 15vdc power rails for the dual opamp devices. The sad part is that the zeners and those resistors which were flameproof ceramic body style were operating at about 1.5X their design rating and failed. One of the zeners failed shorted and the amp quit completely. I replaced the 1/2 watt zeners with 1 watt versions and the 1 watt flameproof resistors with either 2 or 3 watt flameproof. Not sure which I used as I have both in stock. The owner bought a new amplifier in the mean time and I now own the one I repaired.

[2222]

I need to decide how to proceed.

The workaround suggested by member bdunham7 above seems like it should work for my purposes (turning the amp on [labled "Automatic" on the power switch] forces the amplifier to activate).

But that still leaves the overheating resistors, possibly caused by faulty capacitor(s). The board shows a date of 9/96, is that in the range of defective capacitor supplies?

[David_AVD]

I've seen subwoofers with auto (audio sensing) issues where the culprit was some of the small electro caps on in the input board.

The result of the bad caps was low gain in that section causing it to become less sensitive over a long time.

Because subs are usually powered up 24x7 it's very common for the electro caps to be shot in 10 years time.

A fair proportion ( maybe even > 50% ) of the subs I see for repair only have a single power supply, so the "standby" is sometimes nothing more than muting the output.

[Kim Christensen]

I'd look at carefully removing one of those hot resistors. Measure it's value, and since you know the voltage across it, you can calculate the wattage required. Then order proper replacements.
You could get all new electrolytic capacitors and replace them to see if that fixes the issue. At least you'll have all new capacitors to give it a longer lease on life.
If the new caps don't fix the issue, I would then try bdunham7's resistor idea to keep the amp alive all the time.

[bdunham7]

The board shows a date of 9/96, is that in the range of defective capacitor supplies?

Not really, the capacitor plague was later, starting 1999 or so.  But I've seen cases of the small electrolytic caps on various audio products (not a subwoofer guy, so I don't remember any of those) failing in control circuits and things like that from the late-80's onward.  So I would not be even slightly surprised if a bulk recap operation were to fix your subwoofer.  The capacitors probably don't have anything to do with the resistors burning, that is probably just marginal design, drift and time.  You can replace them with better versions and stand them off the board a bit, but I think they are probably a sideshow.  Of course, you never know. 

[2222]

The result of the bad caps was low gain in that section causing it to become less sensitive over a long time.

Yes! That seems to accurately describe the behavior! I have to really punch up the bass and volume on the receiver to "wake up" the subwoofer.

A fair proportion ( maybe even > 50% ) of the subs I see for repair only have a single power supply, so the "standby" is sometimes nothing more than muting the output.

This was a suspicion earlier in the thread. Am I correct in presuming that is a cheap and easy way to do "Automatic?" And possibly that is not as much of a power saver as it could be? (IIRC, it was advertised as a power saving feature. And convenience feature.)

[2222]

Anybody have some recommendations on where I can source suitable replacement resistors and caps? Also probably will need some thermally conductive paste to reseat the IC to its heatsink during the reassemble. Radio Shack near here closed decades ago, and Fry's more recently.

[bdunham7]

Mouser, Newark or Digikey.  There's no local sources anymore and even if there were, you'd be picking from their availabe inventory instead of getting exactly what you want.

[2222]

There are two ways I can think of to go at this.  The first would be to put a 22K resistor from pin 8 to pin 4--this should unmute the amplifier without putting any undue burden on the original switching circuit.

Something like this? 22k 1/4watt?

https://www.digikey.com/en/products/detail/yageo/CFR-25JB-52-22K/1306

[2222]

That is indeed what I installed (22k 1/4W)-- success!    The amp is now always energized when powered on.

It sure was finicky and delicate soldering it to the closely spaced interleaved IC legs. Legs 4 and 8 of course are the legs under and behind, greatly limiting access. Required several tries to avoid bridging to the adjacent odd-number "out" leg. Copper braided wick was useful.

Thanks for all the helpful guidance and recommendations.

I suspect I will need to renew the caps at some point as that resistor section still gets very hot.