Hi all,
After a little help again.
Just purchased a second hand sub which I set about cleaning and checking visually just to make sure everything is ok before adding to my system. (Note I did hear the sub playing before purchasing and all sounded good)
I noticed on the amp board there is a resistor that looks a bit suspect. Could someone please confirm if this resistor is ok or if it would need changing before using?
There does seem to be alot of this the tan coloured goo over the board which in some areas, such as on this resistor, has turned black and charred. Excuse my ignorance as I'm not sure what the goo is actually for?
Many thanks,
After a little help again.
Just purchased a second hand sub which I set about cleaning and checking visually just to make sure everything is ok before adding to my system. (Note I did hear the sub playing before purchasing and all sounded good)
I noticed on the amp board there is a resistor that looks a bit suspect. Could someone please confirm if this resistor is ok or if it would need changing before using?
There does seem to be alot of this the tan coloured goo over the board which in some areas, such as on this resistor, has turned black and charred. Excuse my ignorance as I'm not sure what the goo is actually for?
Many thanks,
The goo is quite common - think it is there to hold things in place whilst being soldered. That board looks a mess - the components are very crowded in and overlapping - but so be it. The resistor is clearly broken and needs replacing. It may be that the exposed core of the resistor is tracking (shorting) and blackening the goo.
1) Replace the resistor.
2) Remove all that bonding muck/glue in case it becomes conductive, easily determined by putting probes on it and checking for any resistance.
2) Remove all that bonding muck/glue in case it becomes conductive, easily determined by putting probes on it and checking for any resistance.
Thanks guys,
Yeah it does look a mess and I wasn't sure if it served a purpose or if I could just get rid of it. Most of it is hard so I'm not sure it's hot glue which tends to stay soft.
Thanks wiseoldtech I'll replace the resistor before using the sub. It is odd that at the demo it worked fine but maybe it wasn't warm enough at the demo.
Thanks again
Yeah it does look a mess and I wasn't sure if it served a purpose or if I could just get rid of it. Most of it is hard so I'm not sure it's hot glue which tends to stay soft.
Thanks wiseoldtech I'll replace the resistor before using the sub. It is odd that at the demo it worked fine but maybe it wasn't warm enough at the demo.
Thanks again
Solid state electronics doesn't rely on a "warmup" in order to run, only tube equipment does.
That resistor is cracked and something is wrong. A cooked resistor is a symptom of a problem, not the problem itself.
Quite possibly.
However, without a service manual, one can only specualate...... endlessly.
The resistors that attenuate the speaker level inputs of these “subwoofers” often get burnt to a crisp. Caused by driving it hard for extended periods - they are usually only sized for playing it rather gently without a lot of distortion, and if you go blasting your music this happens. It may actually still work - the resistor may have just shifted in value rather than open up entirely. It usually won’t affect the line level inputs - they may still work even if the speaker level inputs are dead as a result.
All this cream-colored bastard glue is the main problem.
This become conductive due to thermal conditions(color becomes dark).
Because of this many short circuits occure.
Clear all the glue and use a better material for vibration protection.
After that find and replace all the faulted components.
This become conductive due to thermal conditions(color becomes dark).
Because of this many short circuits occure.
Clear all the glue and use a better material for vibration protection.
After that find and replace all the faulted components.
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Thanks again, great info guys!
I'll replace the resistor like for like as I don't ever plan to drive the sub hard, just want it to integrate well with the current system.
I also only plan to run line level not speaker lever input.
In terms of replacing the glue to stop everything moving about. Would hot glue gun glue be acceptable or is there a specific product I should be looking at?
I'll replace the resistor like for like as I don't ever plan to drive the sub hard, just want it to integrate well with the current system.
I also only plan to run line level not speaker lever input.
In terms of replacing the glue to stop everything moving about. Would hot glue gun glue be acceptable or is there a specific product I should be looking at?
I am not entirely convinced that the resistor is defective.
I would check it first with a multimeter to be sure.
Is the sub still working?
The goo is a nasty thing, it gets hard and conductive over time.
Try to remove it by carefully pushing it of the board with a sharp object.
I would not replace the goo afterwords, maybe only apply some glue on the heaviest components.
The resistor measures 1.484 k ohm but I've not seen black as the first or last band on a resistor so not sure of its value?
Yeah the sub did still work fine when demoing it.
Yeah the sub did still work fine when demoing it.
If you look carefully, you can see that there's no glue on the resistor. The glue is not the issue here.
That resistor has cracked for sore reason, it could be power overload or from mechanical loading.
I would still replace it even if it measures OK, just to be sure.
Jan
That resistor has cracked for sore reason, it could be power overload or from mechanical loading.
I would still replace it even if it measures OK, just to be sure.
Jan
Resistor I think could be 100 Ohm (brown black brown), sometimes the difference between brown and red is difficult to see.
I agee to replace it. Can you see where is it connected?
I agee to replace it. Can you see where is it connected?
Most of the “goo” is yellow. That one bit on the resistor is black. That happened because the resistor got hot. The resistor is suspect, but no big deal to replace. Other than it will continue to generate heat if operating conditions don’t change. That TO-92 transistor sandwiched between it and the cap probably doesn’t like being heated up like that either. The solder joints on the PCB will eventually fail too, as well as turning the board to carbon like the glue was.
Those are most likely the input attenuation resistors, which eat all the power your receiver will throw at them. If you are going to run these hot, they really should be re-located somewhere they can dissipate it. Wires running off to a 10 watt sand cast, glued to the chassis, is not a bad idea.
Then you just have to worry about whether you burn out the output transistors running too hard. At least the sub would have been DESIGNED to run those hot. Since these products are often “designed” by bean counters and Junior Level Circuit Designers (C) there are often little oversights like this.
Those are most likely the input attenuation resistors, which eat all the power your receiver will throw at them. If you are going to run these hot, they really should be re-located somewhere they can dissipate it. Wires running off to a 10 watt sand cast, glued to the chassis, is not a bad idea.
Then you just have to worry about whether you burn out the output transistors running too hard. At least the sub would have been DESIGNED to run those hot. Since these products are often “designed” by bean counters and Junior Level Circuit Designers (C) there are often little oversights like this.
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If you use the low level inputs, the resistor(s) get no power to dissipate and will stay cool and useless.
If you connect the subwoofer with the low level (sub pre out) inputs, these resistor are out of the circuit, broken or not.
All quite true. The low level inputs don’t have this problem. But what happens is someone cranks it waaaaay up with music that isn’t bass heavy (old classic rock) using the speaker level inputs. 20 volts RMS coming in puts 4 watts in the resistors - which is too much when they’re packed in that tight. With bass heavy music, the sub starts farting or bottoming out before things get that far. But with a lot of midrange you don’t notice any distortion, just the smell of something burning.