What resistor do I need?

One thing is certain, they used a high temperature resistor in that location so they were expecting it to get hot, he may get lucky with just replacing it.

 

I'm wondering about that transistor with the metal tab and hole next to your connector. I think the resistor that got hot, works to bias the voltage on the transistor and together controls the power to the rest of the board. I'm not sure from your pictures but the transistor shows heat damage to the board so, it might be bad.

 

Here are some, hopefully, better pics. It doesn't look burned out to me.

 

I wish you the best of luck. I've never been able to resurrect a circuit once a component smoked but I hope you're able to.

 

Did you guys talk to bose yet? They made a million of those. The complete board shouldn't be too hard to find. Call bose and ask for parts dept instead of letting them rope you in for repair.
I bet they still have those boards. I'm a cerwin vega man myself.

or this >>>>>>>>>>>>> https://www.ebay.com/p/Bose-Acoustimass-5-Series-III-Speaker-System/69561754

even better>>>>>>>>>>>>>> https://www.ebay.com/p/Bose-Acoustimass-5-Series-III-Speaker-System/69561754?thm=3000

 

I was able to find the schematic for this board.
I think Al called the component that has the most probability of being bad. That is in a T02 transistor style case. It looks like a transistor but Is a DIAC.
wikipedia.org/wiki/DIAC
It is a form of rectifier that if bad could cause the resistor damage.
If a resistor is destroyed by heat. It was caused by something other than that of the resistor.
On the schematic for this section. I have boxed the components in red and yellow.
The resistor R437 is clearly marked 7.5 K ohm in the picture.
On the schematic it is marked 7.5 ohm.
This is the power supply portion of the board. If this resistor was a 7500 ohm resistor with a supply voltage of 120V if shorted would only have 2 watts dissipated in heat. This would not damage or discolor this resistor in this way. A 7500 ohm resistor would be a bad choice for several reasons.
If it is a miss stamped 7.5 ohm resistor. This would at least explain how a 2 watt wire wound resistor could show this kind of damage at the 120 V supplied if directly shorted. It would be dissipating close to 200 watt of heat, 100 times its rating.
In the diagram. The red box is a known problem. The red with the yellow around them would be usual suspects. With greatest suspicion of it being the DIAC. The transistor Al had pointed out.
D420 would be the next suspect and it is easy to check. Last if all else checks out the problem would be U402 The DIAC driver.
This is all happening after a fuse no larger than 3 amp. The parts in question as Al had mentioned are of a switching power supply that has an output of 12 VDC.
The service manual with the entire schematic is here.
https://elektrotanya.com/bose_am-5p_sch.pdf/download.html
Joe

 

Hi Joe, looking at your schematic, the mains is coming in on R456 (varistor?) and then through the fuse to Q400 which is a TRIAC as it has three wires vs the DIAC's two wires so it's an AC switching device. The mains is also going straight to R437 as well so unless there's some kind of physical power switch then the hot resistor in question is always energized, as a voltage dropper to the standby power supply: the D420, 1N4004 rectifier diode, after the half wave rectification it goes to the D413, 12 Volt Zener which forms a small 12 Volt regulated power supply. So it would be dropping a large voltage at a small current non stop for the standby circuit.

115 Volts - 12 Volts (Zener diode regulator) = 113 Volts RMS across the 7.5K resistor

113 Vac / 7500 Ohms = 0.015 Amps current through the 7.5K resistor

which gives 113V x 0.015A = 1.695 Watts

1.695 x 0.5 (1N4004 half wave rectifier) = 0.8475 Watts power dissipation in the 7.5K resistor when no current is drawn from the 12V supply. So if we assume the 12V standby power circuit draws another watt through the resistor for the microprocessor and remote control circuit, that 2-3 Watt 7.5K resistor in the photo is running at 1.8475 watts: close to it's maximum rating non stop, even when the appliance is turned off.

 

R437 can not drop more than the voltage available to it, 120v. If it is a 7500 ohm resistor and this was the worst case short of the 120 directly across the resistor. The resistor is not alone stand alone component and current must pass through a 12v Zanier for a 12 v drop then overcome the 1+ volt drop of D413 this would give a maximum voltage drop of another 13 volts leaving R437 with 107 volts at 750 ohm would be .042 amp at 1.5 watt.
You are right it is a TRYACK. I am getting old!
D413, D420, along with R437, R408, R407 and R405 set the reference voltage for pin 1 anode on both U401 and U402.
The TRIACK acts as a switch and is controlled by U402 pin 4 main putting power on the transformer.
I do see where you are going with your thought. It is most certainly marked 7500 ohms in more than 1 spot.
On second thought it would not give a reason to strongly suspect the TRIACK. It is just how seeing this much heat damage from a part that if it is 7500 ohms and it most likely is. Would not be at its rated dissipation. I suppose even at 1.5 watt if it is left to cook forever with little to no air flow it might get too hot.
If this is the case even if it checks good. Replace it with a higher wattage resistor or at least when you solder in the replacement. Leave the resistor leads long so it is well above the fiberglass PCB to help with air flow and thermal insulation.
It does not look as if the part is damaged.
Zapin If you check it with an ohm meter and it says 7500 ohm or any thing over 1000 ohms. It is a 7500 ohm resistor Without removing it. It is hard to get a good reading because of the other parts on the board. you will know it is. If it reads 7.5 ohm then it is that. Either way it will read one of the 2 or open. A wire wound resistor does not short. They open.
Joe

 

+1!!

 

I could call them. But figured I'd try the repair first.

The subwoofer says its a series 3 but it looks like a series 4. And has series 4 connections. So it might be a bit harder to source the right parts. Also it's the European 220v version not the 120v american type. So maybe a bit harder again to find.

I got soldering wire and flux ready to go. Will wait for the new resistors to arrive.

So just to confirm the consensus is to solder in the 5 watt resistor? I bought 2 watts and 3 watt versions as well just in case since they were a few pennies more.

 

Yes use the 5 Watt resistor and increase the lead length so the resistor's a bit higher above the board to keep the heat is away. The board schematic shows a 2W resistor but if you're feeding it 220V then the wattage dissipation will be 220-12V=208, 208^2/7500=5.768 Watts. Divide that by two for the half wave rectification and you get 2.88 Watts into a 2 Watt resistor.....going to get pretty hot!!.

 

Those corner cutting bastards! Will do. 5 watts it is.

 

There is usually a switch on the rear to go from European 220v/50Hz to American 120v/60 Hz.
Common setup for items sold to service members at American military bases in Europe.

 

Usually yes but this doesn't have a switch like that. We had it hooked up to a 300w transformer brick to go from 220 to 120v for many years before it fried. We used to live in Europe where we bought it.

 

Resistor came today. Soldered in the 5 watt. Turned it on, nothing happened. Started testing voltages, the resistor had power to it and even got warm to the touch. But something else was busted as you suspected.

Tried several different components d423, and d420 were ok and had voltage but 2 of the 6 leads in U402 (the 2 closest to the edge of the board) didn't have power and U401 didn't get power at all. R412 did not have power and neither did the removable fuse. C402 and c403 had power.

I flipped the board over and kept testing but something happened when I touched the small square boxes on the back of the board under the resistor and one of them exploded off the board at me. I lost the magic smoke so I think the board is toast now. No point trying to chase down the bad component now.

I suppose plan B: just buy a second hand one for 60 to 90 bucks from ebay is the way to go.

 

Once the circuit board gets charred enough it will conduct electricity too especially with around 200 Volts across it.

 

There ya go. Plan B, Full STEAM ahead... Ya tried, can't blame ya on that one. Now go get you a used unit for 70bucks.