Original M-audio bx8 schematics

[musician2001]

I have the original m audio studiophile bx8 monitors and I recently lost the lf output amp in one of my monitors, I bought a new one and intalled it, then a resistor burnt up on me. My other "good" monitor then had the same resistor burn up for no apparent reason. It is a 200 ohm resistor and I have tried 200 ohm resistors all the way up to 10 watt and they're still getting hot. Something is wrong and m audio doesn't support these monitors anymore. If I had a schematic I'd have a place to start on what is causing the resistor to get too much wattage. Any help would be greatly appreciated!

[g1]

Don't know how different they are but here is the 8A : http://music-electronics-forum.com/a...8452-bx-8a.pdf

[mozwell]

from g-one's link the 220 ohm resistors are in the +/-15V power supply.
are the 15V zeners ok, are the transistors Q21, Q22 getting hot, are the 15V rails ok.
are any of the op amp ic's getting hot
are any of the electrolytics bulging
is the voltage select switch in the correct position

[musician2001]

Thanks for responding

from g-one's link the 220 ohm resistors are in the +/-15V power supply.
are the 15V zeners ok, are the transistors Q21, Q22 getting hot, are the 15V rails ok.
are any of the op amp ic's getting hot
are any of the electrolytics bulging
is the voltage select switch in the correct position

Thanks for responding. Here's what I can say so far. I have since replaced the 2 6800uf 35v power caps to no avail, I am not an electronics pro by any means. I can solder in components and use the basic functions of a multimeter, but, most of this is foreign language to me, though I do have a friend that knows quite a bit more and has had me check some things. Unfortunately the bx8a schematic is fairly different from my setup, but, at the resistor that blew on the bottom circuit board, I have alligator clipped to a high enough wattage resistor for testing. The voltage drop at that resistor is 32v dc, 71v ac! The original resistor was a 200 ohm with a 2% tolerance, I now have to use a 25watt to be safe @ 200 ohms and my friend on the first night we worked on this threw on a 250ohm 50watt because it's all he had available and the system functioned with it in there. I checked the transistors and op amps, they do not seem to be getting hot. I do not see any electrolytics bulging or swollen, I replaced one that was right before the LF IC because it had the burst impression in it which I thought may have meant they expected it to fail sooner than the others(the power caps were the only others caps with the x impressed in them). the voltage select switch is on 115. At that resistor it is operating at about 35hz. I think I'm at the end of my rope, ready to accept defeat and throw in the towel . If any of the newly provided info pops on any light bulbs, please let me know. Thanks!

[g1]

Power resistors run hot. What was the wattage of the original resistor? At 32VDC across 200 ohms, you are running about 5 watts. Why do you think you need a 25W "to be safe"?
What kind of voltage do you measure across that resistor in the good working speaker?

[musician2001]

Power resistors run hot. What was the wattage of the original resistor? At 32VDC across 200 ohms, you are running about 5 watts. Why do you think you need a 25W "to be safe"?
What kind of voltage do you measure across that resistor in the good working speaker?

These are running way too hot. my engineer buddy did the math on the a/c d/c voltage swing and he said that the high point of the swing is 18watts (i think), the low point around 5. He said just like you did that it's running at 5watts d/c, but, 25watts a/c and that the a/c rides on the d/c voltage. He said the reason it's getting so hot is the high point of the cycle is well over it's power rating. Unfortunately I never got to check the good one because it burnt up before I could check it (I had no idea there was anything wrong with that monitor). Based on the original resistor's size, I would guess it was no more than 2 watts and now I have a 10 watt in there and it gets extremely hot, too comfortable to hold. Thanks for trying to help, I appreciate it very much!

[Jazz P Bass]

There should not be any Vac after the bridge rectifier/filter caps.
I would look at the bridge rectifier.
If the linked schematic is even close, the 220 ohm resistors are rated at 1/2 watt.
And the caps on the 32 volt rails are rated at 50 volts.

[musician2001]

There should not be any Vac after the bridge rectifier/filter caps.
I would look at the bridge rectifier.
If the linked schematic is even close, the 220 ohm resistors are rated at 1/2 watt.
And the caps on the 32 volt rails are rated at 50 volts.

I got really excited for a minute when I read this reply because it made sense to me (why do I have a/c anywhere after the rec/caps), but, after testing the rectifiers for both failed monitors, they're checking out ok I really wish I could get my hands on the actual schematic but m audio refused to provide it and also will not service my product (thanks guys, I'll be sure to never buy anything with your name on it again as many people have had service issues even with newer equipment) but though the linked schematic is in the ballpark it's got quite a few differences. A couple easy to spot: I have 6800 35v power caps in mine, and a single 200 ohm power resistor on the bottom board. It is the only one that is not 1/8-1/4 size between both circuit boards in the system. Most likely, they improved the later design based off of the gen 1 versions, but, if they won't fix it, they should supply me a schematic so I or someone else can, it would sure make a chance of repair fell less hopeless. Thanks for the effort anyway.

[Jazz P Bass]

O/k, so work without a schematic.
The bridge rectifier & the capacitors must make a relatively ripple free output voltage (30 some volts) This is a basic power supply.
If the + & - rails are Not ripple free then you need to find out why.
If possible, disconnect the high voltage rails from the rest of the circuit.
The secondary low voltage supply utilizes a 'pass transistor' voltage regulator for the +15 & -15 volt dc rails.(on the schematic posted)
Is this similar on the unit that you have?
If so, the zener diode on the base of the transistor is what sets the output voltage.
If possible, disconnect the load (ie: +15 & -15 wires to the rest of the circuit)
Obviously, with the load resistor baking, something is pulling way too much current.

[g1]

I'm starting to doubt this is a voltage dropping resistor for the low voltage supply. If it were, there should be 2 per monitor. Also they would not have AC riding on the DC.
Without a proper schematic (or hand drawn one showing what this resistor connects to) it is all just guesswork.

[Jazz P Bass]

True, true, true.

[musician2001]

I had a band in my studio this weekend so unfortunately I ran out of time to troubleshoot and had to buy some new monitors (I got 8 years out of them at least) but, I would still like to fix the bx8s if possible to have a backup set or a great set of house speakers. I greatly appreciate all the input you all have put in as I am a very basics level electronics repair guy. Here are some developments since I last posted. Since the new better quality power caps I tried to put in the first broken monitor didn't work, I tried them in the one that only had the resistor go bad and viola, it worked. The resistor I made (5 watt 200ohm) for the circuit didn't get hot at all and very oddly (to me), read a drop of 0Vdc . Regardless the lack of heat tells me that something different is obviously wrong with the first monitor that went bad on me, so, i took some ac and dc voltage readings along with capacitance readings on the working monitor and then put some of my old power caps that checked out on a capacitance meter and put them back in the broken monitor. Didn't work. I later determined it was because it burned up the 2nd power capacitor in the circuit. At that point I wasn't sure it really burnt the cap up, but perhaps the old ones were really just that bad. So, I put the new, high temp/quality caps in, again, 2nd power capacitor burned up. I'm thinking that if it's burning up power caps (or anything that early in the circuit--I know...I know..it's a circuit, it could be anywhere) it's got to be the bridge rectifier but it's checking out. I'm going to test the 4 zeners tomorrow and see if any of them are bad. Since I can easily fix the other one by putting 2 good power caps in it, I'd really like to fix the original failed monitor because I've put so much time into this and they're a pile of components if I can't fix them.

[mozwell]

Niot sure if it applies, but check out this link
My BX8a Deluxe speaker died - and how I fixed it [Archive] - The Official M-Audio User Forums
M-Audio Studiophile BX8a reference monitors - Badcaps Forums

Unplug both speakers from the board. Measure speaker output volts from the amplifier, both should be lower than 0.5VDC.
Measure the ohms of each speaker. are they > 2 ohm ?

Measure the voltage across each of the 6800u 35V capacitors. Set your meter to DC Volts, then measure again with meter set to AC Volts.
Measure where the transformer connects to the pwoer supply, do you get teh correct AC volts, is one side of CN2 lower than the other ?
The two bridge rectifiers are cheap, i dont like shotgun approach to repair, but maybe replace them if you have similar ones

[Sintie]

Everyone seems to get into these speaker boxes so easy. How do you do it? The backs are glued tight.

[Jazz P Bass]

"The backs are glued tight. "

Wouldn't want then to leak air now would you?

If you apply heat to the edges of the panel (heat gun/ hair dryer on high) the panel should come free.