Hello all, hoping someone with more electronics knowledge can help me with this issue and little project.
I have a pair of these Fostex 6301 monitors and use them as secondary/reference monitors for mixing music. A while back I replaced the deteriorating old drivers with the modern equivalents from Fostex. While not the fanciest thing, I do like them, their small footprint, simplicity of use, and single driver design.
One of them is giving me trouble, however. It powers up fine, but after playing for a minute or two, loud peaks in the music cause it to distort. If something is loud enough, it will distort and then stop outputting at all. The unit stays on with its power light, and if I wait a while or turn it off and back on later, it will play again and repeat the same behavior.
Any idea what might be going on here? I sadly don't have a schematic, and there appears to be many revisions of these under the same name.
Here are some photos of the speaker and it's insides. I assume the amp board must be rather simple - perhaps simple enough to fix?
Any input is appreciated! Thanks ahead of time.
I have a pair of these Fostex 6301 monitors and use them as secondary/reference monitors for mixing music. A while back I replaced the deteriorating old drivers with the modern equivalents from Fostex. While not the fanciest thing, I do like them, their small footprint, simplicity of use, and single driver design.
One of them is giving me trouble, however. It powers up fine, but after playing for a minute or two, loud peaks in the music cause it to distort. If something is loud enough, it will distort and then stop outputting at all. The unit stays on with its power light, and if I wait a while or turn it off and back on later, it will play again and repeat the same behavior.
Any idea what might be going on here? I sadly don't have a schematic, and there appears to be many revisions of these under the same name.
Here are some photos of the speaker and it's insides. I assume the amp board must be rather simple - perhaps simple enough to fix?
Any input is appreciated! Thanks ahead of time.
Attachments
Hi. This active speaker has amplifier ic, mounted to heatsink, in photos marking is not visible. But looking at board soldering side, i see some suspicious places, probably more clear picture of whole board tracks and all soldering places is required to be exact. If your soldering skills are enough good, try to resolder everything on this amplifier board. Symptoms like you describe, sounds to me like bad soldering somewhere. If you have multimeter and time, you can remove metalic item, who holds the amplifier ic, and read it's marking, then locate datasheet. Then measure voltages on all it's pins , in respect to ground (datasheet will tell you which ic pin it is) while speaker is working ok and when not working. And post here , what we have. Maybe amplifier just misses power supply voltage , it's turned on, but no sound output. It may help to determine is the problem with amplfier part or signal part, like volume control, cable, tone controls ( if present) and so on. Sorry for long post and bad english .
P.S . forgot to mention ,that sometimes special flexible wire in speaker driver is going bad and broken , you can verify that while sound dissapeared ,by touching the speaker cone carefully . If touching restores sound ,then that wire needs to be changed . Had such failure in my speakers ,bass dissapeared sometimes ,with new speaker this happened after 1month of use ( ! ), had succesfully repaired by replacing that flexible wire with mgtf wire from old soviet times . So check if not speaker driver itself is faulty in your case first , you can connect another speaker you have and check if symptoms are still same .
P.S . forgot to mention ,that sometimes special flexible wire in speaker driver is going bad and broken , you can verify that while sound dissapeared ,by touching the speaker cone carefully . If touching restores sound ,then that wire needs to be changed . Had such failure in my speakers ,bass dissapeared sometimes ,with new speaker this happened after 1month of use ( ! ), had succesfully repaired by replacing that flexible wire with mgtf wire from old soviet times . So check if not speaker driver itself is faulty in your case first , you can connect another speaker you have and check if symptoms are still same .
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Thank you for the quick response! That sounds like a good idea to swap the driver itself with the other one that I have to make sure it is indeed the amplifier portion that is having the issue and not the driver itself.
I should also state that it was working fine at a previous point - not sure if a bad soldering could become a problem over time or would be so from the get go. I worry that it was damaged during a period of time where it was playing rather loud for an extended period of time. Feeling the unit it was quite warm after.
I took apart the speaker further and exposed the IC. Looks like its an NEC UPC1188H. This seems to match: https://www.jotrin.com/product/parts/UPC1188H
Not sure if the following line, L9341A, has any significance, or if that is a batch number or something similar.
Would it be likely that if any part were to fail (and do so in the described manner), that it would be this part in particular? My soldering and electronics testing skills are limited, but I think I could manage replacing it. I see the part available online for not too much.
I appreciate you sharing your knowledge on this!
I should also state that it was working fine at a previous point - not sure if a bad soldering could become a problem over time or would be so from the get go. I worry that it was damaged during a period of time where it was playing rather loud for an extended period of time. Feeling the unit it was quite warm after.
I took apart the speaker further and exposed the IC. Looks like its an NEC UPC1188H. This seems to match: https://www.jotrin.com/product/parts/UPC1188H
Not sure if the following line, L9341A, has any significance, or if that is a batch number or something similar.
Would it be likely that if any part were to fail (and do so in the described manner), that it would be this part in particular? My soldering and electronics testing skills are limited, but I think I could manage replacing it. I see the part available online for not too much.
I appreciate you sharing your knowledge on this!
Attachments
Ic is upc1188h , 20w amplifier with dual rail voltage, so no output coupling capacitor needed.
Two of local stores lists as having them in stock, probably old ic and it's old stock. But i don't think you need to replace it right now, try another speaker first. Ic is designed for 8 ohms load, but if you don't play loud, it may handle less ohms . Or you may connect a power resistor ,few ohms, in series with the another speaker you have, to prevent ic overload,if your speaker is less than 8 ohms. Mostly if ic amplfiers fail, they fail permanently, or output a dc to speaker, if direct coupled ( or have bridged output ).
Two of local stores lists as having them in stock, probably old ic and it's old stock. But i don't think you need to replace it right now, try another speaker first. Ic is designed for 8 ohms load, but if you don't play loud, it may handle less ohms . Or you may connect a power resistor ,few ohms, in series with the another speaker you have, to prevent ic overload,if your speaker is less than 8 ohms. Mostly if ic amplfiers fail, they fail permanently, or output a dc to speaker, if direct coupled ( or have bridged output ).
Your hunch was correct! I tried out an older driver and it is playing fine, without triggering the same distortion and shutoff. I had assumed that I had damaged the amplifier section and didn't consider that I might have damaged the brand new driver instead... whoops 🙂
Looking further into the IC's datasheet, it does mention a muting a circuit for thermal and overcurrent protection. So, I suppose it must have been triggering that in response to the load from damaged driver, rather than malfunctioning on the electronics side.
My guess is that it will be easiest to just replace the driver again rather than try to fix this damaged one. In any case, you have saved me a trip down the wrong rabbit hole. Thank you for the help on this!
Looking further into the IC's datasheet, it does mention a muting a circuit for thermal and overcurrent protection. So, I suppose it must have been triggering that in response to the load from damaged driver, rather than malfunctioning on the electronics side.
My guess is that it will be easiest to just replace the driver again rather than try to fix this damaged one. In any case, you have saved me a trip down the wrong rabbit hole. Thank you for the help on this!
Progress is good news . If you can get identical driver, then its simpler way to just replace it. But if you get just similar , better replace both channels then. For stereo to play properly, both channels should be identical as much as possible. If you get different speaker driver, but just same size and mounting, it would have different frequency response and phase characteristics, resonance and so on. And without replacing both channels you will hear that difference as some kinda of localized stereo effect, that some frequencies are not centered , when they should be. Or one channel weaker bass , mids, or highs. So maybe repair wire ( if wire is broken) is option too.
Yes, this was already the second driver to match the other speaker of the pair. Luckily the current replacements are still available new, so I will just order again. Hopefully won't have an issues this time! Thanks again for the help and recommendations.