RT1000i rebuild
so_ein_pech
Posts: 42
I have a set of RT1000i towers with powered subs. Or, more accurately, I had powered subs, but both of the amplifiers died a painful buzzing death. By buzzing, I don't mean ground loop hum that arises from poor grounding, but what sounded like a community of angry bumblebees living inside of the bottom half of my speaker. The amps died at different times, completely independent of each other. And a little bit of searching on this forum reveals that this problem is not uniquely mine.
Also, one of the woofers broke or 'froze up'. (See "Note about Polk Audio Speakers" on this site.)
Instead of trying to repair the speakers so their original condition, I have decided to try to upgrade them. The original subs were a bit anemic sounding. The manual claims, absurdly, that the subs are 80W each. Even the 35W RMS listed on the website are fluff, since the data sheet for the LM1875 (the IC that drives the woofer) lists the RMS output power at 25W.
My goal is to rebuild the amplifier around the LM3875 chip which provides more oomph (around 50W).
--Is there any reason that this would be more difficult than replacing the faulty parts, upgrading the transformer, slapping in a new IC, and doubling up the filter caps?
--Does anyone know anything about the MW7200/7201 woofers. Is it worth buying the replacement from Polk (around $60, I think)? How many ohms are they? How much power they can handle, etc. These woofers in the sub assembly (MW7201) and the main speaker (MW7200) are the same as far as I can tell. Actually, I am using the woofer from the broken sub as the top woofer, since it was the top one that broke. I assume the woofers can handle a quite a bit since the speakers are rated up to 250WPC (although I find that number hard to believe.
--Does anyone know if this the problem with these amps is related to a design flaw, or is it simply a batch of bad parts were used in the RT1000i's. Has anyone on this reading this successfully repaired such a problem? I had one amp repaired at a local electronics shop, only to have it mysteriously fall silent later--no bees or anything. But from the looks of the (sloppy) soldering, he replaced both filter caps, one of the voltage regulators, the amplifier chip (LS1875) and maybe the bridge rectifier. I did a little bit of probing on the amp that still sounds like it has bees in it with a multimeter and my poor-man's oscilloscope (a preamp that I don't particularly care about, an RCA cable with one end cut off, and a set of headphones) I found that the voltage a bit low on one end. The power rails for the amp should be +25V and -25 V are instead +25 and -21. I fail to see why that, in itself should cause such a massive failure since these voltages fall well within the operating range of the LS1875, but it does suggest that there is some leakage going on in either the rectifier or the filter caps.
schematics: RT1000P.sch.pdf
Maybe i'll rebuild the crossovers too.
Also, one of the woofers broke or 'froze up'. (See "Note about Polk Audio Speakers" on this site.)
Instead of trying to repair the speakers so their original condition, I have decided to try to upgrade them. The original subs were a bit anemic sounding. The manual claims, absurdly, that the subs are 80W each. Even the 35W RMS listed on the website are fluff, since the data sheet for the LM1875 (the IC that drives the woofer) lists the RMS output power at 25W.
My goal is to rebuild the amplifier around the LM3875 chip which provides more oomph (around 50W).
--Is there any reason that this would be more difficult than replacing the faulty parts, upgrading the transformer, slapping in a new IC, and doubling up the filter caps?
--Does anyone know anything about the MW7200/7201 woofers. Is it worth buying the replacement from Polk (around $60, I think)? How many ohms are they? How much power they can handle, etc. These woofers in the sub assembly (MW7201) and the main speaker (MW7200) are the same as far as I can tell. Actually, I am using the woofer from the broken sub as the top woofer, since it was the top one that broke. I assume the woofers can handle a quite a bit since the speakers are rated up to 250WPC (although I find that number hard to believe.
--Does anyone know if this the problem with these amps is related to a design flaw, or is it simply a batch of bad parts were used in the RT1000i's. Has anyone on this reading this successfully repaired such a problem? I had one amp repaired at a local electronics shop, only to have it mysteriously fall silent later--no bees or anything. But from the looks of the (sloppy) soldering, he replaced both filter caps, one of the voltage regulators, the amplifier chip (LS1875) and maybe the bridge rectifier. I did a little bit of probing on the amp that still sounds like it has bees in it with a multimeter and my poor-man's oscilloscope (a preamp that I don't particularly care about, an RCA cable with one end cut off, and a set of headphones) I found that the voltage a bit low on one end. The power rails for the amp should be +25V and -25 V are instead +25 and -21. I fail to see why that, in itself should cause such a massive failure since these voltages fall well within the operating range of the LS1875, but it does suggest that there is some leakage going on in either the rectifier or the filter caps.
schematics: RT1000P.sch.pdf
Maybe i'll rebuild the crossovers too.
Comments
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I keep getting share requests for the old link because google changed their security settings. This link should work without having to bug me:
https://drive.google.com/file/d/0B5c0IGpsS0fcOTgyMmM5ZTUtMjcxYS00NTA5LWJjNDAtMzRkMzBlNGViNzVi/view?usp=sharing&resourcekey=0-uUOVjHlaD2ZFeZQ-P7AWJQ
Also, 10-year update: These speakers are still going strong and currently serving duty in my parent's surround system. I'm pretty sure I used the LM1875 and didn't bother with the LM3875 chip. In retrospect, you would want to make sure the other components (specifically the rectifier and transformer) would be up to the task of providing enough current to drive a more powerful amp package. Not to mention, thermal dissipation... Not to mention the LM3875 would provide similar output power with the +/-25V supply voltage... so yeah. Just stick to the LM1875 and ignore my 10-year old ramblings about using a more powerful chip.
It remains true that the driver is woefully underpowered. You could hypothetically take the output of the op-amp NJM2060 (pin 7) and use it to drive your own amplifier and then feed it back to the woofer. That would probably be easier (but less elegant) than building a custom power supply to drive a different chip amp.
Anyway I've spent too much time thinking about this already. Thanks for the trip down memory lane.Post edited by so_ein_pech on