Polk Audio RTi100 Plate Amplifier won't power on and intermittently honks and buzzes

[MrCommunistGen]

My childhood best friend was killed about 9 years ago. His parents are finally letting go of his things, and they offered me his Polk RTi100 tower speakers. The Polk RTi100 towers are one of a handful of designs from the early 2000s that is a full range 3-way tower with a built-in self-powered subwoofer. This is accomplished with a plate amp integrated into the speaker input panel.

I think they used them for a while, but they've been sitting for at least a few years now. After about 40 hours of driving to their house and back (both they and I have moved in the last 9 years) I got the speakers and set them up.

Before even connecting the banana plugs to my receiver, I went to plug in one of the amplifiers into my surge protector and the subwoofer immediately started intermittently thrumming -- making a sort of honking noise -- almost certainly being over-driven. I disconnected the amplifier quickly. Suspecting the worst I tried the other speaker and it behaves in the same manner.

I had to perform some minor amplifier repair on a different (Infinity) subwoofer amp a few years ago, so I have more than 0 experience with these types of things, but I'm very much an electronics amateur. I was prepared to re-cap the Infinity sub, but it ended up being just a burnt out resistor that had to be replaced. The whole process was much simplified because I was able to find the service manual for that amplifier.

I've had no such luck with finding diagrams of the amplifier in the RTi100 tower speaker.

I was able to pull out the plate amplifier and examine it. There's a bit of darkening on the board where a Mosfet is soldered to the board, but it's by no means burnt or scorched. As far as I can see all the caps and resistors look OK.

To try to get to the bottom of what was going on, I disconnected the subwoofer driver from the amplifier's output leads (to avoid damaging the subwoofer driver) and plugged the amplifier into AC power with the amp sitting outside the speaker. What I discovered is that the transformer seems to be buzzing intermittently in a manner coinciding with how the subwoofer was thrumming.

Video of this behavior:
https://youtu.be/-zIFATqpQvY

Based on this, it would be my guess that the transformer has gone bad. Unfortunately, with both amplifiers having what appears to be the same failure, I can't check the correct output of the transformer, and I'm not seeing any markings that would properly identify it.

The sole sticker says:
54-0100-5900
KSE 0139

But an internet search doesn't seem to turn up anything meaningful with any of those terms.

I'm also reaching out to Polk support, but based on various threads around the internet (many in the 2013-2016 range) it looks like Polk has long since stopped providing repair support for this product.

In my searching I found numerous listings on eBay and the like for repair services for the amplifiers, but I'm super leery of a scam. UPDATE: I looked into this while finishing up the post, and it turns out the eBay seller I found is "electronetix" who seems to be recommended on a few AV forums and has mostly positive feedback on eBay.

Since these speakers are more for sentimental value than practical value, I'd much rather keep the speakers with non-functional amplifiers (they still work as 2-way speakers), than mail someone the plate amps and they just disappear or come back even more broken.

Ideally what I'm looking for is guidance on how to repair these amps myself, perhaps with a service manual, or barring that a recommendation for a reputable repair shop to send these off to.

I guess as a 3rd option I'd be interested in figuring out a way to bypass the built-in amp and power each subwoofer with its own independent amp... but that sounds like a lot more work than I'm probably willing to put in.

Any and all advice would be greatly appreciated. I'd be happy to provide additional photos or videos if requested.

Thanks in advance!

[budwich]

The nice people at polkaudio.com sent me schematics for psw202 (old) recently so they have some things around. Of course, that doesn't mean parts might exists or can be found. I found that the caps on associated with the "front end" (ie. power) had issues along with possibly the IC AMP on a chip. I gave up as working with the board was difficult as there was solid thermal glue everywhere.

I attached this herein... it might provide some clues IF it is close to your design.

[Per Hansson]

I would say the inner CapXon in this picture looks both bulged and sits at an odd angle.
In any case with caps of such bad brands it would not be strange with silent failures also (dried up due to heat)
Desoldering all caps and testing them on a cheap eBay "component tester" might be revealing.
Or if you don't have one just ordering new caps and replacing them all as a start with reputable brands.
Also it is well worth to check the PCB for bad solder joints with a good magnifying glass or loupe.

[MrCommunistGen]

First update:
I looked into this a bit more and it turns out that it is NOT the transformer that's buzzing -- which is good news -- as that part would seem to not be replaceable.

Second point:

Originally Posted by Per Hansson

I would say the inner CapXon in this picture looks both bulged and sits at an odd angle.

Good observation. I'd just assumed that it was installed a bit crooked. However, while trying to narrow down what component was buzzing I poked at the tilted capacitor and noticed that the lead on the raised side is completely detached from the capacitor. So yeah, that's dead.


My thoughts:
Ordinarily, I'd be comfortable just desoldering and replacing those caps myself, but after spending FAR TOO LONG trying to free the amplifier board from the rest of the plate so I can access the primary caps, I've given up. The caps are very solidly glued down on one end which, combined with the heatsink directly above them, makes accessing them very difficult. For a novice this is more than a bit intimidating.

I don't know enough about tearing these things apart to non-destructively separate the power board from the rest of the plate with all the connectors, switches, and dials excessively glued together... so I think I'm just going to send these off for repair.

As I mentioned before, these have strong sentimental value to me, and I'd rather not risk break something, so at this point I think I'm just going to send them off for repair.

I appreciate the input from the community. I definitely want to actually repair some more stuff for fun as well as experience, but not these amplifiers under these conditions.

[Boats1997]

I picked up a pair of Polk RTi100 yesterday with the same issue on one of the amps. I'm going to send the amplifier for repair. This shop does the repairs for about $97 shipped. https://www.ebay.com/itm/253344310964

While the amp is being repaired, I'm going to sand and paint the cabinets to match my Polk RTi70's.

[Boats1997]

Here are few pictures from the RTi100 refresh project, one amp was mailed out today for repairs.