I'll post some photos on Flickr when I get a chance, but thought I would share some of my travels through the inner workings of the SoundBridge Radio power supply. I know this has been a constant discussion topic, so I hope some of you find some help here.
A friend of mine brought me his dead SoundBridge radio, along with links to this forum and the PSU problems plaguing SoundBridge Radio users. I have a bit of time on my hands, so I thought it would be fun to tear the thing open and fix the PSU. Little did I know what I was in store for.
Disclaimer: You should not even attempt to try this unless you have a solid understanding of electronic circuits and all the safety precautions needed. There are dangerous amounts of power being manipulated by this PSU. Before you try to open this sucker up and play with it, ask yourself this: Do I know enough to not accidentally die from an electrical shock?
Opening the Case
This requires three things:
1 - A T20 tamper proof star driver (the one with a hole in the driver for a center pin)
2 - A paint scraper or other implement for prying through the copious amounts of glue
3 - A well ingrained sense of patience
There are four tamper proof screws that hold the back of the unit onto the main electronics housing. You can probably find the tamper-proof driver you need at a local hardware store, but you might need to order it online. The one you need is a T-20 tamper proof star driver.
Once the screws are out, the two halfs of the unit are connected by two wire harnesses that have simple clips holding them together. There is no need to open the main electronics housing (with the LCD display), so just set it aside. The back housing (triangular) is where the PSU lives. To get this open, you need to remove four recessed screws as well as 8 small surface screws, some of which may be hidden beneath rubber gasket strips. Thankfully all those screws are standard phillips heads.
Once the screws are removed, the front panel needs to be carefully pried off, as it is held in place with about three tubes of silicone glue. An exacto knife and a thin-bladed putty knife did the trick for me, although it did take about 30 minutes to complete this without breaking the plastic housing.
Once inside, you will find three sets of wires that prevent you from opening the case very far. Two of those wire bundles have chokes wrapped around them (rectangular blocks). The chokes have small clips on the side that allow you to open them, freeing the wires so you have more working room. The third set of wires runs into a small circuit board (I'm guessing a crossover circuit) and eventually over to the speaker in the top of the cabinet. There's isn't much you can do about that one unless you want to cut the wires and re-splice them later.
What you need is the PSU which is contained in a plastic box wedged into the case. It is also glued, so a bit of prying might be needed to get it loose. The PSU is encased in a thin plastic container wrapped with copper tape for shielding. Peel back the copper tape, and the plastic pieces should separate, although you will need to feed the wires through the plastic holes to get the PSU boards out of this little box so you can work on them.
Do I need to mention that you should have unplugged the radio from the wall before doing any of this? Really? Do I?
A Look at the PSU Board
Once you are able to get the PSU board free from the case, what you will find is two boards glued together with enough adhesive to assemble a small commercial jet. There will probably also be giant blobs of adhesive strewn across the components and board in a pattern that makes you think it might have been bring-your-child-to-work-day at Roku industries.
On the unit I dissected, as well as from many on these forums, the most visible problem is a blown capacitor on the board. The one you should look for is a small black electrolytic cap very near to the two huge caps that are adjacent to each other. More than likely you will find the top of that capcitor to be bulging or even burst. There is probably a bunch of blackened goo nearby as well.
It would be easy to say "replace that cap with a fresh 220uF 35V aluminum electrolytic capacitor", but there's a bit more trick to it than that. To be able to do so, you first need to do some more prying.
Separating the two PSU circuit boards
The main board with the PSU components has a secondary PCB underneath it, which appears to do nothing more than provide a ground plane and noise shield. To reach the underside of the component board, you need to pry these two boards apart. Start by removing the several solder connections that penetrate through to the lower board. Once those are desoldered, you will need about thirty minutes with an exacto knife and several thousand words of profanity to pry the two boards apart without snapping one of them. It was also interesting to find the two MOV Varistors tied to the ground plane in a way that tells me they weren't part of the original design. (What? The UL Inspector says we need over-voltage protection?)
Replacing the Capacitor
Unsolder the old one. Solder in a new one. Put it all back together and call it done.
Except It's Not Always That Easy
So during my dissection, replacing the failed capacitor did bring the unit back to life . . . . . . . For about 15 minutes until it failed again. Think opening that case and then closing it all back up is fun? Wait until you get to do it twice.
On the second failure, I found a zener diode (CORRECTION: Schottky Diode SB260, NOT Zener) that had failed, located right next to the capacitor I just replaced. The solder job was strange on this diode, and I soon found out why. The leads on the diode didn't fit through the holes on the board. Apparently whoever designed the board forgot to check the lead diameter and laid out drill holes that were too small. So the assembler had simply snipped off the leads, laid the diode on top of the board and then looped some small bits of wire through the holes to connect it to the pads on the bottom of the board. Nice job, guys. High quality. Nobody at the assembly plant owns a drill? Really?
Before I started replacing more components, I decided to learn more about how the PSU was built. I ran several of the part numbers and found that the basis of this PSU design is the TOP247 switcher. In fact when I pulled the datasheet for that part, one of their example circuits is pretty much part for part the PSU that powers this radio. It's not a bad design at all, so don't blame the circuit designers from Roku. The problem seems to be purely in low quality components used in assembly, as well as some crazy assembly practices.
The problem with this PSU design is that all the pieces play together very tightly to ensure the correct output. So when one component fails, it impacts voltage and current all over the board. There is a reasonably complex feedback circuit to control the switcher, that works through a set of sense resistors and an opto-isolator back to the TOP247. If a component fails, the feedback profile changes, thus causing generalized weirdness all over the board. In the unit I was working with, the failed diode caused some unsafe current loads through the feedback circuit and fried the opto-isolator as well as several of the resistors in the circuit. It also seems to have damaged the voltage reference IC on the board, which pretty much borks the whole thing.
The PSU should have two output voltages (19VDC and 9.5VDC), with the damaged feedback circuit, I am getting 22VDC on the 9.5V rail, and a constantly changing voltage reading on the 19V rail. I can only pray that the Roku people put some regulators inside the main electronics enclosure, otherwise those voltages could have fried stuff in there as well.
In the end I have decided to just replace most of the post-transformer components with new reliable parts. I'm building a list of components available through Mouser Electronics for anyone else who wants to give this a try. Nothing pre-transformer appears to be borked, but I might replace that giant mongo 100uF 400V cap just for good measure.
I'll work on getting some pictures together of the operation, as well as a parts list of replacement components and post those to the forum later this week.
Oh and one last thing. The datasheet for the TOP247 highly recommends the use of an NTC Thermistor to limit inrush current charging the capacitors at first power-on. There doesn't appear to be one of these on the PSU board for the SoundBridge Radio. There is a component that could possibly be an NTC Thermistor, but looks more like a fuse pot. (UPDATE: It's a fuse, no inrush protection on the board). Without that Thermistor, the inrush current is what is probably blowing people's power supplies like fireworks on bonfire night. I make that guess since so many postings state their radio died after they disconnected it from power and then reconnected it.
I'm gonna see if I can determine if that mystery component is actually a thermistor (UPDATE: It's not). If not, I'll try to see if I can hack a way to insert one into the board and provide that protection.
Last edited by spiffy_chimp
on Tue Oct 14, 2008 9:26 pm, edited 4 times in total.