Bose Acoustimass 10 IV Subwoofer No Sound. Help with burnt component identification?

[nobbnobb1]

I picked up this used Bose Acoustimass 10 IV speaker system. Speakers seem to work fine, but the subwoofer does not make any noise. So I took apart the unit and found some obviously failed (burnt) components:

- 1x 3.14 A @ 250V main circuit fuse
- 1x bridge rectifier GBU601. there's actually two of these on the board, one is obviously burnt and the other does not seem to have any anomalies like shorts. However I figure may as well replace both.
- 2x diode. This is tricky because the only markings I see on it are: "617 402". No current rating or anything else. How do I figure what to replace this with?

There are also transistors SGSD200 (PNP) and SGSD100 (NPN). Couldn't seem to do the typical diode test on these, maybe due to how they are wired on the board. There's no obvious shorts, but I wouldn't mind replacing these just to be safe. One of these does seem slightly darkened. The part numbers show up as obsolete on Digikey. What do I look for in order to source a suitable replacement?

So plan is to just replace the components listed above, help identifying what diode and transistor to use appreciated. Everything else on the board otherwise looks fine. If there's other thoughts, let me know? Thanks!

See attached picture.

[Sam Scarbo]

When testing/trouble-shooting, i suggest not to use the original driver speaker. if there is a DC voltage fed to the driver speaker by accident, it will damage the driver speaker ($$$). use a cheap spare speaker or a power resistor to replace the driver speaker during testing.

[CapLeaker]

I think you're gonna have tough luck if one or all of these Darlington transistors are bad. Take them off and check them first before you go do an order and find that you can't replace them.

[nobbnobb1]

Dang, so I just discovered that small SMD resistor below the "O" in "BOSE" may also be blown. It's testing ~50 ohms, but I'm guessing there's almost no way to verify what it should be without the schematic?

I found a similar Acoustimass 10 III schematic (attached), but upon further tracing it's not 100% the same, so I'm a bit lost on how to identify this resistor.

[CapLeaker]

Digikey has some 2stw100 and 2stw200 Darlington transistors limited number in stock, but already marked for obsolete. The blown resistor measures 50 ohms, so you know it couldn't have a higher resistance. What ever the value, either is 50 or less than 50 ohms.

[nobbnobb1]

Digikey has some 2stw100 and 2stw200 Darlington transistors limited number in stock, but already marked for obsolete. The blown resistor measures 50 ohms, so you know it couldn't have a higher resistance. What ever the value, either is 50 or less than 50 ohms.

Thanks. So per the above circuit, I was trying to trace it and was under the impression that it was the resistor labelled R73 which is a 2Kohm resistor. But the schematic doesn't line up exactly based on me trying to trace it from pin #3 on Q209, to Q213, then to R73.

So if I'm measuring 50 ohms right now, by that logic I may be safe just ordering a 2Kohm resistor and seeing what happens?

[CapLeaker]

Nope. You know already that the resistor in question measures 50 ohms and the resistance cannot have been more than that because a damaged resistor would go higher in resistance not lower.
You can try cleaning the resistor with 99 IPA and see if you can find markings on it, do an educated guess or find another picture that shows this resistors value. Also have a look at the markings on the SOT23 device and identify it. I couldn't identify it as the pic was too blurry for me on that spot.

[nobbnobb1]

Where is this SOT23 you are talking about?

I did try to clean/scrape things off...no markings legible at all. Also, per the above schematic for the similar AM10 III, I'm noticing that the two transistor/bridge rectifier sets (top row vs bottom row in the board pic) have very similar components. I thought that in theory, if I could find the similar resistor on the bottom bank in my unit...that it should be the same as the burnt one I'm trying to identify. However, no luck as it seems the AM10 IV is different enough to the AM III that I am unable to find it.

[CapLeaker]

That burnt SMD resistor, follow the trace left. It goes to a 3 legged SOT. What are the markings on it?
added a similar SM

[nobbnobb1]

That 3 legged device has markings: KA6 50 (or could be "SO"). I did try to search it up, but turned up nothing useful.

And thanks for the alternative service manual. That's from a unit in the 90's of which the circuitry appears to be completely different than the AM10 IV. I've already tried sleuthing through Elektro Tanya for anything Bose in the hopes the schematics for similar products of this era may yield some details. I also contacted Bose and they would not disclose any service manuals.

EDIT: WOW..nevermind. Thanks for pointing me in the right direction! I think I was tracing the circuit incorrectly this whole time. That SOT-23 device appears to be an MMBD914LT1 as per the schematic. The burnt resistor is R87 at 49.9 ohms (so I WAS measuring it correctly!). I also traced the other end to +12V which corresponds to the 12V output pin in U808 (LM317 regulator). So strangely this resistor is still measuring correctly despite the burn marks! So strange.

[CapLeaker]

Yep… I guess you are now on the right track! I wouldn't trust that resistor, as it could change resistance now while current is flowing through it since it is damaged. So replace that 49.9 ohm resistor with an equivalent and check that diode as well.