Help repairing Boston acoustic Msubwoofer(no sound output) (pics) - Page 1
Forum section: electronics / repair

author: r2digital
author_rank: Newbie
timestamp: March 23, 2024, 01:27:16 pm
content: Hi everyone, i’m hoping some one has the patience to point me in the right direction to repairing this fairly expensive boston acoustic msubAt first the off/on/auto switch was busted so i forced it into auto mode by bridging a connection on the main board and now the blue light comes on but i don't get any audio output. Does anyone one have any experience repairing this amplifier? I looked it over, checked some the of output transistors, some of the power resistors and cleaned that nasty conductive epoxy glue stuff off but not having much luck in the trouble shooting department. I did however find a part of the PCB that was discolored from heat that may be an area of interest. It consists of two transistors which you can see in the short videos i attached along with some pictures to my google drivehttps://drive.google.com/drive/folders/1fx9ICpo1UH4mJoX8_hJFEv-s0_tVUBKF

author: r2digital
author_rank: Newbie
timestamp: March 23, 2024, 05:38:58 pm
content: In this pic, does it look like parts are broke off and are missing from the preamp board or were these intentionally left off?C263R238C259R282

author: r2digital
author_rank: Newbie
timestamp: March 24, 2024, 06:22:11 pm
content: After searching the web ALOT i finally found the MSUB schematic maybe now some one can help?

author: Grandchuck
author_rank: Super Contributor
timestamp: March 24, 2024, 07:18:47 pm
content: Usually, early on, one verifies the supply and rail voltage. For example, have you checked voltages on sheet 2 of 7.

author: r2digital
author_rank: Newbie
timestamp: March 24, 2024, 07:20:52 pm
content: Sorry, i thought i uploaded the pictures but yes i did

author: Grandchuck
author_rank: Super Contributor
timestamp: March 24, 2024, 07:31:10 pm
content: OK. Do you have the means to safely inject an audio signal?

author: r2digital
author_rank: Newbie
timestamp: March 24, 2024, 08:16:07 pm
content: i don’t have anything to inject audio but if it’s not too expensive i don’t mind purchasing something to help aid in repairing this. All i have is an auto ranging DMM and a ESR capacitor tester

author: Grandchuck
author_rank: Super Contributor
timestamp: March 24, 2024, 08:40:07 pm
content: Since it is a subwoofer, it might respond to hum injection. If it is line isolated, you can probe using a wire long enough to pick up power line noise. Some even use a metal screwdriver thatthey are in body contact with. If it is not line isolated and since you do not have an isolation transformer thatis a terrible idea, I hesitate to guide you where it might not be safe to go

author: r2digital
author_rank: Newbie
timestamp: March 29, 2024, 11:19:50 am
content: I usually just play a source through my AVR with a subwoofer pre out while probing around. Does anyone have any ideas what to look for? I replaced just about every cap on the preamp board and still have the same no audio output. I did possibly manage to find an area of concern. Randomly after 30-45 seconds of powering the unit up this area emits a high pitched whine on its own but if i press my finger gently in this area the high pitched whine will come on constant

author: r2digital
author_rank: Newbie
timestamp: March 31, 2024, 05:06:01 pm
content: Im only posting this so this may help some one in the future as i got very limited help from the forum. I ended up fixing it by replacing all the SMD caps circled in red, more specifically it was the two small ones in the middle. Once those were replaced sound was restored but i continued to finish by putting the rest of the new electrolytic caps in the pre amp section and the limiter as well. Also @C234 on the preamp schematic it called out for a 10u/50V but what i took out was a 100u/16v not sure if this was intentional or what but i put the 10/50v in

author: amanosalva
author_rank: Newbie
timestamp: October 07, 2024, 03:37:02 am
content: Hi. Thank you for sharing schematics. In my case i changed R09, C11 and C12, now the delay relay works fine. The resistors chargle slowly the capacitor and after a fiew seconds the transistor can push up the relay. is a simple delay function. Regards from Chile.