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In Reply to: RE: Bose Wave Radio/CD - Scratchy Noises When Off posted by JonM on May 08, 2016 at 15:40:31
From a guy named "Dan M" on youtube:
"""I had a very similar problem (loud buzzing/ humming sound when turned on). Ending up replacing 4-5 of the larger
electrolytic capacitors that had gone bad and it worked fine afterwards.
I believe it was the largest 4 or so caps, not including the very
largest. Need soldering experience to replace the old ones. It also help to have an
in-circuit (not all are) capacitor ESR meter to double check
which ones are bad, without having to remove them first.
Read elsewhere this is a common problem with the older wave radios because of a design flaw. They never turn off! The power button drops volume to zero, but circuits otherwise are constantly on, meaning they stay warm all the time and the caps dry out faster."""
Not sure if that's exactly the solution to the problem, but of all things, replacing big electrolytic caps is the easiest. Also, I'm pretty sure these are going to be shitty (read cheap) caps, so it won't be an expensive fix. I had to get the Bose dig in somewhere...
Good luck! Here is the link to the video: https://youtu.be/5I8t7tRO2pc
Thanks! Caps are easy, and are on my list of top suspects. However, I am getting no audible hum or buzz even with the radio on. If you put your ear against the speakers, with the volume at 0, you can still very faintly hear the music (but zero hum), but when you turn the unit "off" the music stops (and the scratchy noise starts), so there is a difference between "on" and "off".
I'm also thinking the issue is somewhere else - the output amplifier is misbehaving when it's partially powered off, or the power supply itself isn't fully off when it needs to be (letting small voltage spikes through to the amplifier). Or something else, who knows?
I am totally guessing at this point - without a service manual, it's all I can do. The radio isn't worth the cost of replacing parts till it works, so if the fix isn't simple and cheap, the radio will probably go back in the trash. If I figure it out, I'll post my results, in case it helps someone else.
I initially thought that there was a bad relay on the on/off circuit- but after reading the above post - that it actually does not turn off - but drops the volume - there may be an issue w/ the rt channel of the amp - having been driven into ground for so long- it may have a bad cap or tired transistor...
If you do revive it- you might put in a DPDT power switch - just to make sure it goes OFF -
I found the service manual and schematics on line (thank you Google). The output amp is an integrated part - TDA7375AV - under $6 from Mouser. I have a little more experimenting to do before I plug in my soldering iron, to see if I can isolate the source of the crackling noise. But with the manual and schematics/parts list, at least I'll have an idea what I am looking at.
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