ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi everybody,
thanks to Dillen's great kit my Beovox S45-2 are alright so I was able to move on to another project. I came across a Beomaster 900K in pretty bad shape :
- It was powering up but no sound on the left channel-Huge background noise (with a bit of hum) on the right one-Tuning wires completely messed up- Dirty front panel and badly scratched cabinet
So after a few searches I found that one of the wires connecting the left filter capacitor was broken and fixed it. I cleaned everything and put the tuning wires back together (harder than it sounds) and worked on the cabinet. I replaced the power supply cap and the two filters cap fixed to the back panel.
So now, everything works (fm, am, phono, tape) and it sounds much better but I still got a rather large amount of static noise on every source even with the volume at zero. I have search extensively the archived forum but can't seem to find a thread tackling this particular issue.
Here are pictures of patient :
And I have the original schematics for the 900K and the stereo decoder if anyone needs them. So if you know what's causing this noise please, let me know, I am at a loss :)
Many thanks.
Jean
Nice set and a good job !
The persistent noise typically comes from one of the smaller germanium transistorsin the output stage.In your case, one after the volume control, I would say.
Martin
Thanks a lot, once again, Martin to the rescue !
Ok, I have located three potentials culprits on the mainboard (in red circles on the picture below) :
When I was doing that, I noticed a loud ground noise when I was touching this area on what I think is the stereo decoder (I have circled the area) :
And on the other side of this area is... a germanium transistor (also circled) :
Which one is the most likely to be the troublemaker and with what would you replace it (or them) ?
Thanks again !
The buzzing is normal when you touch the amplifier input and that's exactly the outermost transistoron the amp board.I wouldn't suspect this first transistor since it sits before the volume control (from memory) and you told that the noise was thereregardless of the volume setting.I would check the others, those that comes after the volume control, that'll be the one without marking on topand the ones with "J" and "B" stamped on top.
Thanks,
as always, you're right for the transistor on the left, it can't be the one causing the noise. For the other ones,correct me if I'm wrong but there's no way to check them on board ? So if I'm going to desolder them, wouldn't be easier to change them all while I'm at it ? What would be your choice to replace them ?
Regards
Ok, it's status report time !
Thanks Martin, you were right. I ordered and received all the germanium transistors and proceeded to replace them. I believe the AC128 (the ones with a "J" on them) were responsible for the huge noise with the volume at zero. As soon as I changed those the static noise stopped (I tested between every replacement) of course I changed those ones last :)
So now it's perfectly quiet when the volume is down. It sounds actually quite nice (I wasn't expecting much) and my only issue is that I get a bit of noise when I push the volume to a third (it gets worse higher). It is noticeable between tracks. To be honest, I can live with it but if you have a way to identify the culprit, I'll take it :)
Again, many thanks !
Good job so far !
Can you describe the noise ?
I've tried to capture the sound but the mic on the Iphone is not very sensible so I had to push the volume a bit :
http://www.youtube.com/watch?v=xg-WdCeVTA0&feature=youtu.be
It's just a static sound with some scratches once in a while.
I can refer to that noise on the youtube video.The K version I have in my kitchen sounds exactly the same but, but. connecting a laptop playing music, the BM900 playes VERY load at 9 o clock.,the radio even louder.
The output amp pushies its maxmuim at peaks in music when the volume is at 10 o clock.
If I play the BM with the volume at 10 o clock we almost have to scream to each other , yes it loud.So, the level distance between that noise and when its working is not much of a problem, you wont hear it in the music.Remember its half a century old, and quite impressive for its age.I believe that noise is the the preamp transitors, and yes I will exchange them at some time, but that noise is not a problem when using the BM900, with radio or connected to a laptop. if connecting to a record deck, maybe that nosie will be a problem depending on level from RIIA amp, but for only using it for above described, its not a problemChristian
My re-capped M75 are my precious diamonds.
You are right it is not very disturbing, I was just wondering if there was an easy fix. As for the source I use, I integrated a bluetooth A2DP module inside the BM soldered on the tape input. The output level is almost the same as a laptop and it works great !