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
I've just purchased a lovely Beomaster 901. When I put it on for the first time there was no sound from the right channel at all. No click when turned on or the input's changed. I opened it up and discovered that two of the three contacts on the output capacitor (one at the top and one on the bottom of the cap) had broken solder connections. I secured the cap down so it made contact and turned it back on, the right channel worked. I re-soldered the connections.
I also noted that it seems at some point in the receiver's past something must've leaked in the top of the unit as there was a mark from dried liquid on the green side of the board with the solder joints. The stain does look like it contacted some of the connections in the right channel.
Now the right channel works but has intermittent issues. Occasionally it plays for a few hours without much issue but often the right is noticeably warmer sounding than the left, and after about a half hour of play I start hearing a few light crackling sounds and eventually the sound gets very quiet and muffled with all the bass notes sounding like static. I've sometimes been able to get it back to normal again by gently proding the large capacitor.
I thought maybe I didn't get the solder joint joined right so i re-soldered the two capacitor joints. It hasn't changed anything.
I'm wondering at this point what's going on. Could the break in connection of the cap have caused damage to anything else in the right channel like the amplifying transistor? The issue only seems to happen after about a half hour of use when things have warmed to operating temperature so I would imagine if it was still a bad solder joint the issue would happen at all time? Or would the spill that happened in the past have blown something else even though there isn't any physical evidence of a burnt out component?
Any help or suggestions would be great! I'm a newbie to B&O so I'm at a loss at this point!
Thanks!
-Aaron
If it sounds good while it works, it's unlikely that there are damaged components. It probably is a bad solder joint somewhere. Try to tap other places as well while it's playing with a screwdriver handle or something... you might find a place that responds audibly and then it should be easier to find the offending joint.
Bad solders are sometimes next to impossible to spot with a naked eye. Often it's easier and faster to just redo them all instead of spending time looking for bads.
Of course, the tapping might lead you to a cracked resistor or a cap with a loose leg as well!
--mika
Thanks for the quick reply!
That's a fair point I suppose the bad connections may not only be the two I could see. Do you have any ideas where to target in the circuitry?
It's clear the issue is isolated to the right channel as the left channel is perfect. Also adjusting any of the controls on the front (bass/treble/volume/mono) don't seem to have any effect. Would it be right to assume then that the issue is further isolated specifically to the right channel output circuit since none of the earlier circuits which have effect on both channels? Since the issue only seems to come up once the system has warmed up it seems like the heat expansion is a factor. Should I be checking the transistors since those are probably the source of the most heat?
If the problem is there on all sources, I would start with the power amp of the right channel. I'm not too familiar with the 901 myself, so I can't say if there are any common problems that would cause this.
Thanks Mika.
I got the board cleaned off today more to get through the residue to see the joints better to try to fix it. I noticed there's some thermal grease/paste used near two of the amplification transistors where another component is poking up through the board. Any advice on what kind of thermal compound it is in case I need to replace it in my testing? I know from working in computer repair in the past that there are different kinds so I don't want to put the wrong one on in case it could damage things.
I'm not sure what kind of arrangement there is, but if you think in the context of when the receiver was made, I don't think it can be anything else than bog standard silicon based thermal paste.
It was white in colour which appeared to be just silicon based stuff so I got some of that.
It's seeming perhaps that this might be an issue w multiple things going on. In looking closer there seemed to be a specific region on the board near where the output capacitor and one of the amplification transistors connect that, if the issue was happening, and I gently pressed the board, it would go back to normal for a moment. So I went through and re-soldered the connections in that region.
The issue is still happening though, but it is no longer affected by proding the output cap like before and the gentle board flexing doesn't seem to make it go away reliably ant more. What I'm noticing now is that when the issue happens, if I sort of press on different parts of the amplification transistor it seems to have an effect, to the extent that if the sound issue has started, and i firmly press the transistor body against the heat sync with my thumb, the issue goes away. That "fix" was able to be repeated multiple times now. I tried this with the other right channel transistor and nothing changed, nor with the other three transistors. I tired re-soldering that one transistor thinking the connection is broken. No change still. Is it possible that the issue with the broken capacitor connections somehow damaged that transistor so that once it warms up to operating temp it suffers frequent intermittent failures in the form of very heavy, muffled distortion? If so are transistors like those obtainable still?
Thanks again for your help, Mika!