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
Hello all,
I picked a black keys Beomaster 1000 with very low volume on the left channel as a restoration project while I'm over here in Germany visiting my girlfriend (can't stay away from an audio project for too long!). I noticed this on both the turntable and tape inputs but I decided to focus my troubleshooting on the tape input as it would be easier to manage and that it seemed the issue was affecting both inputs. I discovered the b154 #3 transistor was bad on the left channel and replaced it. The transistor in place was actually soldered in was a BC159B (same on both L and R channels) and I was able to find some NOS BC159B on german eBay. This seemed to have solved the low volume issue but now I can clearly hear a difference between the two channels. The left channel has more high frequencies and significantly less bass than the right channel. It also just causes me to feel uncomfortable as the music doesn't sound natural or how it is supposed to sound like the right channel does. I can adjust the bass eq up about 40% and decrease the treble control a bit that gets it close but it still doesn't sound quite "right". I checked the dc voltages throughout the left channel and didn't notice anything extrordinarly out of line. The only thing I noticed is that between BC154 #4 and BC154 #9 being that although the voltage at the collector on was higher than the schematic value on both channels due to slightly high power supply voltages, the bass and emitter voltages were slightly below the schematic values on the left channel. On the right channel they were proportionally higher than the schematic values as expected. But it was nothing extreme in this case as the bass and emitter voltages were still within 10% of those written on the schematic. Could this possibly be a clue or significant and warrant further investigation? I am wondering how critical the matching is between the transistor I replaced on the left channel and the original in the right channel? The NOS transistor I bought was from a different batch, but I find it difficult to belive this could affect the sound so dramatically. The other thing I thought of was that one of the tantalum coupling caps was bad causing the filtering of the bass. I've only heard of them exploding or shorting so I'm not sure if this failure mode could account for the symptoms I'm experienceing. The last thing I thought of was a parasitic oscillation somewhere or another bad transistor that wasn't exhibiting an obvious failure but still wasn't perforimg correctly. Unfortunately I'm over in Germany visiting my girlfriend and I don't have a scope or spectrum analyzer at my disposal. I just have a multimeter and I can generate an audio signal from my computer. I would be grateful if anyone could help me sort this out so I can experience this beomaster in all of its glory. I just don't want to have to be anymore invasive than necessary, desoldering parts to swap between channels to try and isolate the problem as I have found the solder pads can be a bit sensitive on these old beomasters. I don't seem to have any issues with the switches or pots so I've ruled that our. No crackling or dropouts.
regards,
Ryland
Hello Ryland.
To me the problem that you are experiencing sounds like a capacitor issue. The more so since you mention that there are tantalum caps in the circuit.
Back in the day when the Beomaster 1000 were made, tantalum caps were all the hype as the next greatest thing for consumer electronics... smaller then electrolytics while having the same capacitance and supposedly better performance... but they came out to be more a headache then anything else for those on the "repair" side of the story. This is particularly applicable to the "drop" shaped Ta capacitors and lasted at least until SMD came along. A tantalum capacitor made in the 70's and 80's should be regarded as not trustworthy any more, and even back then they had all kinds of failure modes, not only the "dramatic" ones. So, in a nutshell, my suggestion would be to replace those tantalums with new ones. For completeness I must say that I do not have a Beomaster 1000 and haven't seen the inside of one, so my comment is based purely on the details in your description of the problem.
Best Regards
-Hermann-
Hi Hermann,
Thank you for your response. I totally forgot to update here that I was able to resolve the issue. I was poking around a bit and I found that if I disturbed the polystyrene capacitors in the "loudness" circuit strapped across the volume control, I was able to trigger the issue. I tried replacing the capacitors but there was still an issue and it seemed like there was something making poor contact or shorting. I did my best to dress everything neatly and levered some solder tabs a bit and I was able to make the problem go away consistently for now.
Also it looks like maybe I should have posted this in the "work bench" forum; I'm new over here, but I love B&O.