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It could be a couple of different things like a bad power filter capacitor or a trim pot for the amp bias circuit. It could also be a number of less likely things…
Are you thinking about trying to fix it yourself? If so, what kind of experience do you have and what equipment do you have access to?
Glitch
A quick google search finds one person claiming success using a TIP132/TIP137 combo. I recall seeing other stories where the person used something else on a BM901.
Another tactic is to use the published cross reference charts. That transistor might be old enough that you will have to cross reference the cross references. Verify that the specs (voltage range, gain, etc.) are similar to that of the original part.
A “close enough” part won’t necessarily perform exactly like the original.
I’ve only had to do this with final stage transistors a couple of times. Hopefully, someone with more experience, or experience with this exact situation, can chime in with better advice.
Of course, Martin’s advice of replacement using the correct parts with a procedure that won’t damage them is the best way (if you have the necessary equipment).
Glitch
You might want to consider using modern, readily available transistors while you are trying to sort out your failing transistor issue. This could save you a significant amount of money versus installing rare and expensive vintage transistors. This is especially true if you use the trial and error methodology.
I keep a fairly large supply of cheap Chinese parts on hand for this purpose. I’ll swap them in as part of the debugging process. Once I have the circuit working, I’ll replace them with either high quality modern parts or (expensive) original vintage parts. What I do depends on my goals for the particular piece of equipment.
Glitch
It wears out the motor brushes instead.
Good point. The brushes certainly wear out (and don’t appear to be a serviceable part).
Glitch
But it does seem strange behaviour to me, and can’t do much for prolonging component wear.
For some bearing technologies, the highest wear occurs when the mechanism starts. I think that this is because motion is needed to create and maintain an oil film. Bearing wear is negligible after the bearing reaches equilibrium. B&O probably made this design decision in order to minimize wear.
Glitch
I assume that you are asking about the capstans?
I’m not familiar with the BC5500, but on other Beocords the capstan turns whenever the deck is not in standby.
Glitch
The good news is that the bad part should be easy to find ;-). Determining why it failed could be trickier.
It most likely can be repaired. You will need to open it up to be able to tell how involved the repair will be. Posting pictures of the problem area will make it easier for someone to help.
Glitch
The original caps have values that are not available anymore.
This is not completely true. A quick check of part-express.com will show that you can still buy capacitors that are marked the same as the original caps. One would have to measure the caps that were received to be sure.
There are a few other manufacturers of higher end caps that would likely have the original values (but you might have to wait a while for them to come into stock). The tolerances on these caps are very close to design, but are much more expensive.
In many of the other capacitor brands or product lines they will only make the values similar to what you have in your crossover.
I generally agree with premiumverum about the tolerances (assuming that the original caps were +-20% parts) and the sound.
I’d leave them alone if you like how they sound. Unless of course, that you like tinkering with things as much as listening to music.
Glitch
The former owner may not have been able to find the exact values and substituted the next closest values. Piggybacking the capacitors in parallel is one way of getting a value closer to the original.
Post a table of the installed values versus the schematic values. This will make it easier for those that want to help to give better advice.
Glitch
It looks to me that there are 10 bipolar capacitors on the schematic and “10” on the actual crossover board. I’m assuming that the two capacitors to the
rightleft of the “50” on the board are wired in parallel and should be counted as one.The other caps are (likely) the white boxes, yellow components and the electrolytic on the bottom of the board.
Glitch
You might also find some hints here…
https://forum.beoworld.org/forums/topic/my-beocenter-9500-restoration/
Glitch
Ears are notoriously untrustworthy – they’ll blindly believe anything they hear! ?
Isn’t the problem actually the brain? (i.e. Psychoacoustics) 😉
Glitch
I’m still undecided what to do with the crossovers. Just take the service kit from Martin or maybe do the improvements suggested by Die.Bogener? What do you think?
You might want to measure your existing capacitors and go from there. The caps that came out of my Pentas measured very well. I had some caps (leftover from another project) that I swapped in as an experiment and I couldn’t tell any difference.
I subscribe to the theory that the majority of improvement that comes from recapping crossovers comes from having better matched components. This is in contrast to those that believe that one capacitor sounds better than another. Speakers with well matched crossovers image better than those with mismatched ones. IMHO, this is easier to notice than a minor change in frequency response.
You might want to consider measuring, sorting, and repositioning the mid-ranges. This in in the spirit of having better matched speakers. Doing this noticeably improved my Pentas.
Glitch
My suggestion for anyone collecting these vintage Beocord units is <snip>
I can’t argue with that suggestion. The only thing left to think about is spare parts. I would like to keep a spare set of belts and rubber wheels on hand. I am hesitant to buy them if there is a good chance that they will degrade before I have a chance to use them. I am weighing this against the possibility of the supply of replacement parts drying up.
I am thankful that there are quality parts available right now. I am also realistic about that at some point in time this will not be the case.
And yet some belts seem to last forever: I have a Thorens turntable that has a 20 year-old belt that is still perfectly operational.
Yep. That is what makes this such a difficult topic.
Glitch
Nice work. Really clean 4k you have there! I can’t wait to work through my BM4k. I have a BG4002 with CD4 built in and can’t wait to get the duo playing together one day.
Thanks! Don’t forget to post pictures when you get your duo working. I hope someday to have the same combo but am still waiting for the right turntable to come along.
Glitch
Most companies consider speculation and rumors about their upcoming products as a positive thing. It is arguably better than the alternative of nobody caring if you are making new products.
Usually the truly sensitive product information is carefully controlled as a trade secret or with NDAs. I certainly hope that none of the employees, dealers, or possibly press, leaked something that they shouldn’t have.
One positive thing about living in the age of deep fakes is that even if something sensitive IS leaked, nobody will know if it it real or not ;-).
Glitch
The camera guys seemed to be concerned about stickiness versus degrading into goo. I’m not sure that any of them are able draw a meaningful conclusion without an actual comparison test. For example, they would have to have two lenses that were manufactured at the same time, then use one and store the other. This would be the only way that they could “know” that the difference in stickiness was due to handling. However, they still could come the wrong conclusion. Maybe the any difference in stickiness is due to incrementally removing the sticky substance on the handled lens versus it building up on the stored lens. Both items could have actually degraded the same amount, but “stickiness” is simply a poor metric to use for judging degradation.
Similarly, to make any real judgements about belt life, one would need to have two decks with the same parts installed. One deck would need to be used and the other not. A meaningful conclusion could be made if the belts on one of the decks held up better than the ones on the other. Otherwise, I don’t see how any definitive statements can be made.
The closest I can come to a meaningful experiment is with the rubberized coating on a computer mouse. Many years ago, I set up two office workstations, one for me and another for my wife. They both had the same keyboard, mouse and monitor (all purchased at the same time). I use my workstation daily, my wife uses hers very occasionally. The rubber coating on my mouse turned into a sticky mess, the coating on my wife’s mouse turned into a hard substance. Neither is anything like when they were new. I think I can make a meaningful statement about the coating on this particular model of mouse. I don’t think extrapolating this experience to anything else is necessarily meaningful.
One commonly believed theory (backed by science) is that every rubber part degrades somehow. The degradation clock starts when it is removed from the mold. The nominal speed of degradation is based on the composition and process used to manufacture the part. One can speed up or slow down the clock based on the environment or usage, but the degradation is inevitable.
The more I think about this the more questions that I have… For example, how important is it to buy a freshly manufactured belt? Buying from old stock would be problematic if the belts degrade mostly from not being used. Conversely, it wouldn’t be so much of a problem if the belts degrade mostly from usage.
Glitch
I did a bit more research and found that the correct term for the phenomena is “rubber reversion”. However, I think the term “goo-ification” is more fun ;-).
There seem to be many theories about what is going on. Many of them are based on the supposition that the polymer stabilizing additives migrate out of the rubber. I’m having a hard time conceptualizing how a working the rubber would slow the process versus keeping it in a state of stasis. There is a group of vintage camera enthusiasts that swear by the “use it or lose it” theory when it comes to rubber parts on cameras. Perhaps the rubber should be considered a viscous mixture and working it keeps everything mixed-up appropriately?
Regardless, I’ve already gone deeper down this rabbit hole than I intended. I’m still hoping that someone can shed some light on the original question.
Glitch
Poor storage conditions might accelerate the degradation of the belts, but I don’t think that it is the primary cause of goo-ification. I bought my Beocord new and it has spent its entire life in a climate controlled, living space. Regardless, I ended up with goo.
This issue is not unique to B&O. I’ve read similar stories for many other brands of equipment. The common cause has to be in the base material or in the manufacturing process of the belt. Who knows, maybe all of the goo-belts come from the same plant.
I’m hoping that they have made progress on the longevity of the belts. I believe that they have improved the formulation for other issue prone parts like foam speaker surrounds. It would be a real shame if the plastics industry reserved the “forever materials” strictly for disposable items like water bottles and soda straws. (yes, I know that they are different classes of plastics)
Glitch
I do know that as long as the machine is used every now and then, causing the belts to expand and contract, that reduces the chances of “gooing”.
I’d like to learn more about this. Can you provide any further details about why this would be the case?
I’ve always assumed that the belts degrade due to a process (like oxidation) that either causes the polymers to break down (into “goo”) or form a “new/modified” substance that is less elastic (cracked, brittle belts).
My motivation for having a better understanding of this is to help with a long term preservation plan for the equipment. For example, if it is inevitable that belts will turn to “goo”, it might be prudent to remove them before placing the equipment into storage. An analogy would be removing the alkaline batteries from a vintage toy, draining the gas from an antique car that won’t be driven for a while (or the swapping out the foam inside certain active speakers). I recently pulled apart a vintage computer where the CMOS battery leaked and corroded the nearby circuit board traces into green dust. Had I foreseen this, I’d still have a working computer.
Glitch
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