[Glitch]

… and here(?)

 

[Glitch]

klesolin wrote:

How can they get damaged from within the speaker?

I’ve always assumed that the damaged coils were a result of people pulling their crossovers to check if their coils were damaged. 😉

An alternative (more serious) explanation is that the coils break loose from a hard impact during shipping.

Glitch

 

[Glitch]

1. Remove amp
2. Remove bottom plate
3. Remove wires from crossover(s). Wires are color coded and labeled on the crossover.
4. Pull hard.  The boards are held in place by friction (even though it might feel like they are welded in place). The boards slide into slots in the housing. There might be tape on the board to keep it from rattling. Try not to pull on any of the crossover components.

Hopefully this makes sense. Good luck!

Glitch

[Glitch]

Tuning issues are usually caused by a bad U264B chip (IC8-Board2) or a broken wire on the coax cable between the FM board and the CPU board.

Replacement U264B chips can be hit or miss for working with a BM6000 (depending on the manufacturer). The ones sold at beoparts-shop.com work well. The cheap ones from China sometimes work, but you might need to try a handful of them before you find one that is compatible.

The broken coax wire is usually at the CPU end of the cable where the center wire connects to the connector pin. This can be repaired by soldering in a short piece of wire to reestablish the connection.

Glitch

 

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Evan wrote:

In my experience it seems our human listening isn’t mega sensitive to height, more so side to side. I’m thinking it shouldn’t be a huge issue to place the center relatively low. Not to invalidate your experience at all, Glitch. I have a colleague who has a pair of center channels, one above and one below and he has tuned the [acoustic] image between them.

Different people are sensitive to different things. What one person doesn’t notice, really bothers another. Room acoustics can be quite different also. I could see where the bottom-center location would work someone with a dead floor and a live ceiling. The stronger reflection from ceiling would move the apparent image upward.

I’ve considered the over-under setup. I just can’t figure out a way to make it look “right enough” to be a permanent solution.

Glitch

 

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Evan wrote:

I think for home theater it is best that there is a dedicated center channel physically centered below the screen. This will anchor the sound stage and center channel content to the image really well.

Yep, I understand why most people like the “centered below the screen” location. I like the center of my TV screen to be below eye level, which keeps the TV low. The center speaker is almost on the floor. Now the sound is coming from well below the actors faces (which are usually higher on the screen) and things like coffee tables are in the direct path to the speaker. The bigger the TV’s get, the worse this problem becomes. The “dual mono on the side” addresses some of these issues, but admittedly causes other issues.

The good thing about the lack of a perfect solution is that it opens the door for experimentation.

Have you thought about mocking up your original concept? Possibly something like pulling the tweeter/mid module and pairing it with a couple of conventional speakers as surrogates for the woofers?

Glitch

 

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Evan wrote:

The problem with putting the same signal through both speakers is that you only serve a listener exactly in-between them.

This is only true if the radial dispersion is constant, which is not the case for the higher frequencies.  This explanation isn’t quite correct. What I meant to say is something more along the lines of… This does not have to be the case if the system was designed to not have a small, narrow sweet spot.

Consider the sound system in your car. The driver is much closer to the driver’s side speaker than the passenger side speaker, and vice versa for the passenger. Despite this, both driver and passenger can simultaneously experience a (mostly) centered image. The car audio engineers achieve this by carefully manipulating the dispersion fields of the higher frequency drivers. An overly simplified explanation is that a speaker is louder on-axis than off-axis. The speakers are arranged such the the far speaker is on-axis and the closer on is off-axis by the exact amount that it needs to be to get an equal sound level at the listener’s position. Look up some of the papers from Dr. Bose for a detailed explanation of the theory behind this.

It would be awesome of Geoff Martin could chime in and provide the dispersion data for the Pentas. I have to believe that B&O has this, but is likely wishful thinking on my part that they would make it available.

Glitch

 

[Glitch]

Evan wrote:

et me know what you guys think.

From a general sound perspective, I don’t see any reason why it wouldn’t work. As long as you keep the cabinet volume and reflex tube length/diameter the same you will, in general, have a similar frequency response. Even if you mess up the very low frequency response, this won’t be critical for voices since they don’t contain those frequencies.

Would you actually be happy with the form factor? I’m having a hard time imagining what this would look like under a TV.

Did you plan to have the bottom-most mid-range close to the floor? Not having the tweeter and mids at ear level may mess with the vertical dispersion. I don’t know if this is a better or worse trade-off versus having the mid-range array horizontal.

Have you considered using a (well matched) pair of Pentas (in the original configuration) placed tight to sides of the TV? The mono pair would provide the a solid centered image and the apparent sweet spot could be roughly controlled by the angle. The other “surround” speakers could also be Pentas for the full overkill effect. 😉

Glitch

 

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It has been long enough where I’ve forgotten most of the finer details of recovering these grills. Hopefully you can see what you need from the picture.

Taking the grill off reminded me of how precarious the procedure feels. No matter how many times I’ve done it, it always seems like something is about to break. 😉

Glitch

 

[Glitch]

I used iCraft Supertape based on several recommendations in another forum. It has been seven years since I changed the grill cloth and it is still holding strong.

Glitch

 

[Glitch]

It is good to hear that you have made progress.

One tip related to changing the grill cloth is to use two sided tape to attach the new material. This makes it much easier to get the tension just right versus using glue.

There are a many other tips that people can provide for Pentas. You probably have already figured out that B&O enthusiasts are pretty passionate when it comes to Pentas ;-).

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thomash wrote:

Meantime I sliced the front off this cartridge where the cantilever had broken off.

Pictures please.  I, and likely many others, find it very interesting to see the inner workings of things that people normally don’t cut apart.

It also might be worth a search of the forum history to see if anyone has tried this before. One of the more recent similar threads is here.

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Audionerd87 wrote:

These decks can be made functional (although longevity is not a guarantee) with a parts machine as a donor

This has never worked for me… Eventually, I always fix the donor machine and it becomes a spare/backup machine ;-).

thomash wrote:

It would be nice to find a OK lower end cartridge with intact needle for this at a cheap price out of dead broken machine.

This is possible, but you will need to be very, very patient while waiting for something to become available. I think that many of the old B&O turntables get taken out of service when something happens to the cartridge. For the rest of the broken machines, people seem to remove the cartridge before they sell the rest for parts.

Additionally, when something does come up for sale, you have to quickly buy it before one of the other zealots from this forum snaps it up from under you.  😉

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NickNS wrote:

First I checked polarisation with voltmeter, it was correct on two drivers and incorrect on other two.

I’m not quite sure what you mean by this… Did you mean to say continuity or resistance?

It looks like your the polarity of the drivers are marked with the “+” stickers. Check the wiring against the schematic.

NickNS wrote:

If you have any further suggestions on my next step, I’m here to listen

There are spade connectors at the crossover boards. One (or more) of these may be either disconnected or misconnected. Also, many possible crossover issues could be identified if you provide pictures.

All of the drivers look to be original, which is a good start.

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Die_Bogener wrote:

… and we did not talk about the amp. Just the box.

Nice list.  An addition is …

• other intentional modifications intended to make the speaker “sound better” (i.e. substituted drivers, changed cap values, etc.)

The positive news is most everything can be resolved. Pentas are worth the effort to make right.

Glitch

[Glitch]

I assume that you didn’t have the opportunity to audition them before purchase. On the off chance that you did, do they sound different now?

NickNS wrote:

Seller told me that amplifiers were re-capped, and mids have new (rubber) surroundings.

Did you get the feeling that the seller knew what they were doing for these repairs?

The first thing that I would test is the speakers and amps separately. Remove the Penta amps and use them to drive the S45.2’s. It sounds like you have already done the “BeoVOX Penta” test by driving them directly from the BM5000 amp?

Check that the all of the internal speaker driver connections are correct. You might have one or more drivers that are hooked-up with reverse polarity. You should check ALL of the connections.

I would also try to test each of the mid-ranges individually. All eight midranges should sound exactly the same. If they don’t, the refoam job was likely botched. Hook up two drivers directly to a amp (at low volume), run a frequency sweep, and listen/compare to the results of left versus right. Of course, if you have better test equipment like microphones, etc. you can run better tests.

filip_kbh wrote:

Perhaps the crossover filter needs a recap

Maybe, but not as likely as something else being messed up.

As a note, the more detailed the explanation/description of your test/results that you provide will make it easier for people to provide meaningful advice.

…and, of course, pictures are always appreciated…

Glitch

 

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thomash wrote:

I would have a better chance fixing it if the service information included scope traces and circuit description theory.

Unfortunately, the service manuals often exclude any “theory of operation” information. However, the datasheets for the individual chips frequently provide the needed information (or at least strong clues). It is common to see B&O using a reference design shown in the datasheet. This may or may not help, but is something else to try if you get desperate.

Glitch

 

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quattttro wrote:

Looking at the schematic, it seems sometimes the positive and sometimes the negative end of the cap goes to signal ground

I assume that you are referring to things like C2 & C204? The schematic makes sense since the “negative” lead of the capacitor is connected to a voltage lower than ground. This would keep the electrolytic capacitor (properly) forward biased.

I can’t tell if how the capacitors are installed on the circuit board are correct. Markings on the board are usually accurate, but not always. Mistakes happen in printing the boards and I don’t blame any manufacture for not scrapping boards for this kind of mistake. Same goes for printed schematics. Many times they “fix” the issue by an obscure errata statement.

The best practice is to install new capacitors with the same orientation as the original ones. Taking “before” pictures of the circuit board can be really helpful as part of the quality control or debugging process.

If all else fails, verify that the electrolytic capacitors are forward biased with a voltage measurement.

Glitch

 

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PM sent…

[Glitch]

I recently repaired a BC8000 with similar symptoms, a loud hum on one channel that pegged the VU meter. After debugging efforts, that were shamefully intensive in hindsight, I found a fairly simple cause for the problem…

Broken pins on the tape head connector to the tape transport. Pins 1 & 4 had broken off in the connector.

Here is a picture of a broken pin next to its replacement.

Based on my experience, I would check the connections to the tape heads for cold/cracked solder joints, broken connector pins, and broken wires before moving on to other things.

Hopefully your issue is as simple to fix.

Glitch

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