Beolab Penta, 13cm speaker distortion.

14 replies
J(
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Jamian (archived2)
Bronze
Posted January 7, 2013 at 05:59 GMT
#1986651

Hi,

Sorry for the long winded description skip down to the CAPATILZED headings if you want to get to the point.

I have just picked up a pair of Beolab Pentas locally for a GREAT price.  Of course there are caveats to that price.  They were shipped to the previous owner from their parents (they had fond memories of the Pentas from their younger years).  They were crated, but one of them arrived slightly beaten up (small dents on some panels).  The owner said that when they connected them to their BC9000 the 'batterd' one, '...smoked a little and stopped working.'  After that they were stored for years and only brought back out recently.

They had one running its amp and the other was rigged with a lead running through the amp housing to the direct speaker connections (Beovox style); strange set up in my opinion.  Leaning in to them at low volume I could hear the 13cm speaker just below the mid ranges sounded distorted.  Still for the price I was game for a gamble and they wanted them to go to some one that would use them.

My plan - Get the speakers up to scratch, then consider new caps, then maybe tackle the amp.

SPEAKERS

13cm - I took off the front panels (those release details don't work very well!) and removed the broken amp from the slightly battered Penta.  I then hooked it up, without the amp, to my BM2000 (only 25watts, but  it was on my work bench already) and played various genres at low volume (it was late at night).  The speaker just below the mid ranges was distorting frequently and the one just above them was slightly distorted as well. I tried switching the units around to see if the problem was electrical, but the distortion followed the speakers.  Gently/slightly pushing on their cones resulted in grinding; more immediate than pushing on the 'good' speakers.  Certain points around the cone seamed to be more probe to grinding than others.  There was no visible damage to the cones, surrounds and spiders.  Could the bad shipping have misaligned them?  Are they blown?

Today I decided to hook them up to my BC9000 to see how bad they sounded at higher volume.  Turning the volume up seamed to eliminate (or maybe mask) the problem.  Obviously I can not get in close to really listen to the individual speakers at high volume ;) Lowering the volume I felt that each 13cm speaker sounded different, or at least different levels of vibration (am I just paranoid?) How can I conclude if I need to replace the 13cm speakers, or not?

8cm - Will re-foam these, once the 13cm speakers are sorted out.  I have done my CX100s and am planning on following Martin's advice to shim on this project as well (to avoid any disasters where possible).

Would the general advice to replace the caps solve any of these symptoms?  I am game to replace eventually, but would ideally like to sort out the speakers out first.

AMP

I do not have a clue where to start.  I checked out the fuses and they do not look burnt out.  I do not imagine they would create too much smoke anyway. Due to the mention of smoke I have not plugged the amp in my self, yet (of course I do not know if it actually smoked, or not).  Advice welcomed.

If I want to use them as Beovox, until I sort out the amp, I am guessing I need some shorter thread pieces (Beolab has long ones that run from the bottom of the speaker, through the amp, to the base)?  If so any idea where I can find some?

I did run the good amp on the other speaker.  Only used regular speaker cable (which cable do I need to get the best out of these and the display working, Beolink?), but it sounded pretty dam good.  I can not wait to get these in good working order and try them out for real.

Thanks for reading; sorry there are so many questions in one post.  Happy listening (and tinkering),

Jamian

 

J(
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Jamian (archived2)
Bronze
Posted January 21, 2013 at 22:47 GMT
#1986652

Hmm no replies...

I decided 3 of the 4 woofers were blown - No sound from the first; second was very distorted, third was slightly distorted.  I now have 3 functioning used woofers on their way from eBay.

I plugged the amp that was said to have been broken in to the mains.  The light comes on green and stays on.  I am afraid to hook it up to any of my speakers to test it for fear of repeating the damage inflicted on the 3 woofers.  Is there a way to test the amp output without using speakers?

If I need to connect a speaker could I simply connect one of the already faulty woofers directly, or?

Thanks for the advice

Jamian

V(
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Vienna (archived2)
Bronze
Posted January 21, 2013 at 23:24 GMT
#1986653

If you want to use the amp section of your Pentas and want to be on the safe side not to
ruin more speakers, I'd recommend to bring the amps to a qualified service tech, beeing able
to check if there is no DC offset, no hum caused by damaged caps, ...

Afterwards it is up to you to try "eBay used", a professional re-coning  or buy new woofers from
B&O. These woofers are still available from B&O, order/ item#  is 8480403 and they retail here
in Austria for approx 87 EUR (each) including 20% VAT (sales tax)

A friend of mine blew 1 of the 8 woofers of his pair a few weeks ago - he ordered an original one ...

JÖ
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Johan Östlund
Bronze
Posted January 21, 2013 at 23:29 GMT
#1986654

My guess is you have a blown output stage in the amp. If this is the case you will have to replace the output transistors (at least). The likely culprit are the no load trimmers, so these must be replaced as well, and adjusted according to the service manual. While you're in there you should replace the electrolytic capacitors. Which Penta version is it?

Now, the speaker elements is the other problem.. Start by checking the connections on the x-over pcb (open the bottom plate of the "beovox" and check that the cables correspond to what's printed on the pcb. If those are wrongly connected the elements could easily blow. I suppose it could also be the x-over caps, but that seems less likely.

/  Johan

J(
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Jamian (archived2)
Bronze
Posted January 22, 2013 at 00:13 GMT
#1986655

Thanks for the responses.

They are Mk1.  I have played both without the amps attached, to figure out which speakers needed replacing.  I am guessing that no more issues (when swapping drivers back and forth) would indicate that the cross-overs are not wrongly wired.  I will take them out and visually inspect them for any damage.  I know that many say to replace the caps on the cross-overs.  I may do that if I do not like the sound, once the woofers are in place and the mid ranges are re-foamed.

I really want to save the apparently faulty amp, just trying to figure out how much of a mountain it will be to climb (skill/time/money).  My love of vintage B&O is much greater than my current electronic competence; though I am pretty handy and know a few EEs.

 

Cheers

Jamian

 

 

J(
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Jamian (archived2)
Bronze
Posted January 24, 2013 at 06:51 GMT
#1986656

Received the three woofers and they are working fine.

 

Decided to take a look inside at the x-overs.  Everyone likes pics, so here we go...

Looking at the components of the 'damaged' speaker, that had the blown speakers, I only noticed that R3 on board 14 has a deteriorating exterior.  Is this just old age.?

Then I opened up the 'good' speaker.  L2 was hanging off board 15.  Board 15 was not even correctly located in the position slots (one edge out).  Is it simply fine to glue back in place or does the warped wrapping of the wire effect performance (noticeably)?

Thanks for the advice!

Jamian

TO
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tournedos
Bronze
Posted January 24, 2013 at 11:10 GMT
#1986657

The resistor doesn't look like it has been excessively hot, so it's probably OK. Measure its value, and if it's correct I believe you can leave it alone.

Form the coil back the best you can and glue it back on. It has enough turns that a couple missing or added won't make any real difference. Make sure it doesn't short circuit though, the insulating lacquer may have been damaged.

J(
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Jamian (archived2)
Bronze
Posted January 25, 2013 at 06:00 GMT
#1986658

Thanks - I was very happy to see that the cross-overs were not in a really bad state. Any recommendations for glue?

S(
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Severian (archived2)
Bronze
Posted January 25, 2013 at 06:34 GMT
#1986659

I've generally used standard glue for a hot glue gun for sticking xover components to boards. A good hot glue gun is invaluable around the house anyway, so you can rationalize a purchase pretty easily if you don't already have one. 😁 Not expensive either. Your wife or significant other can also use it for crafts projects, just don't ever cheat on her as it can glue things you'd rather not have glued!

I've wanted a set of Pentas for a while now, just haven't found any locally I can look over, the eBay ones tend to be damaged, bad mids, and just too pricey. So I'm following your progress vicariously.

TO
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tournedos
Bronze
Posted January 25, 2013 at 08:15 GMT
#1986660

Yes, hot glue. But don't use it to attach components that get hot 😁

(I don't think these coils do - that one has probably gotten loose simply because it is heavy)

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