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Beocenter 5000 Blow-up

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TonyN
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TonyN posted on Fri, Jan 24 2014 4:29 PM

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I’m new to this forum, have no knowledge of electronics but am used to dealing with delicate and intricate equipment. As an ex-jeweller I am used to soldering. Last year I bought a Beocentre 3500 with P50 speakers. After digging down to the switches and treating them with cleaner/ lubricant it works well and is giving me a great deal of pleasure, but there were no purely electronic problems, so I got off lightly.

Now I’ve acquired a Beocenter 5000 with Beovox S45 speakers from a neighbour who has had it all stored under a bed in the spare room of a stone built Welsh cottage for 14 years. When I plugged it in there was a loud crack followed by a smell of burning. Now, when I switch it on, there is some initial activity on the VDU followed by a click, then nothing. The phono goes through the motions of playing a record but there is no music, just a hum. I can find no signs of life in the radio or tape player.

Careful inspection revealed that R226, R227, R219, R212 on the Power Supply and Output Ampl. circuit board are  blackened or worse, and TR210 has exploded. I’ve downloaded a service manual to get the specs for these parts and they seem to be acaillable.

I would be very grateful if anyone can spare the time to help with the following questions:

Is the damage likely to be far more extensive than what I have identified? I think I can manage to replace these parts OK, and I’ve ordered a digital multimeter, but expert diagnostics would obviously be beyond me.

Should I be looking for the cause of the blow-up? And if so where?  Or might it just be old age and, perhaps, damp (hopefully!)? A carful inspection of all the CBs I can see with both chassis open hasn’t revealed any nasty surprises.

The clicking I hear sounds like a switch. Is this some kind of cut-off device designed to prevent further damage after a fault has occurred?

Although I’m a complete newbie so far as electronics are concerned, I’m keen to learn and this could be an opportunity, or am I likely to be biting off more than a beginner can chew?

It's a lovely machine, the speakers  are great, and it has a styus that probably hasn'e seen a lot of use. It would be great to get it working again.

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TonyN
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TonyN replied on Fri, Jan 24 2014 4:45 PM

Serious typo in the second paragraph; the one I've just got is a Beocenter 5000  as the headline says, not another 3500. Very sorry about that! I've corrected it now.

marco
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marco replied on Fri, Jan 24 2014 4:54 PM

Hi

First of all in vintage gear is much better change all electrolytic cond in particular in power supply, after verify and change if rusted or corroded all fuses and after you can replace transistor.

Hope this help.

MArco

 

TonyN
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TonyN replied on Fri, Jan 24 2014 6:56 PM

Thanks Marco, and I get the idea I think. The problem may have been caused by too much power from the power supply and if I don't fix that it will just blow up again. Does 'electrolytic cond' mean the same as capacitor? Sorry if that's a really stupid question.

The only  sign of rust I have found so far is at the earth connections on the underside of the steel plate that covers the bottom of the Beocenter. All the fuses I have found so far have been OK.

Peter
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Peter replied on Fri, Jan 24 2014 7:12 PM

The red capacitors in this age of kit go a bit crusty - you can sometimes see a bit of goo on the top of them. A reasonable plan is to just replace them all. The other simple bits that go wrong are the variable resistors (the components with the adjusting slot) Marti (Dillen on site) often can do a kit of capacitors for many B&O devices.

Peter

TonyN
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TonyN replied on Fri, Jan 24 2014 7:52 PM

Thanks Peter. Do you mean all the capacitors on that board or throughout the whole Beocenter? It's certainly an option I'll consider, but if I replace adjustable resistors I'm wondering how I'd learn how to adjust the new ones. Sounds like a bit demanding.

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Peter replied on Fri, Jan 24 2014 8:31 PM

I agree - quite a job.

Peter

Dillen
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Dillen replied on Fri, Jan 24 2014 10:03 PM

The burned output stage emitter resistors points towards at least one shorted output stage transistor.
Most likely the pair blew together and in most cases they will also take the driver transistors, the
constant current generator and idle current circuits down with it.

The initial cause for the damage could be a shorted speaker or speaker cable so check this BEFORE connecting
to anything else.

The amplifier can be repaired but for even hardened tech guys it could take a couple of evenings or even
a couple of takes to make sure every damaged component has been identified.
The damage-avalanche problem is common for DC coupled amplifiers like this. Believe me, it's not a task
for beginners (with all due respect).

I would grab a known good replacement module and with that the easiest and potentially cheapest repair.
We are not allowed to promote our own Ebay and Bonanza listings but I can tell you that the same module is
found in Beocenter 7000.

Martin

TonyN
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TonyN replied on Sat, Jan 25 2014 9:18 AM

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Martin

 Very many thanks for that response, and sadly you’ve confirmed what I suspected. I was looking forward to wielding a soldering iron and looking wise.

At least if I swap the module as you suggest then I can mess about trying to identify the blown parts on the broken one and perhaps learn something.

I will surely follow up on your advice in the last paragraph.

 Tony

marco
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marco replied on Mon, Jan 27 2014 9:38 AM

Hi TonyN

Yes you are right electrolytic cond' mean  capacitor Big Smile

Power supply after many years needs fresh capacitor because them can be shorted.

Bye bye

MArco

TonyN
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TonyN replied on Tue, Jan 28 2014 6:00 PM

Thanks Marco. I've ordered a replacement module now.

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TonyN replied on Mon, Feb 3 2014 9:58 AM

 I’ve now got a replacement Power Supply and Output Ampl. module on it’s way from Martin, and I’m hoping that will get the Beocenter 5000 up and running again, although there is at least one mechanical problem with the cassette player that I know about which needs sorting.

In the meantime I’ve been working on the S45 speakers that came with the 5000 and have refoamed them.

Looking at the crossovers, replacing the capacitors seems fairly straightforward to this newbie, providing I can work out what are the most suitable components to use.  (See photo below)

Parts available here in the UK seem very limited compared with the US, so I’m trying to decide between the following:

1)  6.8 µfd Electrolytic Capacitor
Bi-polar reversible smooth-foil electrolytic capacitor with 100v DC/35v AC working. (Wilmslow)

2)  Axial Polypropylene Film & Foil capacitor ±1% Tolerance. Very High Stability. Epoxy Coated. Body length 15mm. 6.8nF (6800pF) 160V (Cricklewood)

3)  6.8 µfd MKP Capacitor
MKP Polypropylene Capacitor
250v capacitors use metallised polypropylene films (Wilmslow)

Am I right in thinking that I can us a cap with up to three times the voltage of the original without problems and that polypropylene is better than electrolytic? In which case I’d go for option 3.

Dillen
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Dillen replied on Mon, Feb 3 2014 12:13 PM

Yes, this is definitely one of the easier speakers to recap.
One single capacitor in each cabinet.

Option 1 or 3 as you please.
No need to go for audiophile class in these speakers.

Option 2 is useless unless you parallel 1000 of them....

Martin

marco
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marco replied on Mon, Feb 3 2014 1:08 PM

Hi

I would choose cheapear option1 (Bi-polar ) and I would do some listening test after if you aren't satisfied choose option 3 (MKP Polypropylene ) and don't worry about voltage someone says that higher voltage give better sound Big Smile

Have a nce day..

MArco 

TonyN
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TonyN replied on Mon, Feb 3 2014 4:56 PM

Thanks again Marco. I always like to hear that the cheap option is the best option!

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