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missing BG4000

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Craig
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Craig posted on Tue, Oct 19 2021 11:56 AM

Following a protracted delivery process Justin was finally able to get this to me, seems it had fallen off DHL's radar a week or so ago and we feared the worst........


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manfy
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manfy replied on Thu, Nov 25 2021 9:41 AM

Now I read the whole thread from the beginning and things are starting to get clearer. Sorry, I should have done that before posting my first answer!

There's one post that doesn't add up, though:

Craig:

Craig:

I've just had an epiphany, or as some may say "a lightbulb moment" regarding the strobe lamp.....can anyone come up with a reason why I cannot connect across the 240vac incoming supply fuses through the reed relay and then into the strobe lamp? I seem to recall something about back EMF generated when disconnecting a coil i.e. an ignition coil on a car engine....could this be an issue bearing in mind the reed relay would open when the set is switched off?.........any thoughts would be welcome Wink 

well....so much for that bright idea, tried it out and it didn't work......when I connected the neon lamp to the incoming fuses I found that the voltage dropped to around 106vac when the transformer was powered up, well below what's required for the neon lamp. I tried measuring the voltage at the fuses on a working unit and found the same reading........could a faulty bridge rectifier be responsible for this loss of 240vac on the secondary side (Martin.....I'm looking at you) Wink

Something must have gone very wrong in that measurement - it's physically not possible to see what you're describing here!
The circuit diagram shows that 0R2 isa 27k 1/2Watt resistor, so the max current it can bear without burning up is 4.3mA and a 4.3mA load cannot pull down your mains voltage from 235VAC to 106VAC.
Even if the neon lamp were shorted, you'd get a maximum of 235VAC/27k ohm = 8.2mA. If that happened the resistor would go up in smoke because it would have to dissipate good 2 Watts, but again you surely wouldn't see any dip in the mains voltage!

You can easily test the function of your neon lamps by connecting a 27k resitor in series with the lamp and then hook it up to your variac and turn it up to the nominal 220VAC.
I didn't find any datasheet for the GL90 neon lamp on the net, but on digikey I found some modern miniature neon lamps and most 220V versions show an ignition voltage of around 180V +/-20% and a running voltage of around 80-100V. The lifetime of such modern lamps is usually spec'ed with 12000 to 25000  hours.

 

manfy
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manfy replied on Thu, Nov 25 2021 9:55 AM

Craig:

manfy

Another thought just struck me.....when I made the above measurement I was under the impression that the service manual circuit diagram was correct....

Yes - thumbs upYes - thumbs up Good thinking!

If the first image on page 6 is the actual correct diagram, then you connected the lamp only across the first primary winding, and that is less than half the mains voltage!
Test the lamps you have with your variac as described above. Then you know for certain which one is working and which one is not.
Also take note of the actual ignition voltage. That way you can assess the expected lifetime of each lamp. Ignition voltage keeps rising over the course of its life and when it's getting close to the line voltage (235VAC in your case), end of life is near.

Craig
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Will confirm this evening as per the drawing below Surprise


Craig
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Little later than I anticipated the transformer is back in the deck, all wired up with the 230vac incoming supply, correctly wired up this time, fed through the reed switch and 27k resistor


Craig
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how about that..........


Søren Mexico
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Craig:

how about that..........

Applause

 

Collecting Vintage B&O is not a hobby, its a lifestyle.

chartz
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chartz replied on Sat, Nov 27 2021 3:48 PM

Well done Craig! ☺️ 👍🏻

Jacques

Craig
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Craig replied on Sat, Nov 27 2021 5:18 PM

These turntables can be so entertaining......seems like I learn something new every time a look inside one, and the input form the site is priceless Smile

Craig
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That saga complete its time to start replacing the trimmers and capacitors...........


manfy
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manfy replied on Sun, Nov 28 2021 5:43 PM

Congrats on your successful fix! Beer

If it were my machine, I would have checked the lamp current...just for peace of mind, you know! After all you have a damaged trafo coil and the only component that could have caused this is the lamp...
That's just food for thought and I don't want to alarm you, but if the lamp for some reason draws more current than it should, it will shorten its lifetime and it may overheat the 27k resistor. If I understand this correctly, the light is permanently on whenever the platter is turning and you connected it now to mains voltage instead of the 220V that the circuit was designed for. Neon lamps don't necessarily have a linear V/I characteristic!

Craig
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Craig replied on Mon, Nov 29 2021 8:15 AM

Thanks for that Manfy....however bear in mind the neon lamp currently installed is not the one that the machine arrived with...it's one of mine from a working machine, A new one will be fitted when it is delivered, actually I will test the new one out with my variac just to find the ignition voltage and compare it with one of mine. The 230vac that is now wired to the lamp is wired downstream of the fuse so we have that protection....I dont think there is any cause for concern.....unless anyone can advise differently of course

Regards Craig

manfy
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manfy replied on Mon, Nov 29 2021 2:35 PM

Craig:

....I dont think there is any cause for concern.....unless anyone can advise differently of course

Hi, Craig! Yes, that's alright. There's no immediate danger in running the lamp temporarily at higher than normal currents - in worst case it would just burn out faster than normal.
Unfortunately we don't even know what the specified max current of this GL90 neon lamp is because no datasheet can be found on the net. We can only assume, based on the B&O original design, that the current should be well below 4.3mA. If the actual current does get dangerously close to that value, you can lower it by connecting this circuit to the yellow and white wire on the trafo primary, which will give you <=220V instead of the 235V mains voltage between yellow and green.

 

Craig
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Ok....this arrived in the post today, time to get a bit of a shake on......


Craig
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As my working deck still had the wiring from the transformer disconnected I thought I would tie in my variac and (with the deck switched on to close the reed switch) I tried each neon lamp in turn to determine the ignition voltage, my original neon first, coming in at 163vac


Craig
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And now the neon lamp I received today........which is igniting at a somewhat  higher voltage, I think what Manfy was saying was that the ignition voltage got higher with age.....doesnt look that way? however.....now we have two working lamps ;¬)


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