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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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Craig
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John

The toroid transformer looks very nice too.......however i recall you saying it was an expensive option? I have cleaned up the drawing I made last night to reflect the actual wiring....it now looks far more plausible and all the measurements tie in, doesn't help Justin of course but I'm happy to have got to the bottom of it, it more or less resembles the schematic shown in the service manual for a 240vac supply machine 


manfy
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manfy replied on Tue, Nov 23 2021 12:21 PM

What makes you think that the transformer is spoiled?
The trafo in your picture looks good and so do your resistance measurements. You have two 0.25A fuses in the primary and if they don't blow, an overload is rather unlikely.

Now that you have it removed you can make some simple measurements in idle condition. You'd have to wire up the primary so that you can connect it to 240VAC and then you can measure the idle voltages at the output windings.
The 220V secondary winding should show some 220V +/-10%. Then connect your neon lamp to this output and see what happens to the voltage. If it drops to 160V without the lamp lighting up, the neonlamp is spoilt - replace it. Don't worry about back EMF in that circuit because there's no voltage-sensitive component in it. Besides, the series resitsor of 27k limits the max. current to some 8 milliamps, thus also the back EMF will be negligible.

Craig
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Craig replied on Tue, Nov 23 2021 1:01 PM

Manfy

Thanks for your input.....as I said  earlier in the post when I installed a working neon lamp from one of my working decks the neon lamp wouldn't energise, however it works fine in the working deck. When I measured the voltage at the neon lamp terminals of the suspect transformer I was only getting 165vac whereas I get 230vac at the same points on my working machine.....also when I measure the resistance of the secondary windings of the suspect transformer I get 0.9k ohms....working transformer gives 1.3k ohms. 

Those are the only differences I can find.....I will however do as you say and power up the transformer and measure the idle voltages just to be sure, I'm not up on transformer winding calcs or loadings and I did ask the question earlier if anyone thought a fault in one of the bridge rectifiers could pull down the secondary voltages across the transformer......but nobody seems to know (do you?) Wink

Regards Craig

manfy
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manfy replied on Tue, Nov 23 2021 1:50 PM

Ah, ok! I skimmed your posts but didn't quite get all of it.

If you used a confirmed working neon lamp on this transformer but got the same symptom, it points in a different direction. My first guess in this case is a corroded contact in the voltage selector switch. Corrosion causes increased contact resistance and can cause a significant voltage drop as soon as current flows.
But since you removed the transformer, you can now exclude this switch as the source of the issue when you carry out the idle voltage tests.

Generally speaking, a faulty rectifier could theoretically cause a voltage drop on the transformer if it overloads the trafo just a bit above its working point. But it is unlikely because the fuses would probably blow before you see a significant voltage drop.

There exist only 2 primary failure modes in such low-volt transformers: an open, ie. a broken or burnt wire in a winding, or overload, which practically always leaves some visual marks on the unit. Another effect of overload could be degraded wire insulation, which would cause a short circuit between multiple windings and thus a reduced voltage, but you would recognize this by measuring distinctly different or odd resistance values.
(Of course, additional fault scenarios do exist, but they only happen in high-power grid transformers and for all practical purposes, this can be excluded in our case.)

Craig
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Craig replied on Tue, Nov 23 2021 2:06 PM

Great stuff.......I will tie the primary windings together this evening and do the voltage checks with the selector switch out of the equation, It would be nice to find that this was indeed responsible.

Craig 

manfy
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manfy replied on Tue, Nov 23 2021 3:59 PM

Yes - thumbs up

Just make sure, you wire it up correctly (as per your diagram above). The current must flow in the same direction in both primary windings or else you're creating a magnetic short circuit. It's probably best to keep at least one 0.25A fuse in the supply line...just in case...

Craig
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Ok.....wired up as per the diagram above, connected RD & BK and fed 240vac into GN and YW....WHT was left floating....and using the variac injected 240vac whilst carefully monitoring the current......the transformer pulled very little current as one would expect at idle


Craig
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Happy with this I next confirmed I was delivering 240vac into the primary windings....


Craig
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Moving onto the secondary windings and unfortunately I'm still low on the neon lamp supply...


Craig
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The 24v feed looks good........


Craig
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As does the 6v feed........


Craig
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As a final word I checked the secondary winding output to the neon lamp on my working deck and as suspected im getting 230vac........


manfy
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manfy replied on Wed, Nov 24 2021 1:25 AM

Thanks! That leaves no doubt: at some point in time the 220V coil was overloaded and that caused a meltdown of the transformer wire insulation. That also explains the difference in resistance compared to your working unit.

So the 8V and 24V secondary windings are fine but the 220V winding is spoilt. A repair is probably not possible, at least not in a cost-effective way.
I'm not really sure what this neon lamp 0IL3 does in this machine, but sonavor suggested that it's the strobe lamp for speed control.

You have two options: either you replace the neon strobe with a LED version or you get a low-cost, low-power transformer to replace that 220V winding.
240V/240V insulation transformers do  exist but they are usually costly and bulky and hard to come by, thus don't make much sense. I'd probably go for a cheap low-power PCB transformer.
Reichelt is one of those low-cost electronics suppliers in Europe and <here> you can find a 5VA transformer for some 3-4 Euros that'll do the job just fine.
The 27k resistor in the neon lamp circuit shows that you can expect a load of less than 10mA on the 220V side, which gives a maximum of 100mA on the 24V side. I don't know the wire gauge or load on the 24V side of your current transformer, but I think it should be able to take some extra milliamps - the circuit shows a 400mA slow blow fuse on the 24V output winding and you can assume that the normal nominal current on that winding is well below 400mA.

-----------------------------------------------------------------

postscript:

I did some more thinking this morning and I asked myself: What's actually the point of isolating the neon lamp supply from mains??

Answer to self: No point whatsoever!
If you're not planning to move across the pond and take the BG4000 with you, there is no real technical reason why you can't connect this lamp to the primary! I guess the main reason for adding this 220V winding was to make it compatible with the US market. If the grid supplies only 120V then, of course, you need to step up the grid voltage to drive a 220V neon lamp.

Bottom line: the best and most cost effective solution for you is to simply wire this strobe lamp to the primary of the transformer and all is good!
You won't have to worry about getting a replacement transformer or any add-on components. Just make sure that the wiring of the strobe lamp is in good condition and that there's no danger of any part of that circuit shorting out to the chassis.

Craig
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Craig replied on Thu, Nov 25 2021 9:20 AM

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

Manly

I tried that, when i measured across the fuses i got the volts drop described above.....

Craig
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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....


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