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Old 12th Mar 2023, 2:04 am   #1
Uncle Bulgaria
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Default B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

I've been dismantling this drill of my grandfather's with the intent of making it useful again. The mains flex was crumbly, but the business end seems OK. The live and neutral leads to the stator both have a capacitor in, marked as per the title. They start passing 15mA at 2V so they're toast.

They have little brackets on that keep them out of the way in the drill's handle, which is going to be difficult to replicate with a modern component, so I thought for the first time it might be beneficial to restuff the metal case.

They're marked 0.005mfd at 250V working, so I thought I'd replace them with a couple of 4.7nF 630V Vishay film types, which should fit in the can.

Has anyone taken one of this type apart before and have top tips beyond accurate drilling or sawing off one end?
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Old 12th Mar 2023, 8:49 am   #2
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

You could try a small pipe cutter if it's 3mm diameter+, they give a good clean edge.

About £7 from the likes of Screwfix etc.
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Old 12th Mar 2023, 9:14 am   #3
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

That looks like a feedthrough suppressor capacitor. One end connects to the other end and the capaciting happens from the through-wire to the earth tag. If it's just re-stuffed with the new capacitor's leads sticking out the ends, the capacitor is now in series with the power feed to the motor, no longer just shunting it.

Feedthrough capacitors minimise lead inductance and give better HF/VHF and higher filtering.

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Old 12th Mar 2023, 1:04 pm   #4
Lucien Nunes
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Don't re-stuff! Save the originals as curios, but fit a modern, purpose-made mains-rated suppressor in the best-engineered way you can. This is a hand-held 230V power tool with a metal case and there is no room for concessions to visual appearance.

As David says, the capacitor is not between one end and the other, it's between the ends and the fixing tab so that the capacitor bypasses interference from the L & N to earth, making all three conductors of the mains lead common at high frequencies. A wire-ended suppressor is not quite as good in theory because of the parasitic inductance of the leads, but in practice probably good enough.
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Old 12th Mar 2023, 1:44 pm   #5
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Thank you - just the useful specialist knowledge I was looking for! I wasn't concerned with visual appearance, just that as they were screwed to the handle it seemed foolish to have any new ones dangling about on live wires. As it seems they require that earth connection, even more reason to do it properly.

I will say I couldn't work out why there'd be a tiny capacitor in each leg in series with the motor windings, so I'm glad that thought wasn't completely stupid. I must remember that the can is often a contact not just a case...

I have seen an 'Instructable' on this very drill, but a US version which has no suppression capacitors at all - the live wires go straight to the motor via the trigger.

Is this design using two what we would call 'Y' capacitors from L and N to earth then? Or is there a separate product available as a suppressor?
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Old 12th Mar 2023, 4:37 pm   #6
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Please be very careful. Early versions of these drills were only single insulated, and that relied on the varnish insulation of the wire on the armature and stator. Over the years contaminant ingress will have occurred. Do do a proper insulation test, ensure good earthing, and always use an RCD protected supply if you intend to use it. Several have passed through my hands, and almost all have had bad earth leakage.

Shame as they were very well made mechanically.

Sorry to be a pessimistic fuss, but I like reading your posts and wish it to continue!

As for the capacitors, they're not a Y* suppression network as there are only two, but if you can find a suitable one that fits then it would be ok, though not strictly needed, they do limit the RF interference from the motor.

*Edit: do use suitably rated safety capacitors that are made to fail open circuit.

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Old 12th Mar 2023, 6:32 pm   #7
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Quote:
Originally Posted by G.Castle View Post

As for the capacitors, they're not a Y* suppression network
Of course, I meant Delta suppression: one cap between L and N, one between L and E, one between N and E.

Sorry.

Dont buy Rifa ones though!
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Old 12th Mar 2023, 7:49 pm   #8
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Agree on the safety front. I can't concentrate properly on drilling unless i've got confidence that the drill isn't going to bite me- so i have an RCD devoted to my old B&D. (Intend to upgrade to a 13A plug c/w RCD.) The contact area between sweaty hand and drill is obviously quite large.

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Old 12th Mar 2023, 10:26 pm   #9
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Thanks for the concern, chums. I'll be sure to hook it up to the 1kV Megger. I intend to use the thing, otherwise what's the point? If the winding insulation is suspect, then I suppose more varnish is an option. The fit between the stator and the casing is also loose enough that I think I could sleeve the whole stator with heatshrink or large diameter silicone tubing if required.

I hadn't considered the RCD aspect, but have checked and discover the house has MCBs instead, although shortly to be rewired.

Anyway, before that Rubicon is reached, I'll put this back together (I've washed all the casing pieces now) with some new capacitors and run some tests. I've got some Y2 types of the same value here as the originals, and am planning crimp terminals and heatshrink, if space permits.
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Old 12th Mar 2023, 10:50 pm   #10
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Absolutely, it should work for a living (or at least be capable of doing so)
I retired my other half's B&D 2 speed because the switch started to jam in the ON position and out of respect for the pretty green flames that were visible from the comm...but i saved the carcase as if the brushes are OK they would fit my own B&D!
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Old 14th Mar 2023, 5:52 pm   #11
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Quote:
Originally Posted by Uncle Bulgaria View Post
Thank you - just the useful specialist knowledge I was looking for! I wasn't concerned with visual appearance, just that as they were screwed to the handle it seemed foolish to have any new ones dangling about on live wires. As it seems they require that earth connection, even more reason to do it properly.

I will say I couldn't work out why there'd be a tiny capacitor in each leg in series with the motor windings, so I'm glad that thought wasn't completely stupid. I must remember that the can is often a contact not just a case...

I have seen an 'Instructable' on this very drill, but a US version which has no suppression capacitors at all - the live wires go straight to the motor via the trigger.

Is this design using two what we would call 'Y' capacitors from L and N to earth then? Or is there a separate product available as a suppressor?
Reminds me of my first B&D drill I bought with my grade school graduation money at the age of 14YO. The drill was a Home Utility model! It was sold at a store that sold close-out items, as the new models had a three conductor mains lead. For $10 USD, it was a real bargain, as I used it many years.
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Old 14th Mar 2023, 7:10 pm   #12
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

That cap looks like a coaxial RF feed thru type to me.
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Old 14th Mar 2023, 8:28 pm   #13
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Yes they are coaxial feed-throughs.

Pragmatically, I would put RF suppression and modern electrical-safety considerations ***way*** ahead of any sort of sentimental aspects in re-working this drill.

Unless you want to get an early re-acquaintance with its original owner.
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Old 31st Mar 2023, 12:06 pm   #14
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

1kV Megger reports infinite or >100M between the stator windings, and between each winding and the armature. I mused on taking the windings off and adding a layer of kapton tape, but they're held on very strongly and I resisted fixing them until they were broken. Perhaps a future means of repair if leakage occurs to the armature but the windings are OK. There does seem to be room between the armature and the case to insulate the whole assembly.

I've added a new mains cable and gland, and after much teeth-sucking have made a start on the new capacitors. As the original neat bracket-and-self-tappers is out with the old capacitors, I've gone with toothed washers and nyloc nuts to obviate loosening with vibration while making a good contact with the case.

The trigger assembly has a small rivet which has lost its staked end, so I will probably drill the rod and tap it for a small screw.
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Old 4th Apr 2023, 11:44 am   #15
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Did you test the old capacitors, once you had established that the end-wires were meant to be continuous?
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Old 18th Apr 2023, 1:33 pm   #16
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

I hadn't until you mentioned it - I was being blinkered! I've now done so and one shows 0µA and the other a couple of microamps. This was with my tester cranked up to 630VDC, which meant ~550V on the terminals. I only wonder whether there were requirements to fail open in those days?
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Old 20th Apr 2023, 1:57 pm   #17
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

The self-healing process of Y-caps that seals off damaged areas depends on the behaviour of metallised film dielectrics. I don't think that was possible with older forms of capacitor on this small scale, although some limited self-protection was incorporated into large capacitor banks for PFC duty etc.

Quote:
There does seem to be room between the armature and the case to insulate the whole assembly.
Presumably you mean between the field assembly and the case? Surely it must be clamped in reasonably rigidly somehow? I can't quite follow your observations about the insulation resistance but I would be cautious about applying 1kV to the armature especially, also the field to a lesser extent. One wants a thorough test but once you start getting up to the kilovolts, you are in 'flash test' territory where exposure time must be limited to prevent insulation damage through partial discharges.
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Old 21st Apr 2023, 2:21 pm   #18
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Thanks Lucien. I'm sorry for my lack of nomenclatorial awareness! The field assembly (if that's the static bit with the coils on - first picture in post #14) sits inside the case with two long screws that tie it back to the rear of the handle. I think it would be possible to put a large cylinder of tubing or heatshrink over the whole thing to insulate it from the case, and similarly sleeve the screws so there was no contact between the assembly and the case.

There are four wires that come off the field assembly - two to the supply and the others to the brushes. My only means of insulation test is a nominally 1kV hand-cranked Bridge Megger (repaired on another thread) so I used that to see if there was any leakage between the coils and the laminations the coils are wrapped around.
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Old 21st Apr 2023, 5:04 pm   #19
Lucien Nunes
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

The terminology is a little confusing. In any machine (motor or generator) one can theoretically refer to the part that rotates as the rotor, and the part that stands still as the stator. However, in normal usage those terms are mainly used for induction machines where either the primary or secondary winding can rotate (typically the secondary) and synchronous ones where either the field or armature can rotate (typically the field.)

But, for conventional commutator machines there is no such ambiguity; the field system invariably stands still while the armature rotates inside it, therefore instead of stator and rotor one normally just speaks of the field and armature. So it is in the case of universal (=AC/DC series-connected brush) motors found in appliances such as this. This drill motor has a 2-pole field with one coil on each pole piece (some 2-pole universals, e.g. Hoover junior 1334 motor, have only one coil between the two.)

The field system is normally rigidly mounted to the body as it has to withstand the reaction torque imposed on it magnetically by the armature. Obviously the motor shaft torque, and hence the reaction on the field, is not as high as the output torque of the drill because of the reduction gearing, but it will still fret and worry at its mountings or any inserted insulation if it is not clamped solid.

It is a bona-fide Class I appliance with an earthed case which should not need additional untested fudges to change the method of protection against shock. Class I is no longer the preferred approach for a handheld tool; Class II offers the advantages of independence of the efficacy of the earth connection (which is vulnerable in a tool due to flex fatigue and damage), avoids the user being firmly in contact with the system earth when possibly working (or drilling into) an area not bonded into the equipotential zone, and insulates the user from direct contact with any live parts that might accidentally be drilled into (e.g. a live cable in the wall.). However, especially as we now have the advantage of residual current protection which is much less dependent on a low fault loop impedance to clear an internal line-earth fault, earthing will serve the purpose in the drill just as it does in your kettle or toaster.
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Old 9th May 2023, 1:03 am   #20
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Default Re: B&D "Utility" 1/4" drill - Restuff Dubilier SBC7 capacitors?

Thanks again, Lucien. More stock for the knowledge library.

Alas, this evening I thought I'd finally be able to get everything together, but there was a deafening silence on switch-on. It turns out one of the field coils is open-circuit, so I think this is now a goner.
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