Series 1 Evershed Testing Set Bridge Megger Repair - UK Vintage Radio Repair and Restoration Discussion Forum

Uncle Bulgaria · 21st Jan 2022, 2:11 am

A beautiful object, reading low. I read how some models of Megger have a capacitor that's had a hard life which can cause low output.

They put the capacitor in a very awkward place, screwed down on the bottom of the wooden box beneath both the generator and meter assemblies. Some unsoldering, unscrewing, gentle persuasion and clamp removal later, I have enough brass to open a Victorian laboratory, and a Dubilier 0.05µF/1000V capacitor that shows zero microamp leakage at my tester's 600V limit.

I have found two apparently open-circuit wire-wound bobbin resistors, but can't see how they relate to the overall circuit. It's complicated by the fact this can be a bridge as well as a megger, and all the wires have black sleeves.

I guess I need to replace the bobbins anyway. Is it terribly hard to re-wind the things? They sit on such neat brass rod fixings it seems a shame to swap them out for some little metal oxide object.

Does anyone have a circuit diagram or top tips? I've searched the forum a few times but not found this exact model.

Ed_Dinning · 21st Jan 2022, 10:02 pm

Hi, I may have some suitable resistance wire you can have if you wish to rewind them; see if you can find the gauge and ohms/ yd figure and I'll have a look

Ed

trobbins · 21st Jan 2022, 11:11 pm

The resistor/bobbin units look like precision resistors made with Manganin wire. The break in the wire may be at the outer lead connection if you are lucky, or on the outer layer - although the wire gauge looks pretty thin - you may want to try a careful inspection on a faulty unit to see if you can resurrect it. I've repaired some fine gauge resistors of that style - its fiddly.

As a work around, you could just initially bypass each faulty one with a fixed resistor (or two to tweak the value) to see how that modifies your measurement.

I'm guessing the voltage generated by your instrument is AC at a low frequency? Have you been able to confirm the frequency (which may be a concern for any DVM you use) and the variation in the applied rms voltage (eg. with crank speed and loading) ?

Uncle Bulgaria · 22nd Jan 2022, 4:35 pm

Thank you both. Are the bobbins sealed with shellac? I can't see much around the top, in case it's broken right at the end, because of the difficulty of seeing through the lacquer/varnish. Should I soak it off in acetone or something?

I guess from Ed's offer that it's not impossible to rewind them - my idea of the procedure being that if the wire's insulated from itself and of a known resistance per length, then it's 'just' a case of winding that length onto the bobbin.

I don't know what the instrument is supposed to put out, apart from the '1000V' on the plate. On cranking the handle the needle read 10M with the DMM attached, but the DMM only read a very fluctuating signal of about 100V maximum and mostly lower. The 250V hand-cranked Bridge Megger I did the same thing with showed 200+V on the meter and also 10M on the Megger.

If I were to bypass the bobbin resistors with placeholder modern ones, could I meter the result of turning the handle without the assembly in the case? Getting it back in, screwing up and soldering all the connections, and the meter assembly, would be a real bind!

Ed_Dinning · 22nd Jan 2022, 7:23 pm

Hi UB, the resistance wires are available in similar insulation's to copper, enamel, silk etc. Some have an oxide coating. Most of the fine gauges that I have are silk covered

Ed

Uncle Bulgaria · 22nd Jan 2022, 8:20 pm

Thanks Ed. I'll put the rewinding to one side for the moment until I know the Megger's working. I've put some 12K resistors in place of the O/C bobbins with no change in function.

Attached is the simple route by which the Earth and Line terminals are connected to the generator. The bridge connections are omitted for clarity. I assume if everything's working correctly then I should get about 1kV between the two points on the generator that feed out to the Earth and the paralleled resistors before the Line. I guess the resistors are there in case the terminals are shorted.

On the picture the connections are marked with coloured and numbered clips. The brush connection is white 9; the mounting bracket/brass frame is grey 8; the Bridge/Megger switch and resistor connection is purple 7.

Connecting a 'scope probe to 7 and 9 gives an extremely dirty AC signal of about 60V P-P. I couldn't read the period while cranking.

trobbins · 22nd Jan 2022, 11:31 pm

Have you tried to clean all electrical contacts? The rotating bushes look like a concern from oxide build-up.

Barnmead · 23rd Jan 2022, 9:32 am

Silly question
Have you checked that both parts of the moving coil are OK?
In bridge configuration the terminal voltage will not be 1000V
I think the wirewounds might have been 100K

Uncle Bulgaria · 29th Jan 2022, 7:02 pm

trobbins - that wasn't a bad idea. I've rubbed off all the deposits that I could from the copper bushes, and also discovered that one of the carbon brushes wasn't springing back onto its bush as it should. Slightly loosening its containing guide sorted that. Now it spins up to 850V on the model 7 Universal AVO, though it did get to 1016V on the DMM once. It's now consistent at 800V+. Is this because of the AC component, and I shouldn't be expecting a nice round 1kVDC output?

Barnmead - the only silly question is the one not asked! Before disassembly the switch was set to 'Megger' rather than 'Bridge', and as the piece that engages with the knob on the casing has quite a positive engagement, I think all the internal connections are still in that position.

The non-bobbin wirewounds are indeed 100K Welwyn parts, and they all measure OK. They've been disconnected for these tests, which I'm doing only with the generator part.

Refugee · 30th Jan 2022, 2:42 am

I have got a megger from the same period. It is the basic model without the bridge.
It was being sold for the value of the wooden cabinet.
I had a much better idea....
The first problem I came across was that it has no crank handle

The crank handle problem was solved in the end

It was at a time when quite a few users stopped posting here for a while so the photos are on a link.
https://golbornevintageradio.co.uk/f...d.php?tid=3637

trobbins · 30th Jan 2022, 2:57 am

Is there some kind of mechanical governor hidden away in there? Have you been able to add a light machine oil (I use a lock mechanism spray) to all the bearings? It may be worth checking the voltage waveform on a scope to better appreciate the waveform shape and frequency.

Ed_Dinning · 30th Jan 2022, 10:33 pm

From the Meggers used for power line testing I think they employed a slipping clutch to regulate the speed.
Ed

Uncle Bulgaria · 30th Jan 2022, 11:48 pm

Yes, it has a slipping clutch. The readings I'm getting are after cranking at the slipping speed for a few seconds so it can't go any higher. I'll have to put it on the 'scope again. It's hard to hold it on the desk and crank without touching the nasty bits!

trobbins · 31st Jan 2022, 3:49 am

I'd guess there is some form of adjustment of the clutch, or some indication of what may have worn, that could vary the slip rpm.

The AVO ac voltage accuracy may vary (compared to true-rms) due to waveshape and if the frequency gets above about 2kHz.

Uncle Bulgaria · 1st Mar 2022, 10:52 pm

It appears to be up to 1kV on the DMM, so it looks like cleaning was all it needed.

Now I'm stuck with what to do with these O/C bobbin resistors. It would be neat to unwind them to find the break, but I don't know how to remove the varnish they're encapsulated in. I've tried acetone, IPA and gentle heat without success. Is it too naive to expect the varnish to be removable, the break repaired or bobbin re-wound and new varnish applied?

Uncle Bulgaria · 2nd Mar 2022, 11:19 pm

Soaking the resistor overnight in IPA did result in some softening, and I could scrape off the varnish coating by rubbing with the wooden shaft of a swab.

There are a couple of breaks in the first layer. Some thicker wire goes to the terminal, wraps a few times around the bobbin and is then soldered to some very fine wire which forms the bulk of the first layer. I've unwound some of this back to the next break.

It measures 0.08mm diameter with a micrometer, but equally just scrapes through the "36" slot in my SWG plate. The table suggests that diameter is 44SWG. Anyhow, I need to follow it through and attempt to solder it back together.

Any tips for soldering these things thinner than hairs?

joebog1 · 2nd Mar 2022, 11:39 pm

If its Nichrome it is very difficult to solder.
If its Eureka wire it will solder easily ( part of the alloy is copper ).

Keep trying !!!

Joe

joebog1 · 2nd Mar 2022, 11:47 pm

Maybe I shoulld have added. I have seen big precision power resistors available on the various auction sites. They do take a bit of digging. However, they are the right price.

Joe

Ed_Dinning · 3rd Mar 2022, 6:23 am

Hi UB, it can be difficult to get the wire into the solder pool due to surface tension effects !

have some wire of a similar diameter, what are the ohms/ foot spec for this wire as I may have some at home.

Cheers, Ed

trobbins · 4th Mar 2022, 12:56 am

That bobbin type resistor was typical for precision manganin parts in vintage resistor decade boxes - but not sure your wire is copper or manganin.

Have you been able to confirm continuity from the other end of the winding to the last observed break on the top layer? If you have then I'd suggest soldering in a TCW link from the 'broken end terminal' to where the last confirmed break is - to act as a rigid wire to then try and wrap a turn or two of the broken end to. The aim is obviously to have the fine wire somewhat floating in air, which can then be latter constrained with a dab of flexible glue. The end of the TCW link can then be soldered, and hope that with sufficient time and perhaps a highish soldering tip temp any enamel on the 44 SWG will heat itself off sufficiently to allow continuity. Any lost resistance due to wire breaks is then better added as a fixed resistor as it may become a 'select on test' value.

21st Jan 2022, 2:11 am	#1
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Series 1 Evershed Testing Set Bridge Megger Repair A beautiful object, reading low. I read how some models of Megger have a capacitor that's had a hard life which can cause low output. They put the capacitor in a very awkward place, screwed down on the bottom of the wooden box beneath both the generator and meter assemblies. Some unsoldering, unscrewing, gentle persuasion and clamp removal later, I have enough brass to open a Victorian laboratory, and a Dubilier 0.05µF/1000V capacitor that shows zero microamp leakage at my tester's 600V limit. I have found two apparently open-circuit wire-wound bobbin resistors, but can't see how they relate to the overall circuit. It's complicated by the fact this can be a bridge as well as a megger, and all the wires have black sleeves. I guess I need to replace the bobbins anyway. Is it terribly hard to re-wind the things? They sit on such neat brass rod fixings it seems a shame to swap them out for some little metal oxide object. Does anyone have a circuit diagram or top tips? I've searched the forum a few times but not found this exact model. Attached Thumbnails

21st Jan 2022, 10:02 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Series 1 Evershed Testing Set Bridge Megger Repair Hi, I may have some suitable resistance wire you can have if you wish to rewind them; see if you can find the gauge and ohms/ yd figure and I'll have a look Ed

21st Jan 2022, 11:11 pm	#3
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Series 1 Evershed Testing Set Bridge Megger Repair The resistor/bobbin units look like precision resistors made with Manganin wire. The break in the wire may be at the outer lead connection if you are lucky, or on the outer layer - although the wire gauge looks pretty thin - you may want to try a careful inspection on a faulty unit to see if you can resurrect it. I've repaired some fine gauge resistors of that style - its fiddly. As a work around, you could just initially bypass each faulty one with a fixed resistor (or two to tweak the value) to see how that modifies your measurement. I'm guessing the voltage generated by your instrument is AC at a low frequency? Have you been able to confirm the frequency (which may be a concern for any DVM you use) and the variation in the applied rms voltage (eg. with crank speed and loading) ?

22nd Jan 2022, 4:35 pm	#4
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Series 1 Evershed Testing Set Bridge Megger Repair Thank you both. Are the bobbins sealed with shellac? I can't see much around the top, in case it's broken right at the end, because of the difficulty of seeing through the lacquer/varnish. Should I soak it off in acetone or something? I guess from Ed's offer that it's not impossible to rewind them - my idea of the procedure being that if the wire's insulated from itself and of a known resistance per length, then it's 'just' a case of winding that length onto the bobbin. I don't know what the instrument is supposed to put out, apart from the '1000V' on the plate. On cranking the handle the needle read 10M with the DMM attached, but the DMM only read a very fluctuating signal of about 100V maximum and mostly lower. The 250V hand-cranked Bridge Megger I did the same thing with showed 200+V on the meter and also 10M on the Megger. If I were to bypass the bobbin resistors with placeholder modern ones, could I meter the result of turning the handle without the assembly in the case? Getting it back in, screwing up and soldering all the connections, and the meter assembly, would be a real bind!

22nd Jan 2022, 7:23 pm	#5
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Series 1 Evershed Testing Set Bridge Megger Repair Hi UB, the resistance wires are available in similar insulation's to copper, enamel, silk etc. Some have an oxide coating. Most of the fine gauges that I have are silk covered Ed

22nd Jan 2022, 8:20 pm	#6
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Series 1 Evershed Testing Set Bridge Megger Repair Thanks Ed. I'll put the rewinding to one side for the moment until I know the Megger's working. I've put some 12K resistors in place of the O/C bobbins with no change in function. Attached is the simple route by which the Earth and Line terminals are connected to the generator. The bridge connections are omitted for clarity. I assume if everything's working correctly then I should get about 1kV between the two points on the generator that feed out to the Earth and the paralleled resistors before the Line. I guess the resistors are there in case the terminals are shorted. On the picture the connections are marked with coloured and numbered clips. The brush connection is white 9; the mounting bracket/brass frame is grey 8; the Bridge/Megger switch and resistor connection is purple 7. Connecting a 'scope probe to 7 and 9 gives an extremely dirty AC signal of about 60V P-P. I couldn't read the period while cranking. Attached Thumbnails

22nd Jan 2022, 11:31 pm	#7
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Series 1 Evershed Testing Set Bridge Megger Repair Have you tried to clean all electrical contacts? The rotating bushes look like a concern from oxide build-up.

23rd Jan 2022, 9:32 am	#8
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: Series 1 Evershed Testing Set Bridge Megger Repair Silly question Have you checked that both parts of the moving coil are OK? In bridge configuration the terminal voltage will not be 1000V I think the wirewounds might have been 100K __________________ Richard

29th Jan 2022, 7:02 pm	#9
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Series 1 Evershed Testing Set Bridge Megger Repair trobbins - that wasn't a bad idea. I've rubbed off all the deposits that I could from the copper bushes, and also discovered that one of the carbon brushes wasn't springing back onto its bush as it should. Slightly loosening its containing guide sorted that. Now it spins up to 850V on the model 7 Universal AVO, though it did get to 1016V on the DMM once. It's now consistent at 800V+. Is this because of the AC component, and I shouldn't be expecting a nice round 1kVDC output? Barnmead - the only silly question is the one not asked! Before disassembly the switch was set to 'Megger' rather than 'Bridge', and as the piece that engages with the knob on the casing has quite a positive engagement, I think all the internal connections are still in that position. The non-bobbin wirewounds are indeed 100K Welwyn parts, and they all measure OK. They've been disconnected for these tests, which I'm doing only with the generator part. Attached Thumbnails

30th Jan 2022, 2:42 am	#10
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Series 1 Evershed Testing Set Bridge Megger Repair I have got a megger from the same period. It is the basic model without the bridge. It was being sold for the value of the wooden cabinet. I had a much better idea.... The first problem I came across was that it has no crank handle The crank handle problem was solved in the end It was at a time when quite a few users stopped posting here for a while so the photos are on a link. https://golbornevintageradio.co.uk/f...d.php?tid=3637

30th Jan 2022, 2:57 am	#11
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Series 1 Evershed Testing Set Bridge Megger Repair Is there some kind of mechanical governor hidden away in there? Have you been able to add a light machine oil (I use a lock mechanism spray) to all the bearings? It may be worth checking the voltage waveform on a scope to better appreciate the waveform shape and frequency.

30th Jan 2022, 10:33 pm	#12
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Series 1 Evershed Testing Set Bridge Megger Repair From the Meggers used for power line testing I think they employed a slipping clutch to regulate the speed. Ed

30th Jan 2022, 11:48 pm	#13
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Series 1 Evershed Testing Set Bridge Megger Repair Yes, it has a slipping clutch. The readings I'm getting are after cranking at the slipping speed for a few seconds so it can't go any higher. I'll have to put it on the 'scope again. It's hard to hold it on the desk and crank without touching the nasty bits!

31st Jan 2022, 3:49 am	#14
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Series 1 Evershed Testing Set Bridge Megger Repair I'd guess there is some form of adjustment of the clutch, or some indication of what may have worn, that could vary the slip rpm. The AVO ac voltage accuracy may vary (compared to true-rms) due to waveshape and if the frequency gets above about 2kHz.

1st Mar 2022, 10:52 pm	#15
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Series 1 Evershed Testing Set Bridge Megger Repair It appears to be up to 1kV on the DMM, so it looks like cleaning was all it needed. Now I'm stuck with what to do with these O/C bobbin resistors. It would be neat to unwind them to find the break, but I don't know how to remove the varnish they're encapsulated in. I've tried acetone, IPA and gentle heat without success. Is it too naive to expect the varnish to be removable, the break repaired or bobbin re-wound and new varnish applied?

2nd Mar 2022, 11:19 pm	#16
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Series 1 Evershed Testing Set Bridge Megger Repair Soaking the resistor overnight in IPA did result in some softening, and I could scrape off the varnish coating by rubbing with the wooden shaft of a swab. There are a couple of breaks in the first layer. Some thicker wire goes to the terminal, wraps a few times around the bobbin and is then soldered to some very fine wire which forms the bulk of the first layer. I've unwound some of this back to the next break. It measures 0.08mm diameter with a micrometer, but equally just scrapes through the "36" slot in my SWG plate. The table suggests that diameter is 44SWG. Anyhow, I need to follow it through and attempt to solder it back together. Any tips for soldering these things thinner than hairs? Attached Thumbnails

2nd Mar 2022, 11:39 pm	#17
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Series 1 Evershed Testing Set Bridge Megger Repair If its Nichrome it is very difficult to solder. If its Eureka wire it will solder easily ( part of the alloy is copper ). Keep trying !!! Joe

2nd Mar 2022, 11:47 pm	#18
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Series 1 Evershed Testing Set Bridge Megger Repair Maybe I shoulld have added. I have seen big precision power resistors available on the various auction sites. They do take a bit of digging. However, they are the right price. Joe

3rd Mar 2022, 6:23 am	#19
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Series 1 Evershed Testing Set Bridge Megger Repair Hi UB, it can be difficult to get the wire into the solder pool due to surface tension effects ! have some wire of a similar diameter, what are the ohms/ foot spec for this wire as I may have some at home. Cheers, Ed

4th Mar 2022, 12:56 am	#20
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Series 1 Evershed Testing Set Bridge Megger Repair That bobbin type resistor was typical for precision manganin parts in vintage resistor decade boxes - but not sure your wire is copper or manganin. Have you been able to confirm continuity from the other end of the winding to the last observed break on the top layer? If you have then I'd suggest soldering in a TCW link from the 'broken end terminal' to where the last confirmed break is - to act as a rigid wire to then try and wrap a turn or two of the broken end to. The aim is obviously to have the fine wire somewhat floating in air, which can then be latter constrained with a dab of flexible glue. The end of the TCW link can then be soldered, and hope that with sufficient time and perhaps a highish soldering tip temp any enamel on the 44 SWG will heat itself off sufficiently to allow continuity. Any lost resistance due to wire breaks is then better added as a fixed resistor as it may become a 'select on test' value.