Creating a fault for demonstration purposes - UK Vintage Radio Repair and Restoration Discussion Forum

SurreyNick · 9th Jun 2015, 4:11 pm

Recently when I was assembling a lamp limiter my youngest son expressed some interest in how it worked and so I gave him a brief explanation. A verbal explanation has it limitations though and doesn't illustrate well so I told him I would also demonstrate how it works at a later date when I unearth one of the receivers currently buried in the garage.

I know this receiver works and I should therefore get the 'dim bulb' scenario to demonstrate, but I am trying to think of an easy fault to create first which will illustrate the 'bright bulb' scenario which we can then track down and correct together before demonstrating the dim bulb fix.

Any ideas?

The receiver is my McMichael 808AC superhet.

Thanks

Nick

Nickthedentist · 9th Jun 2015, 4:29 pm

You need a fault which won't wreck anything or be dangerous. A short on the HT line might be a bit too risky.

A dead short on the mains would fit the bill, albeit a bit unrealistic, though something like a stray strand of wire across the mains switch terminals (assuming it's double pole) would suffice.

N.

kalee20 · 9th Jun 2015, 5:44 pm

Add a leaky mains suppression capacitor? Then you can watch it fizz, smoke and pop while the lamp flickers and brightens.

KeithsTV · 9th Jun 2015, 8:29 pm

A short on the heater winding of the mains transformer? Should increase the current enough to cause the lamp to light up.

Keith

SurreyNick · 11th Jun 2015, 4:58 pm

Thank you for the suggestions.

I would like to try the leaky mains suppression cap and also the short on the heater winding. Could both of you provide me with a detailed explanation of how I achieve both? Sorry to be a pain, but I don't have enough experience to know how to do it without some guidance.

Many thanks

Nick

wireful3 · 11th Jun 2015, 5:55 pm

The 808 is not an easy set to use but the short on the heater winding is easy to simulate. Remove one of the valves (not rectifier). Link the heater pins by wrapping wire round them near the base . Replace the valve, the frequency changer is probably the easiest to use.

This will simulate a genuine fault without having to disturb any components or wiring. The fault might also be "found" by noticing that the valve heaters did not glow. This was used in the past by some instructors in the services teaching fault finding.

SurreyNick · 11th Jun 2015, 7:50 pm

John, that's perfect and easy too!
Thanks very much. I think that's all I'll need actually

Many thanks
Nick

Malcolm G6ANZ · 11th Jun 2015, 8:21 pm

For a leaky HT smoothing cap you could put a resistor across the cap. Fairly easy and reversible.

Malcolm

SurreyNick · 11th Jun 2015, 9:52 pm

Ah! That simple eh Malcolm?
OK. I'll have a go at that too

Thanks
Nick

Malcolm G6ANZ · 12th Jun 2015, 7:22 am

Possibly. I haven't done it myself. Let me know the result.

Malcolm

KeithsTV · 12th Jun 2015, 7:48 am

Don't forget that the resistor will dissipate a lot of heat. A 4.7k resistor across the cap would need to be rated at a minimum of 15W and a 1k resistor at over 60W (that would almost certainly light up the lamp limiter).

Keith

SurreyNick · 12th Jun 2015, 6:12 pm

Thank you Keith, I shall bear that in mind.
Nick

Malcolm G6ANZ · 12th Jun 2015, 9:35 pm

That's why leaky smother caps get hot. A lot of power is dissipated in a sealed can.

Malcolm

Herald1360 · 13th Jun 2015, 10:30 am

The heater short is also perhaps the safest one for lighting the limiter. Only the transformer is overloaded and it has the benefit of a fair bit of thermal inertia before anything irreversible happens. Leaky suppressor caps if genuine tend to go bang and to simulate (other than with a direct short) could be hairy. Also, plugging direct into the mains with the heater short would also be short term survivable

ms660 · 13th Jun 2015, 10:40 am

I've never tried the heater short as I don't use a lamp limiter but it sounds a safer option, plonking stuff across the HT line could damage the rectifier.

At least the heater winding is usually made of sterner stuff than the HT winding and should dissipate heat quicker.

If it's just for a demo then perhaps an outboard arrangement with two switchable resistive loads and a multimeter to show wot's wot.

Good luck whicheverways.

Lawrence.

9th Jun 2015, 4:11 pm	#1
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Creating a fault for demonstration purposes Recently when I was assembling a lamp limiter my youngest son expressed some interest in how it worked and so I gave him a brief explanation. A verbal explanation has it limitations though and doesn't illustrate well so I told him I would also demonstrate how it works at a later date when I unearth one of the receivers currently buried in the garage. I know this receiver works and I should therefore get the 'dim bulb' scenario to demonstrate, but I am trying to think of an easy fault to create first which will illustrate the 'bright bulb' scenario which we can then track down and correct together before demonstrating the dim bulb fix. Any ideas? The receiver is my McMichael 808AC superhet. Thanks Nick

9th Jun 2015, 4:29 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Re: Creating a fault for demonstration purposes You need a fault which won't wreck anything or be dangerous. A short on the HT line might be a bit too risky. A dead short on the mains would fit the bill, albeit a bit unrealistic, though something like a stray strand of wire across the mains switch terminals (assuming it's double pole) would suffice. N.

9th Jun 2015, 5:44 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Creating a fault for demonstration purposes Add a leaky mains suppression capacitor? Then you can watch it fizz, smoke and pop while the lamp flickers and brightens.

9th Jun 2015, 8:29 pm	#4
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,574	Re: Creating a fault for demonstration purposes A short on the heater winding of the mains transformer? Should increase the current enough to cause the lamp to light up. Keith

11th Jun 2015, 4:58 pm	#5
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Creating a fault for demonstration purposes Thank you for the suggestions. I would like to try the leaky mains suppression cap and also the short on the heater winding. Could both of you provide me with a detailed explanation of how I achieve both? Sorry to be a pain, but I don't have enough experience to know how to do it without some guidance. Many thanks Nick

11th Jun 2015, 5:55 pm	#6
wireful3 Retired Dormant Member Join Date: Sep 2003 Location: Bristol, UK. Posts: 808	Re: Creating a fault for demonstration purposes The 808 is not an easy set to use but the short on the heater winding is easy to simulate. Remove one of the valves (not rectifier). Link the heater pins by wrapping wire round them near the base . Replace the valve, the frequency changer is probably the easiest to use. This will simulate a genuine fault without having to disturb any components or wiring. The fault might also be "found" by noticing that the valve heaters did not glow. This was used in the past by some instructors in the services teaching fault finding.

11th Jun 2015, 7:50 pm	#7
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Creating a fault for demonstration purposes John, that's perfect and easy too! Thanks very much. I think that's all I'll need actually Many thanks Nick

11th Jun 2015, 8:21 pm	#8
Malcolm G6ANZ Octode Join Date: Feb 2006 Location: Bristol, UK. Posts: 1,042	Re: Creating a fault for demonstration purposes For a leaky HT smoothing cap you could put a resistor across the cap. Fairly easy and reversible. Malcolm

11th Jun 2015, 9:52 pm	#9
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Creating a fault for demonstration purposes Ah! That simple eh Malcolm? OK. I'll have a go at that too Thanks Nick

12th Jun 2015, 7:22 am	#10
Malcolm G6ANZ Octode Join Date: Feb 2006 Location: Bristol, UK. Posts: 1,042	Re: Creating a fault for demonstration purposes Possibly. I haven't done it myself. Let me know the result. Malcolm

12th Jun 2015, 7:48 am	#11
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,574	Re: Creating a fault for demonstration purposes Don't forget that the resistor will dissipate a lot of heat. A 4.7k resistor across the cap would need to be rated at a minimum of 15W and a 1k resistor at over 60W (that would almost certainly light up the lamp limiter). Keith

12th Jun 2015, 6:12 pm	#12
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Creating a fault for demonstration purposes Thank you Keith, I shall bear that in mind. Nick

12th Jun 2015, 9:35 pm	#13
Malcolm G6ANZ Octode Join Date: Feb 2006 Location: Bristol, UK. Posts: 1,042	Re: Creating a fault for demonstration purposes That's why leaky smother caps get hot. A lot of power is dissipated in a sealed can. Malcolm

13th Jun 2015, 10:30 am	#14
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Creating a fault for demonstration purposes The heater short is also perhaps the safest one for lighting the limiter. Only the transformer is overloaded and it has the benefit of a fair bit of thermal inertia before anything irreversible happens. Leaky suppressor caps if genuine tend to go bang and to simulate (other than with a direct short) could be hairy. Also, plugging direct into the mains with the heater short would also be short term survivable __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

13th Jun 2015, 10:40 am	#15
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Creating a fault for demonstration purposes I've never tried the heater short as I don't use a lamp limiter but it sounds a safer option, plonking stuff across the HT line could damage the rectifier. At least the heater winding is usually made of sterner stuff than the HT winding and should dissipate heat quicker. If it's just for a demo then perhaps an outboard arrangement with two switchable resistive loads and a multimeter to show wot's wot. Good luck whicheverways. Lawrence.