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Old 23rd Jan 2020, 9:05 am   #1
Test Desk
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Default I've lost it! (Lamp Limiter details).

My family wil happily confirm 😄
I recently rescued a Murphy A128M from our local tip and i plan to make it my first restoration. It's in pretty good nick but has a missing knob and an obvious mod in the form of an extra switch taped to the mains lead? What I've lost is a sticky somewhere on this site which details the use of a lamp limiter and what to expect when metering across the mains lead etc. I can't find it now and i want to approach this sensibly. Thanks in advance.
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Old 23rd Jan 2020, 9:08 am   #2
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Default Re: I've lost it!

100W normal bulb in series with the mains lead before turning on.If it is very bright then it is drawing too much current.

Think the details are on Paul Stennings site?
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Old 23rd Jan 2020, 9:18 am   #3
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Default Re: I've lost it!

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Originally Posted by HamishBoxer View Post
Think the details are on Paul Stennings site?
https://www.vintage-radio.com/projec...p-limiter.html
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Old 23rd Jan 2020, 9:48 am   #4
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Default Re: I've lost it!

Thank you Dave.
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Old 23rd Jan 2020, 10:04 am   #5
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Default Re: I've lost it! (Lamp Limiter details).

Most mains radios are around 40 to 60 watts mains consumption. So I use a 60w bulb, I feel it gives a better indication of what is going on.
Fairly bright initially, - cold heaters
Fading to a yellow then dim orange glow - as the heaters warm up
Then building slightly brighter orange - as the HT starts to work.

This will start the set up fairly slowly and give the electrolytic capacitors an easier time as they try to reform at about half their normal working voltage.

If the lamp comes on very bright and stays bright there is a problem that needs investigating. But at least as you were using a lamp limiter nothing too serious should burn out. (famous last words)
Make sure there is a 3 amp fuse in the mains plug to the radio and the plug is wired correctly. If you are not sure check where each wire goes to in the radio as part of your preliminary visual checks. Nearly all radios I have restored have come with a 13 amp fuse. (does anybody want any 13 amp fuses I seem to have hundreds)

Of course it all depends on the number and type of the valves but I find this works well for the average 5 valve radio.
It is always handy to have a 100w bulb to swap to as well.

I normally change the audio coupling capacitor to the grid of the output valve, check the output transformer winding and the mains transformer windings for continuity and check valves in correct sockets, before applying any mains.

You may be surprised quite a few radios will start to work, if only a bit quietly.

Mike

Last edited by crackle; 23rd Jan 2020 at 10:15 am.
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Old 23rd Jan 2020, 10:13 am   #6
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Default Re: I've lost it! (Lamp Limiter details).

Quote:
Originally Posted by Test Desk View Post
My family wil happily confirm 😄
I recently rescued a Murphy A128M from our local tip and i plan to make it my first restoration. It's in pretty good nick but has a missing knob and an obvious mod in the form of an extra switch taped to the mains lead?
Dave
That might indicate the switch in the Tone control is faulty. Nice radios if a little awkward to place in the house!

Andrew
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Old 23rd Jan 2020, 5:00 pm   #7
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Default Re: I've lost it! (Lamp Limiter details).

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Originally Posted by crackle View Post
Nearly all radios I have restored have come with a 13 amp fuse. (does anybody want any 13 amp fuses I seem to have hundreds)
I too have a large stock of 13A fuses for much the same reason, though I would substitute "everything" for "radios". It is very much the exception to find anything fitted with an appropriate fuse - and rarely do I need to use a 13A fuse.
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Old 23rd Jan 2020, 5:27 pm   #8
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Default Re: I've lost it! (Lamp Limiter details).

At Granada Rentals we bought 10,000s of plugs from MK, and specified they came with 5A fuses. Many were further de-rated to 3A.

Someone usually chimes in to say the fuse only needs to protect the cable, not the appliance, but I've seen too many charred up on/off switches, Rifa capacitors etc, to believe a 13A fuse is always appropriate when the mains lead allows it.
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Old 23rd Jan 2020, 6:02 pm   #9
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Default Re: I've lost it! (Lamp Limiter details).

Like many others I have a jar full of new 13A fuses removed from plugs that I am fitting. And used (but good) ones removed from plugs on devices that don't draw anything like 13A.

At one point (maybe still) MK listed their normal mains plug fitted with a 3A fuse in the catalogue. I bought a load from a shop that had ordered them by mistake (!) and thought they were useless. I confirmed with MK that the plug was otherwise identical to the one supplied with a 13A fuse fitted and that it would be perfectly safe to fit a 13A fuse to a plug originally supplied with a 3A one (provided the cable, etc was suitably rated).

Some of my minicomputers need 13A fuses (large mains transformers, 500VA or more and large induction motors in the disk drives). But they have their own internal fuses, circuit breakers, etc. So I am happy to fit such a fuse to the plug knowing there are other safety devices in the circuit and that the plug fuse really does just protect against cable faults.

Other things that get 13A fuses are heating devices (2kW, for example), my lathe (the induction motor draws a lot more than 3A and the cable/switch box is designed to carr the current) and of course extension leads (which might have multiple devices plugged into them).

But for something drawing a small current (like a radio, tape recorder, etc) I fit a 3A fuse (or even a 2A fuse if I have one) no matter what the cable is. Yes the fuse might be there to protect the cable, but that doesn't mean it can't be rated much lower than the cable could carry.

My lamp limiter (and I use it a lot!) is an MK metal clad switched socket, a similar switch alongside it to bypass the lamp if I want full current and a batten lampholder (again MK, one of those that retracts the contacts when you remove the bulb). Mounted on a small wooden chopping board (I spotted that in a local shop and thought it would be ideal for a lamp limiter (!)). I normally have a 100W bulb in it, that seems reasonable for most things I work on. I have a hook in the workshop where I hang it when not in use, that way there is no effort in using it so I always do
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Old 23rd Jan 2020, 6:04 pm   #10
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Default Re: I've lost it! (Lamp Limiter details).

I.too, have a stock of 13A plug fuses for the same reason. Whenever I'm asked to repair a mains powered item, I check the plug fuse, which in over 90% of cases is a 13A one. Whilst appreciating that the fuse is primarily to protect the cable, I always replace the 13A fuse with an appropriately rated lower one, and to that end, keep a small stock of 1A, 3A, & 5A plug fuses. These are certainly useful, given that the majority of mains powered radios, record players, etc., of UK origin, do not have internal fuses, whilst pretty well all European made equipment does. This of course could be that in at least some other countries, mains leads & plugs are not fused. My lamp limiter iwas made by me on a piece of wood about 15" square, and consists of swutched sockets and a batten lampholder. It is wired to bypass the lamp once I'm satisfied that full mains can be applied to the item being tested. The problem now, of course is getting (say) conventional 25, 40, & 60 watt filament bulbs.
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Old 23rd Jan 2020, 9:30 pm   #11
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Default Re: I've lost it! (Lamp Limiter details).

'@LIVEWIRE?':
Quote:
The problem now, of course is getting (say) conventional 25, 40, & 60 watt filament bulbs.
I have been able to buy the old-fashioned type from Amazon, sold as 'rough service' bulbs.

These were Chinese, fairly cheap and they varied quite a lot in terms of longevity. I used to buy lots of 10. About half would fail early. All would trip the circuit breaker upon failure!

I've now changed over to halogen bulbs, bought locally.
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Old 23rd Jan 2020, 11:46 pm   #12
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Default Re: I've lost it! (Lamp Limiter details).

Quote:
Originally Posted by 'LIVEWIRE?' View Post
The problem now, of course is getting (say) conventional 25, 40, & 60 watt filament bulbs.
Traffic light bulbs. https://www.tlc-direct.co.uk/tcl/sea...&Submit=Search

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Old 23rd Jan 2020, 11:55 pm   #13
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Default Re: I've lost it! (Lamp Limiter details).

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Originally Posted by rambo1152 View Post
At Granada Rentals we bought 10,000s of plugs from MK, and specified they came with 5A fuses. Many were further de-rated to 3A.
Those would be the ones with the Granada "G arrow" logo moulded into the top.

I'm still miffed that my parents threw away the one on my WH Smiths Data recorder!
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Old 28th Jan 2020, 10:03 am   #14
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Default Re: I've lost it! (Lamp Limiter details).

Thanks for all the advice. I was told it was " just about working " so hopefully I can improve on that. I've found couple of 100watt bulbs but I see there are still supplies lurking out there. There's only one thing that is spoiling its appearance, which is generally good, and that is a missing knob. I've trawled the web without luck. Any suggestions?
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Old 28th Jan 2020, 10:59 am   #15
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Default Re: I've lost it! (Lamp Limiter details).

Yes, I was at Granada and think I still have a few G plugs.The rule was ,3 amp for black and white and 5 amp fuses for colour back in those days.

Only thing 13 amp was good for was kettles and fan (3KW) heaters. MK plugs were superb back then.
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Old 28th Jan 2020, 11:00 am   #16
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Default Re: I've lost it! (Lamp Limiter details).

No problem getting 60 or 100w bulbs these days, just look for "rough Service" types.
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Old 28th Jan 2020, 11:11 am   #17
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Default Re: I've lost it! (Lamp Limiter details).

Traffic light bulbs! Now that's a new one...

How to test your lamp limiter? Install a diode backwards like I did the other day substituting a 1N4007 for a selenium rectifier
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Old 28th Jan 2020, 12:12 pm   #18
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Default Re: I've lost it! (Lamp Limiter details).

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How to test your lamp limiter?
Personally I'd just short the output and check that the lamp glows at full brightness without blowing or tripping anything.

If you wanted to go further you could plug a table lamp into the output and check that both lamps glow dimly.
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