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Readings from a lamp limiter

 

Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 7:14 pm

Hello all.
I've been using a lamp limiter for a few years now and today got a response that I am not so familiar with. Specifically - what to examine first to track down the cause of the symptoms.

I have a set on the bench which looked in good shape: no burning, scorching or obviously distressed areas but full of old Hunts and wax caps plus a few perished wires. All of this was meticulously replaced and repaired before even attempting to put any power through the set but when I did the lamp glowed at its full 60w brightneess. Something I've not seen before. No alarming smells or smoke from the set in fact nothing after about 60 seconds so switched off. Had a quick check round then tried again same result. I'm wondering what this reading could point to. Obviously the set is drawing the full amps to make the bulb glow (or none?) but what is the first obvious place to look. The mains transformer? Quite nervous to try anything without sounding this out with people first. Any help you might have will be gratefully taken.

 
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Re: Readings from a lamp limiter

Post by Red_to_Black » Sun Feb 08, 2015 7:42 pm

You must have a reasonably heavy short early on in the supply circuit, I would be cold (dead) testing with a multi-meter (set to ohms) particularly around where the mains enters.

 
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Re: Readings from a lamp limiter

Post by crackle » Sun Feb 08, 2015 7:52 pm

Hi Reg
You are correct, this indicates an almost short circuit fault. Unplug the radio from the mains.
If this happened to me I would be checking the wiring to the set and the mains switch. Check the mains is not wired across the wrong terminals on the switch if it is a double pole switch.
Operate the switch and measure the resistance across the contacts when turning on/off to check you have the correct terminals on the switch.
I would then disconnect the wires from the switch to the mains side of the transformer and measure the resistance of the transformer. You should get a reading on the ohm meter of something like 20 to 100 ohms.
If you are getting only a few ohms reading, it looks like there is a short in the mains transformer primary, which is not good news.

Let us know what you find.

Mike
typed while R2B was posting

 
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Re: Readings from a lamp limiter

Post by sideband » Sun Feb 08, 2015 8:03 pm

Probably a silly wiring error. Resistance checks with a meter as already outlined will soon show where the fault is.

Crackle wrote:Check the mains is not wired across the wrong terminals on the switch if it is a double pole switch.


I did that once in my very early days of radio. At the time, I was trying to get a homebrew radio to work and was using my table lamp as the mains supply ...(typical 60's practice of two-pin 5A plug into a bayonet adaptor). When I switched on, there was a pop from the table lamp and a puff of smoke. The fuses remained intact but the bayonet lampholder was ruined....the springs in the contacts had melted and had effectively disconnected the power from the adaptor which now had a black splodge across both contacts.....we live and learn!

 

Re: Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 8:11 pm

Curious. Resistance on the primary (mains in) of the transformer is 26.
Strangely the mains cable is a new replacement- a sealed plug unit with a figure-eight socket at the other end clipped off and the live/neutral soldered to the switch in the same configuration as the old mains input wire. The cable is sound for continuity and measurements across the switch dip when switched off. At this point it is worth telling that this is another Bush vhf61 set (I figured, based on previous knowledge of this model, that I'd have another go at another).

 
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Re: Readings from a lamp limiter

Post by Red_to_Black » Sun Feb 08, 2015 8:30 pm

unclereggie wrote: measurements across the switch dip when switched off.


Could you expand and clarify this bit please Reg ?

 
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Re: Readings from a lamp limiter

Post by PYE625 » Sun Feb 08, 2015 8:38 pm

Hello, Just to mention the ohms reading of the mains primary would seem to be correct.
Just measured my VHF61 and it is the same.

Hope this is of some help.
You could try increasing the wattage of the limiter lamp to 100w.

 

Re: Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 8:40 pm

R2B - I think that was a clumsy way of saying: if I put a probe on the two incoming contacts of the switch I get a .3 reading when switched on which dips to no reading when switched off. And before I go any further I felt it useful to post a couple of photos.

75.jpg


75-1.jpg

 
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Re: Readings from a lamp limiter

Post by Red_to_Black » Sun Feb 08, 2015 8:44 pm

Ok thanks, What do you get on the other pair of switch contacts when switched on and off ?

I am trying to determine which side your problem is, as 0.3 is a S/C to all intents and purposes.

 

Re: Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 8:51 pm

The other two contacts exiting the switch give the same reading (0.4 when on, zero when off).

I'd have assumed that a blown switch would have had some exterior signs, around that component. Looking at it from a forensics point of view such a failure might have blown the original fuse in the original plug perhaps. The original plug looked as though it had suffered a blow (scorching) but that's not uncommon for people to reuse any old plug they find. The fuse which was in there as it came to me was a perfectly fine 5A - HOWEVER that is not to say that it blew, someone fitted a new fuse - which failed to restore power to the set - then they consigned the radio to the attic for the next 40 years!.

 
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Re: Readings from a lamp limiter

Post by Red_to_Black » Sun Feb 08, 2015 8:56 pm

Okay, that is what I partly expected, your short must still be fairly near the incoming mains though, as you said you got 26 ohms on the primary of mains TX, something must be amiss between the switch and the transformer, I/we would need to see the circuit for this bit, unless it is something mis-wired.

 
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Re: Readings from a lamp limiter

Post by PYE625 » Sun Feb 08, 2015 8:57 pm

With reference to your pictures, I would be very tempted to temporarily remove the live and neutral from the transformer and connect mains to it directly, therefore by-passing the switch.
From what I can remember when restoring my set, and your pictures reminded me, the mechanics of the switch are rather close to the chassis metalwork.

(Sorry if I'm stating the obvious, typical of me !! :ccb )

 
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Re: Readings from a lamp limiter

Post by crackle » Sun Feb 08, 2015 9:05 pm

So if you measure the resistance of the transformer primary, whilst connected to the switch and the switch is OFF you get 26 ohms and if you turn the switch on, you get 0.4 ohms.
The switch poles are wired across the mains instead of in series with the mains, so when you turn the switch on the mains is shorted.

Mike

 

Re: Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 9:07 pm

Luckily I have a scrap version of this set and making that same reading across the contacts of the spare switch brought no readings across live to negative on or off.

 
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Re: Readings from a lamp limiter

Post by crackle » Sun Feb 08, 2015 9:11 pm

If you have no load on the switch that will be correct. But if you connect the transformer primary to the other side of the switch you should get 26 ohms with switch on and infinity with switch off.


Mike

 
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Re: Readings from a lamp limiter

Post by cobaltblue » Sun Feb 08, 2015 9:17 pm

Is it just my imagination or has that mains transformer been oozing wax?

Its hard to tell :aab

If so its either suffered an overload or it's faulty.

Cheers

Mike T

 
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Re: Readings from a lamp limiter

Post by Red_to_Black » Sun Feb 08, 2015 9:18 pm

Crackle wrote:If you have no load on the switch that will be correct. But if you connect the transformer primary to the other side of the switch you should get 26 ohms with switch on and infinity with switch off.


Mike


That is what I was trying to confirm Mike :aad

I will bow out for now as it will just create more confusion if too many are offering guidance, following with interest though.

I would be interested in a snippet of the circuit diagram around this part however,

 

Re: Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 9:21 pm

Hm. With one probe contacting the positive tag of the primary on the transformer and the other probe on the negative tag of the outgoing (as well as the incoming tag) of the switch I get exactly as you say: 26 and infinity.

Cobaltblue - yes there has been quite a dribble of wax but often this seems relatively normal for a well used set?

I'm also thinking that the use of a lamp limiter in this situation not only averted a disaster but also highlighted a serious short circuit. The steady bright glow of the bulb indicating a clear circuit for the light at full power.
Last edited by unclereggie on Sun Feb 08, 2015 9:32 pm, edited 1 time in total.

 
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Re: Readings from a lamp limiter

Post by PYE625 » Sun Feb 08, 2015 9:30 pm

Well, one thing is proven yet again....The use of a lamp-limiter is a vital requirement to avoid damage.
Incidentally, the mains TX on mine had dripped a fair bit of wax but I think it is normal.

 

Re: Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 9:34 pm

Edited just as you posted the same thought there.

Just thinking with my forensics head on - if the switch is damaged: what might have caused that? If I were to wire up the transformer direct to mains and the fault was in there am I not just inviting that to blow a fuse or worse? I think what I mean is - fault diagnosis should take into account what has happened prior to the fail. However, should I bypass the switch to measure what happens?

 
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Re: Readings from a lamp limiter

Post by PYE625 » Sun Feb 08, 2015 10:28 pm

The switch needs to be eliminated and then the rest of the set can be checked with mains directly to the TX (via the llamp-limiter)
It could be possible that the switch is broken internally and the sprung parts inside shorted together, or to the chassis.
I assume it is wired correctly?
Again, forgive me if this has been covered earlier.

 
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Re: Readings from a lamp limiter

Post by crackle » Sun Feb 08, 2015 10:54 pm

Reg
Your switch looks similar to this switched pot which I found in my spares. If it is, and I have read your photos correctly then it would appear yours is wired correctly.
So this would indicate as Andrew suggested there may be a fault inside the switch.

Disconnect the wires from the switch and check for reliable on/off operation with a ohm meter.
My switch is riveted together and riveted onto the back of the pot, so if it is faulty it is going to be almost impossible to fix.

Mike
Attachments
double pole switch.jpg

 

Re: Readings from a lamp limiter

Post by unclereggie » Sun Feb 08, 2015 11:26 pm

Mike. As I mentioned I have a spare set. Tonight I took the switch off that and it is easy to get inside. In fact as mentioned earlier the insides are only protected from the chassis by a disc of cardboard. The plastic unit is bolted to the chassis. Tomorrow I'll try the live bypass trick and also see if the switch will come out to see what has happened inside there. Photos of course. Still eager to learn why this failed and thanks for your confirmation and pic.

 
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Re: Readings from a lamp limiter

Post by Terrykc » Mon Feb 09, 2015 3:42 pm

unclereggie wrote:The other two contacts exiting the switch give the same reading (0.4 when on, zero when off) ...

Taking this at face value, this does not make sense! Image

What you are saying is that with the switch 'on' you are measuring 0.4Ω, which is reasonable and is comprised of two parts - the resistance of your test leads plus the 'on' resistance of the switch contacts in series.

However, you quote a reading of 0Ω when the switch is 'off' which is not only a dead short but is also less than the resistance of both test leads connected together! Unless you have a zeroing function on your meter to null out the resistance of the leads, I would expect to see some residual resistance reading, even if you are using nice thick test leads!

What I think you meant to say is that in the 'off' position, there is no reading - which means ∞ (infinity) or open circuit and is the exact reverse of 0Ω!

I think in successive posts that some have interpreted you literally and assumed a short circuit whereas others have mentally corrected your error and treating it as open circuit (infinity) which causes even more confusion!

You probably won't be surprised to find out that you are not the first to cause exactly the same confusion!

ImageRemember, when posting, to get these things right - if what you say isn't clear, my computer screen can't answer back any better than anyone else's when I mutter "What the ...?" :aaq

 

Re: Readings from a lamp limiter

Post by unclereggie » Mon Feb 09, 2015 8:32 pm

:ccg

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