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Author Topic: Pilot Light Burnout  (Read 14432 times)
KB1WSY
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« Reply #15 on: April 18, 2013, 06:40:40 PM »

Voltmeters are ammeters with multipliers. A multiplier is a series resistance. A shunt is a parallel resistance used to increase the full scale current reading of the meter.

I stand corrected.

KB1WSY
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KE3WD
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« Reply #16 on: April 19, 2013, 07:30:15 AM »

Taking the time to insert AC mA meter in series with a single bulb should shed light on the light.

I don't have an AC mA meter -- just a rebuilt Eico VTVM; a $2 DMM from Harbor Freight; and a Heathkit EK-1. (Plus a whole pile of vintage panel milliameters for future TX and RX projects.) None of them have AC current ranges. Time to increase the menagerie of meters, eh?

I know that all DC voltmeters are basically ammeters with shunts, but I don't think you can play that trick with the AC volt ranges on a VTVM!! Not to mention the puzzle of converting the numbers on the voltage scale to a current scale. From my hazy knowledge of meters, there were traditionally two ways to measure AC: either with a movement that is mechanically designed to do so, or by rectifying the AC then measuring it on a DC movement. Don't know how DMMs do it....

73 de Martin, KB1WSY


All you need is a 1 ohm, wirewound resistor with suitable power handling capability.

1% tolerance resistor is good, but not mandatory here, a 5% tolerance would likely be good enough for determining the situation with the pilot lamp. 

Insert the resistor in series with the load you wish to measure. 

Measure the Voltage Drop across the resistor. 

Since the resistor is 1 ohm, the Voltage Drop will read out exactly the same as the Current value. 

Ohm's law. 

And, in this case, works the same for AC or DC Current Measurement. 


73
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AA4PB
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« Reply #17 on: April 19, 2013, 01:33:33 PM »

How about one of these LED replacements?

http://bcspinball.com/44-47-type-led-bulbs/
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KB1WSY
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« Reply #18 on: April 19, 2013, 05:25:23 PM »

All you need is a 1 ohm, wirewound resistor with suitable power handling capability.
Ohm's law. 
And, in this case, works the same for AC or DC Current Measurement. 

Cool, makes sense. Plus, now that the governor has allowed me to step outdoors (Boston area) I can traipse to the store tomorrow and get the resistor. Don't have that one in the junkbox.

73 de Martin, KB1WSY
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KB1WSY
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« Reply #19 on: April 20, 2013, 10:22:57 AM »

Since the resistor is 1 ohm, the Voltage Drop will read out exactly the same as the Current value. 

So, using a 1 ohm 5W, 5 percent wirewound, I get:
--0.2 volts voltage drop measured on the Eico VTVM's lowest scale (the 1.5VAC scale).
--0.1 volts voltage drop measured on the two-dollar DMM.

Given that the #47 is rated at 0.15 amps I would say that the bulbs appear to be OK (I measured two of them). Admittedly my test gear is crude, at these low current levels! (That's why I did not include a bogus second decimal place.)

Meanwhile the new #47 bulb that I put into the Heathkit EK-2B three days ago, in series with the 10-ohm resistor, is still working fine. Prior to installing the resistor, all the bulbs burned out after a few hours.

I suppose one could surmise that the bulbs I've been buying, all from the same source, may not be particularly good quality, but that the difference in quality doesn't have much of an impact except if the pilot lamp is in a badly ventilated and already hot place (from the surrounding tubes) which is the case in the Heathkit. With the resistor installed, it is now running at 5V and while quite hot to the touch, it is noticeably less hot than it was before. Prior to installing the resistor, I could run a finger along the top of the thin wooden cabinet and feel a big difference in heat just above the place where the bulb is installed. After installing the resistor, the difference in heat is only barely discernable.

Thanks for the tip about measuring AC current!

73 de Martin, KB1WSY
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KE3WD
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« Reply #20 on: April 20, 2013, 02:22:21 PM »


The old D'Arsonval meter movements wth ballistic needles were not mean to be accurate on readings below somewhere around the top 2/3rds of full travel, so I'd be inclined to go with the reading on the DMM, regardless of purchase price. 


Presumably the current measurement was done *before* installation of the new resistor.


73
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KB1LKR
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« Reply #21 on: April 20, 2013, 03:35:52 PM »

They may be unbranded off shore manufactured bulbs, you might try Mouser's cat. no, 606-CM47 Chicago Miniature Lamp brand #47's at $1.05 ea or $9.20 for ten.

Better still 606-CM1847 a #1847 (3/4 the brightness better life) $0.89 ea, ten for $7.80 or, best (2/3 the light, much longer life, cited above) 606-CM755 #755 at $0.89 ea, ten for $7.00. All are 6.3 V @ 150 mA.

A series resister to knock the voltage down a little, (0.5 - 1 volt perhaps?) should make a profound difference in life.
Digi-Key may also carry CML or GE bulbs.

I'm all for LED's in newer gear for lighting but they seem an anachronism in a piece of gear like this, and would be as much work as they'd require a current limiting resistor resistor anyway.
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KB1WSY
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« Reply #22 on: April 20, 2013, 03:52:04 PM »

Presumably the current measurement was done *before* installation of the new resistor.

I went one step further and used a separate bench power supply. Thus these test bulbs were not actually connected to the Heathkit radio, only to the test power supply. I wanted to isolate the possible "bad bulb" problem from any issue with the Heathkit itself. The only resistor in the circuit was the 5W wirewound in series with the bulb.

73 de Martin, KB1WSY
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KE3WD
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« Reply #23 on: April 20, 2013, 06:25:16 PM »

Outstanding methodology there, Martin. 

It is still possible for a filament bulb to be faulty as to filament life even if the Current draw is correct for the bulb. 

It could simply be a case of the filaments being bad as to tungsten content or some other metallurgy anomaly, or even a case of the vacuum (which is often really an inert gas in filament lamps) to have allowed a leak and atmospheric gases such as oxygen are contributing to the shortened filament life. 

Use of the resistor in the rig and time should proof it either way. 


73
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KB1WSY
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« Reply #24 on: April 21, 2013, 07:52:55 AM »

Outstanding methodology there, Martin. 
It could have been better ... because this morning I realized I was reading the wrong scale on the VTVM -- the 5V scale instead of the 1.5V scale! Using the proper scale, the reading is 0.08 volts and this time I am giving you the rather bogus second decimal place because the scale is more spread out. Thus, once corrected for "Martin Error," the DMM and VTVM basically match (the DMM gave me 0.1 volts and did not display a second decimal so I assume it is rounding).

The old D'Arsonval meter movements wth ballistic needles were not mean to be accurate on readings below somewhere around the top 2/3rds of full travel...
Good point, so I tried this: rotating the Zero Adj pot so that, with the meter leads shorted, on the 1.5VAC scale the needle was on 1.0 volts which is exactly 2/3 of scale. Then I did the same measurements described above and in fact got exactly the same deflection i.e. 0.08 VAC. I found this quite impressive! (Eico 232 VTVM, date-stamped June 19, 1962, completely rebuilt by yours truly a year ago.)

I *think* this way of "bracketing" the VTVM readings using the Zero Adj pot makes sense but it is something I made up on the spur of the moment. Looking at the Eico circuit diagram, the Zero Adj pot is wired across the two cathodes of the 12AU7 dual triode. The arm is fed with a negative DC supply. Rotating the arm thus creates a range of voltage differences between the two cathodes and the meter movement is itself connected between the two cathodes. By using the Zero Adj pot to set the meter to read 1.0VAC, I am unbalancing this "bridge circuit" to add 1 volt to the deflection. This is all very theoretical knowledge as I still lack the time to do enough hands-on homebrewing, although it's fun to see how much you can do with a thread discussing a problem with a humble pilot bulb.

Interestingly, and assuming the two meters (VTVM and DMM) are in the right ballpark, it seems that my 0.15A #47 pilot lamps are actually drawing not much more than half that much current.

73 de Martin, KB1WSY
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AC5UP
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« Reply #25 on: April 21, 2013, 09:04:48 AM »

With a traditional VTVM it's good practice to zero the DC voltmeter (leads shorted) and check the ohmmeter for infinity (leads open) whenever the range switch is changed.

On some meters there is a compensation adjustment to minimize the spread between ranges, which is a good thing, but for routine troubleshooting you don't need spot-on accuracy for every measurement. If the schematic says +150 vdc on pin whatever and you read +153 volts that's more than close enough. If you really want to know, zero the meter then measure again. It's also very easy to read the wrong scale if you don't stay in practice with an analogue meter and I freely admit I've been spoiled by the autorange feature on a Fluke 77 purchased 30 years ago..............
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KB1WSY
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« Reply #26 on: April 21, 2013, 09:05:56 AM »

Further experiments.

Because I do have a couple of *DC* milliameters (analogue and digital) I wired one of these "Linrose" brand bulbs to a supply consisting of 4 C-cells in series. The current draw when inserting the meters into the circuit was 140ma on both meters, which is close to the bulb rating of 0.15A.

Then I pulled a 60-year-old #47 bulb from a dismantled old signal generator lurking in the back of the shack. Unlike the "Linrose" brand it actually has a manufacturer stamp on the base: GE. Using the GE bulb, I repeated all of the above described tests (from this and earlier posts) and got almost exactly the same results as with the new "Linrose" bulbs, amounting to a little below 0.1A in AC tests and 130ma measured on the DC milliameters.

I am not sure why there is a difference in readings between AC and DC (I would have thought that these little bulbs were almost perfectly resistive loads) but in any case the two bulbs match, regardless of provenance or age.

As WD pointed out, that doesn't mean my new-sourced bulbs aren't faulty (filaments, poor vaccum); it just means that their current draw appears to be normal.

Footnote: right at the end of these tests I apparently broke the mA range on my $2 DMM, by accidentally connecting the probes directly across the 6VDC battery source (forgetting to put the bulb in series)! Now, it reads zero all the time even when I insert it in series with a current source. The other ranges still work ... I hope that if you spend more than $2 on a DMM, it will include more robust protection against that kind of stupid Martin Error!

73 de Martin, KB1WSY
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AC5UP
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« Reply #27 on: April 21, 2013, 09:18:22 AM »

Open up the $2.00 meter to look for a $3.00 fuse protecting the Ma range................  Tongue
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KB1WSY
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« Reply #28 on: April 21, 2013, 09:49:21 AM »

Open up the $2.00 meter to look for a $3.00 fuse protecting the Ma range................  Tongue
Well I never, there it is, he says, prying off the back and ignoring the warning to "Wear ANSI-approved safety goggles." A blown 0.5A fuse, looking very low-tech amid the sea of SMT components. Now I can get into my $17,000 chariot and burn $1 of gas driving to You-Do-It-Electronics to buy a replacement fuse. Might as well get several of them, and window-shop for various other things too, the XYL seems magnanimous at the moment. Or I could order it via Amazon Prime and have it delivered with "free" shipping from a hulking UPS truck in a box whose cubic capacity will probably be about 1,000 times that of the fuse.

73 de Martin, KB1WSY
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AC5UP
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« Reply #29 on: April 21, 2013, 11:44:54 AM »

Late last year I discovered that applying 120vac across the test leads of an entry level Centech DMM from Harbor Freight while the big knob was in the Ohms position was not a good idea.

No smoke, no fire, and no Ohms from that moment forward.  The fuse survived without a scratch.
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