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Author Topic: Pilot Light Burnout  (Read 12017 times)
KB1WSY
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Posts: 615




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« on: April 17, 2013, 07:07:46 PM »

I have a rebuilt Heathkit EK-2B receiver. The pilot lamp that lights the tuning dial (#47) always burns out after a few hours -- I've now gone through half a dozen of them. The measured voltage checks out as correct (6.3v). Suggestions?

73 de Martin, KB1WSY
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W9GB
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Posts: 2583




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« Reply #1 on: April 17, 2013, 08:00:50 PM »

The Heathkit EK-2B is the 1950s - 1960s educational kit for Basic Radio course.
As I remember, this is an All-American Five style design ---
Tube filaments and pilot lamp are wired in series with the AC Line Voltage (no isolation transformer).

When this kit was designed (1950s), USA line voltage of 110 to 115 Volts was common,
today 120 to 125 Volts is more common.
For example, my residential line voltage, this evening, is 125 Volts !

The #755 is a well-known replacement (longer life) for the #47 lamp.
Very inexpensive at $1.35 for 2 lamps.
http://www.2bradioparts.com/page22.html

(Longer Life)
#755 Bulb /Specifications
Size  T-3  1/4 Lamp Size
Volts 6.3V
Current .150 Amp
M.S.C.P. 330
Average Life 20,000 Hours
===
Factory Original Bulb
#47 Bulb Specifications
Size  T-3 1/4
Voltage 6.3V
Current .150 Amp
M.S.C.P .500
Average Life 3,000 Hours
« Last Edit: April 17, 2013, 08:39:43 PM by W9GB » Logged
KA4POL
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Posts: 1864




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« Reply #2 on: April 17, 2013, 10:11:32 PM »

The dial lamp uses the same voltage as the heating coming from a transformer. Put a resistor in series with the lamp thus reducing the voltage.
A 3 Ohm resistor would reduce the current by 10 mA. That should be sufficient. A more radical approach would be a LED in place of the bulb. This also would require a resistor and a diode due to AC being used.
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G3RZP
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« Reply #3 on: April 18, 2013, 01:56:31 AM »

I've used 47 ohms in series with a#47 in my Hy Gain beam rotator boxes. Still on the original bulbs from 1985.....
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KB1WSY
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Posts: 615




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« Reply #4 on: April 18, 2013, 04:39:06 AM »

GB: it is a fairly ancient kit but there is a heater secondary on the transformer, and as I said the voltage checks out at 6.3 although I agree that with today's line voltages it is probably pushing the envelope. When I measure the line voltage here it is usually around 122-125.

POL, RZP: I had considered installing a resistor and will do so now; s/b a 1W I presume (6.3 x 0.15).

GB:... and if that doesn't work I'll then try the #755; sounds like it would be worth getting in a stock of those anyway.

POL: I'd also thought of using a LED but I try to maintain authenticity in these sets if possible ... call me weird. It is true that the pilot lamp is awkwardly located (right at the top of the front panel, almost touching the underside of the cheesy wooden cabinet) and gets very hot both because of the accumulated heat from the tubes and the proximity of the cabinet. Using a LED would solve that issue as I assume they are not very heat sensitive and give out almost no heat.

I've set up an ultra-low power AM broadcast band transmitter (this is legal here) so that I can transmit Internet radio stations throughout the house and listen to them on my ancient AM radios. The Heathkit EK-2B is next to the desk where I run my home business, so that pilot lamp is getting a lot of use! This "mucking about with radios" is standing in as a proxy for later, when I will have more time to do "real radio." The stash of parts for my very first homebrew RX and TX is sitting forlorn in the shack, if you can call it that, since it has not originated a ham signal yet. Sigh.

Thank you everyone for the advice.

Martin, KB1WSY
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KB1WSY
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« Reply #5 on: April 18, 2013, 06:22:56 AM »

I tried 47 ohms but the bulb didn't really illuminate the dial at all, which is not surprising with a measured potential of just over 2V. So I changed it to 10 ohms, which lights the dial nicely with just over 5V and should extend lamp life. Ohm's Law in action.

While I was in there I noticed that all may not be well with the 6C4 tube which would explain why the BFO isn't working; but I resisted the temptation to do anything about it (lack of time). I also ought to replace the 50-year-old PS electrolytics but because of the tight space around there, that will be a bigger job, for later (the new caps are sitting impotently on the workbench). Impressively, there is zero hum even at full volume which I assume indicates they are still functioning as designed (and when I check the B+ voltage it meets spec) but that doesn't mean they won't blow any minute now....

The "pilot lamp fix" is about the extent of "mucking about with radios" that I will be able to do this year, it seems. Too much "real work" in my home office. Re-sigh.

73 de Martin, KB1WSY
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KA4POL
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« Reply #6 on: April 18, 2013, 06:43:37 AM »

POL, RZP: I had considered installing a resistor and will do so now; s/b a 1W I presume (6.3 x 0.15).
P=U*I=R*I*I  => 3*.14*.14=.06W so 1/2 W will do. 47 Ohm is more than the bulb itself has. That means about half the voltage, so very dim if at all is no wonder.
Quote
... but I try to maintain authenticity in these sets if possible ... call me weird.
I call you reasonable  Grin  I fully understand that.
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KE3WD
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« Reply #7 on: April 18, 2013, 06:53:23 AM »

*If all bulbs tried were from the same source, mfr and lot, in other words, "out of the same box" and you KNOW that the type 47 is the specified lamp for the unit, suspect that lot of bulbs.  This caveat goes double for ebay purchases and imports, although it is also possible for some of today's rebranders to simply stuff poorly made or wrongly identified filaments into the package as well. 

Since you've already measured the supply voltage to the lamps and proofed that it is at 6.3V and the only thing that could be causing premature failure is then going to point the finger at the filament itself, it is likely that the filaments in this lot of bulbs are simply being over-volted due to their resistance not being what it should. 

AC milliammeter in series with a single bulb, powered with exactly 6.3VAC, to see the actual Current draw of the bulb and check it against the published spec for that bulb type would prove things out for that lot of bulbs.  If it is drawing more than the specified max mA, then the bulb is just plain wrong for the job. 

So try a bulb from another source in there. 


73
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KB1WSY
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« Reply #8 on: April 18, 2013, 07:13:20 AM »

So try a bulb from another source in there. 

They are all the same brand -- purchased from the local full-range independent electronic parts store (I am lucky to still have one of those in the neighborhood). However they were bought at wide intervals over the past two years, so they are probably not from the same lot. The brand is "Linrose Light Lines," the corporate address on the packaging is in Plainview, NY, and they come in packs of two. No indication as to country of origin on the packaging, the base simply has a stamped "47" and that's it. Linrose has a nice catalogue on their website, but again no indication as to origin of the wide range of bulbs/LEDs that they carry

If they keep blowing even with the resistor in place, I will try a different brand!!

73 de Martin, KB1WSY
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AA4PB
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« Reply #9 on: April 18, 2013, 07:20:45 AM »

Is the socket in good shape! A high resistance contact might cause the bulb to overheat.
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KB1WSY
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Posts: 615




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« Reply #10 on: April 18, 2013, 07:51:06 AM »

Is the socket in good shape! A high resistance contact might cause the bulb to overheat.


I sprayed it with De-Oxit and cleaned contacts with emery paper, so yes, I think it is OK. I have some new #47 sockets but substituting would be hard because the original one has an integral bracket/clip to fasten to the dial.

Wow, we are really going in depth on this little bulb.

One of the interesting things is that I also have a restored Eico 320 RF signal generator that's even older (mid 1950s) and it, too, has been burning out the #47 bulbs regularly (there are two of them in the large dial). In that case though I did find that the sockets were bad (intermittent contact) and I've set that problem aside to solve later (same issue with the built-in clip, so I have to find a solution).

73 de Martin, KB1WSY
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KB1WSY
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« Reply #11 on: April 18, 2013, 07:52:37 AM »

Sorry, make that Eico model 315 (not 320).

KB1WSY
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KE3WD
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« Reply #12 on: April 18, 2013, 01:08:53 PM »

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


73
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KB1WSY
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« Reply #13 on: April 18, 2013, 01:33:24 PM »

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
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AA4PB
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« Reply #14 on: April 18, 2013, 04:32:09 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.
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