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Author Topic: replacement light bulb for pilot light on a Neumann N52a power supply?  (Read 6164 times)

brucekaphan

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I've got an N52a power supply and the pilot lamp is no longer functional.  I've never seen this type of bulb before (I'll attach a photo); I have no idea how to look for a replacement other than asking the knowledgeable folks here. I'd love to purchase one if anyone has a spare, but if no one has a spare, then I'd love to know if anyone has any idea what this type of bulb is called (i.e., bayonet, etc.) and/or any ideas where I might find a replacement... Very clearly printed on the bulb: 12V.0.02 A
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klaus

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I looked at a schematic I found online (a copy of an old schematic from my files, coincidentally) and all it says is "Reference Type 2821". I will trace this down, but UWE may have the exact type number in his Neumann files (Uwe to the rescue!).
From there I can give you a few German specialty dealers who may have an equivalent in stock. I have in the past replaced these with the "Stecksockel" type that is similar to U.S. automotive instrument panel bulbs- the raw glass type where its filament ends are pressed into the socket.

Here is the schematic with the part number 2821 for the lamp La 1:
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

panman

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The lamp is the same as is used in e.g. V72, V77, V73....It is RAFI L. Nr. 2821. Other info mentioned in an old part list of Siemens 6S Ela 2303(v72) is: ML 26, 12V. I managed to get two of those lamps two years ago from an ebay-seller, but that was all he had. I never found out any company selling them anymore. Lately I have replaced these lamps with leds.
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Esa Tervala

usattler

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Sorrry, but have have no leads to a source for these obsolete bulbs. I also would replace them with a contemporary LED. I did find a schematic of the N52t showing the 12V/0.02A indicator light rather than the 4V/0.25A of the N52a. The other differences are obvious! Notice the connection across the filament secondary of the mains transformer:
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Uwe Sattler
Technical Director - Neumann|USA (retired)

brucekaphan

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Thanks to you all for sharing your knowledge/expertise! My expertise lies more in using these tools than in troubleshooting and working on them—though capable of quite a bit of maintenance and repair on my tools, component level expertise is usually a little beyond my grasp... Uwe—I'm gathering from your post (and Klaus, from the schematic you posted) that it appears someone must have incorrectly substituted a 12V/.02A bulb in my N52a, and as such if I decide to replace the bulb with an LED (which seems a logical alternative), I should be shopping for an LED rated 4V/0.25A? Again, thanks to all contributing to the knowledge here!
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Jim Williams

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Most leds are rated between 1.7 and 3 volts. The higher output leds tend to want a little more voltage than standard low millicandella leds.

For a 12v bulb to led conversion, first pick the led you want to use, select the color. Most high output leds have a clear case, I sand them to difuse the light.

Then use a 1N4002 rectifier diode and a 1k resistor in series with the led, observe polarity.
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klaus

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Given that Bruce is not technically versed (especially when dealing with the tightly-packed N52 electronics) I will hunt for a suitable filament bulb, now that Panman has identified it (thank you).

Uwe pointed out something very important: On most N52a, the lightbulb is in series with the heater voltage supply. That means, if the lightbulb fails, the heater voltage fails. This was not remedied until the N52a with the embossed "t" after the "a" came out (this was a modified N52a with solid-state heater voltage stabilization added), and all units of the final N52 version, N52t.

What complicates this issue is that TAB (maker of the N52) delivered early N52a/t with the old lightbulb wiring (my schematic), before switching (Uwe's schematic).

Bruce: what is the model designation on your supply? And, more importantly: does your supply still work without the lightbulb? If it does, you have the transistor-stabilized N52, and you can safely operate the supply without lightbulb, until you find one.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

brucekaphan

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I'm blown away by the generosity of time and knowledge everyone is showing here! THANK YOU! And Klaus, in particular, thank you so much for hunting for a suitable bulb—that's really above and beyond!

I purchased this N52a (I don't see any indication of a "t" anywhere) from James Gangwer not too long ago. It's been working with no pilot light since I took delivery of it.  I'll email James to ask him if he was aware of this unit having been modified (with solid-state heater voltage stabilization added) as Klaus indicates.
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brucekaphan

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James replied but doesn't remember details about the unit, so since it's working and since the graphics on the unit indicate that it's an "N52a", it appears it's been modified to be of the "transistor-stabilized" variety.
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usattler

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Bulb #2821 is the 4V/0.25mA version, NOT the 12V type! Make sure you find the 12V bulb, since the 4V type 2821 will blow immediately if connected to the 11.5V transformer secondary.
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Uwe Sattler
Technical Director - Neumann|USA (retired)

klaus

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Point well taken, Uwe, BUT the secondary connected to the "old" (4V/0.25A) bulb is rated as "11.5 VAC". What gives?
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

usattler

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The minute potential difference in brightness when powering the 12V/0.02mA bulb with 11.5Vac is totally inconsequential and well within standard tolerance.
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Uwe Sattler
Technical Director - Neumann|USA (retired)

panman

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I just ordered these lamps: https://www.banzaimusic.com/Bulb-12V-20mA.html.
You can replace the broken lamp with this. First you need to solder off the contacts and then you can remove the lamp from the base using vinegar to dissolve the glue. Be carefull, because the base cracks easily. It should be easier, if the glas is not broken. I`ll report back, when I have got the lamps.
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Esa Tervala

brucekaphan

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thanks again to everyone who has posted here!!!
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klaus

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I just sent an email to Rafi in Germany, asking whether they still manufacture or have stock of the Rafi ML26.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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