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R/E/P => Klaus Heyne's Mic Lab => Topic started by: brucekaphan on May 04, 2016, 10:38:54 pm

Title: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 04, 2016, 10:38:54 pm
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
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: klaus on May 05, 2016, 02:23:56 am
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:
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: panman on May 05, 2016, 09:18:09 am
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.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: usattler on May 05, 2016, 10:19:12 am
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:
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 05, 2016, 10:37:27 am
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!
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: Jim Williams on May 05, 2016, 11:07:23 am
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.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: klaus on May 05, 2016, 11:51:10 am
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.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 05, 2016, 02:01:26 pm
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.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 05, 2016, 03:43:55 pm
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.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: usattler on May 05, 2016, 04:47:27 pm
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.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: klaus on May 05, 2016, 07:48:27 pm
Point well taken, Uwe, BUT the secondary connected to the "old" (4V/0.25A) bulb is rated as "11.5 VAC". What gives?
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: usattler on May 06, 2016, 08:53:13 am
The minute potential difference in brightness when powering the 12V/0.02mA bulb with 11.5Vac is totally inconsequential and well within standard tolerance.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: panman on May 07, 2016, 11:41:30 am
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.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 07, 2016, 02:04:36 pm
thanks again to everyone who has posted here!!!
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: klaus on May 12, 2016, 11:51:40 am
I just sent an email to Rafi in Germany, asking whether they still manufacture or have stock of the Rafi ML26.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 12, 2016, 12:11:56 pm
Thanks Klaus! If this lead doesn't pan out, then it appears that panman has a manageable alternative.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: usattler on May 12, 2016, 01:44:23 pm
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 immediately blow if connected to the 11.5Vac transformer secondary.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: panman on May 13, 2016, 08:09:26 am
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.

We can forget that! I received the lamps. Transplanted one into a cleaned base and tested with a V72. Could not see any light on the front-plate spot, where the light should shine through. The lamp works, but there is only a tiny glimmer of a glow :(. What we are looking for is a lamp with 1 w, but there was not enough info on the link. 
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 13, 2016, 10:49:33 am
Thanks panman, for letting us know.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: panman on May 13, 2016, 04:57:52 pm
We can forget that! I received the lamps. Transplanted one into a cleaned base and tested with a V72. Could not see any light on the front-plate spot, where the light should shine through. The lamp works, but there is only a tiny glimmer of a glow :(. What we are looking for is a lamp with 1 w, but there was not enough info on the link.

You must excuse me for beeing too hasty! Yes, it does not work on V72, because the voltage(c. 6.3V when correct) is too low. Then I remembered that the secondary of a N52a(t) is 11.5V and I just tried it and it works perfectly on one of my N52:s. Comparing another N52 (with the orig. Rafi in it)with this shows no difference in brightness at all.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 13, 2016, 05:09:38 pm
I'm deeply impressed with and grateful for the persistance, knowledge and expertise at this blog!
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: klaus on May 25, 2016, 03:13:34 pm
I have found a supplier of original RAFI bulbs in 12V:
http://illi.eu/listen/Miniaturlampen.pdf
info@illi.eu
The socket type is specified as "R0728 F9 (f. Rafi R2821)"
and the part number is: 0901212.
Price: €5.40

I have contacted them for technical info and quantities smaller than 100 pieces
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 25, 2016, 03:31:55 pm
That's some super sleuthing there Klaus! Thanks!!!
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: panman on May 25, 2016, 05:45:29 pm
and the part number is: 0901212.

This one is 12V 120mA, shouldn`t it be 12V 20mA? There is no such on the list. 120mA might affect the filament voltage too  much or not?
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: klaus on May 25, 2016, 08:59:27 pm
Good point. A concern is that the 120mA bulb rating maybe too dim and/or draw too much current for a transformer designer for a 100mA tube heater current draw. I am waiting to hear from the company to take this to the next step.

Uwe?
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: klaus on May 27, 2016, 01:02:54 pm
Illi in can sell 10 of them @ €8.50 each + shipping.
But the discrepancy in wattage needs to be solved first.
Title: Re: replacement light bulb for pilot light on a Neumann N52a power supply?
Post by: brucekaphan on May 27, 2016, 03:36:24 pm
Thanks once more Klaus, for your persistence. I can imagine wanting at least a few for backup, anyone else out there interested in splitting the purchase, in the event that its specs will be found to be within tolerance?