SABA FREIBURG WIII

      So far, only my WIII has been fitted with a set of ersatz Tastenlampchen.
      This radio had required zwei stueck of the original type at the time of restoring the chassis and I could not find any more of the original type, so I decided to substitute all the button lamps at once.
      All the spare 8.5V lamps I had in my stock were previously used to repair other Saba models.
      With the wholesale Tastenlampchen substitution in the WIII, once again I had gained five precious spare lamps for future use in the other Saba models still fitted with original lamps .

      As info for those who will source lamps from North America,the Tastenlampchen substitution for this WIII was accomplished using type Nr. 378 lamps.
      (6.3V @ .20a, 20K stunden)

      In order to reduce the brightness of the replacement lamps to come close to matching the original lamp brightness, I decided to use a single high speed diode (1N4148) in series with the lamp source feed wire at the common circuit connection.
      This particular diode type may be operating with little margin for forward current rating in this application, but so far- so good here.
      Also the fact this particular diode type was already in my parts stock at the time encouraged my use of them...No real harm if it fails.
      Comparing the output of the ersatz 6.3V lamps with the series diode to the original 8.5V lamps at full filament voltage as supplied from the WIII, I find it difficult to see any difference in the brightness of either type.
      Experimenting here with a resistor to reduce the supply voltage of the WIII to the Tastenlampchen circuit caused an observable dimming of the bulbs of the Freiburg WIII when both the Unterdrucken/Antenna Taste and UKW Taste were selected. Thus my decision to use a diode to gain a voltage drop.

      Cheers!

      1N4148 in series with with a lamp 0,2A?

      Dangerous!
      Use something like 1N4004, a rectifier diode. The 1N4148 is a small signal diode, max. 100 mA. You are right, the 1N4148 is oririginally a high speed diode, suitable for (old) AM detectors and HF-mixers. I have some from ITT, not the (chinese) plagiates you buy today as "versatile diode".

      Andreas, DL2JAS
      Was bedeutet DL2JAS? Amateurfunk, www.dl2jas.com

      dl2jas postete
      1N4148 in series with with a lamp 0,2A?

      Dangerous!
      Use something like 1N4004, a rectifier diode. The 1N4148 is a small signal diode, max. 100 mA. You are right, the 1N4148 is oririginally a high speed diode, suitable for (old) AM detectors and HF-mixers. I have some from ITT, not the (chinese) plagiates you buy today as "versatile diode".

      Andreas, DL2JAS
      Hello Andreas,
      Yes, I know the 1N4148 is marginal for this particular application, as I already mentioned. Dangerous? How?
      According to Philips data sheets it is rated for steady 200 ma. forward current. It is glass. I would imagine if it would fail to a shorted junction, then it would simply place 6.3V onto a 6.3V rated lamp.
      Otherwise, it would fail open. Then I know it isn't good anymore. I do have many 1N4007 in my stock. I don't recall why I chose not to use two of those series. The chassis is still not assembled into the cabinet.....Perhaps I'll take a few minutes to revisit this....
      Thanks for making me think again to reconsider this idea.

      *** (edited minutes later).....I am embarrassed! I posted that I used a _single_ diode! Not true! :shame:
      I'm getting so forgetful and stupid!
      I had actually placed _two_ diodes, reversed polarity..On the single common line feed to all sockets under the buttons...I apologize for my gross error ..I'm changing to 1N4007 now....***

      Janusz:
      Thanks for the eBay link......Those 8.5V lamps are never available when you want them.
      I wonder how many bidders there will be for them now?

      Cheers!

      Hi Dennis,

      the small bulbs are normally operating at 6.3 V AC (Heating circuit of the tubes).
      One side of each bulb´s socket is connected to the "hot" side via the brown wire that spans over all bulbs, the second side is grounded by the pushbuttons in use (which doesn`t mean the bulbs have house arrest of course ;)
      You intend to reduce the bulbs´operating voltage, so you put a serial diode at each bulb. If this was DC there would be a drop of the voltage of about 0.7V but in this AC case you have a rectifier so you are operating the bulbs with DC intead of AC which is no problem if the brightness is right.

      Edit: Aah! You´re using two diodes antiparralel

      Instead of soldering 2 single diodes to each socket I´d rather use one single pair of diodes like the BY299 (which can take 3-4A) and put it in the common "hot" wire.
      Achim

      Hi Achim,

      Yes I was- and am- using only the single common path to the lamps. I had completely forgotten that I had used the two 1N4148 diodes in parallel, reversed polarity. (See above) until I actually looked under the chassis now.
      I am going to remove all doubt and use something better until I obtain more original type lamps.
      Thanks! ...and I apologize once more and am truly sorry for having posted my erroneous chatter. I promise for the future to not trust my memory alone.

      Cheers!