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Tek Graticule Lights Revisited

 
Anonymous
 Anonymous
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Once again a 'half-hour project' turned into a full evening. Not a problem as I was keen to get back to the workbench after last weekend's forced absence.

The goal was to do a bulb swap in the newly arrived Tek 1781R PAL Video Measurement Set, which like most of these instruments had burned out graticule bulbs. The bulbs are "unobtainium" - only one vendor and a ten-times price mark up on remaining NOS inventory.

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Three open original bulb holders

The correct bulbs have female pins and sit on gold-plated stiff wires, with white plastic spacers. Over time these over heat and most fall apart when trying to remove the bulbs. Also, heat from the bulbs affects the PCB material, resulting in further difficulty getting them out. 

I considered replacing these bulbs with a more common (and less costly) type on the last instrument I repaired, but at the time I had just enough of those special bulbs to get it working.  The type 7183 (6V 200mA bi-pin) would probably work. The challenge would be how to mount the new bulbs, and would I find a suitable bulb socket? Answer is no, I tried.

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Replacement Candidates?

It was therefore something of a surprise when I was unable to remove three of the six bulbs, from the front access and not knowing what was behind the PCB. I ended up smashing one. These live just under the CRT face-plates, and damaging a CRT would be very sad. The CRTs are certainly "unobtainium" as Tek stopped making these bottles in the late 1980s.

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Arrangement of bulbs, light-pipe, and CRT faceplate.

Further investigation revealed that someone beat me to it! Soldering bi-pins bulbs directly to the old sockets. Was this a bodge? Perhaps it was done in a hurry and only from the front, possibly while this instrument was still in a rack cabinet?

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Previously attempted repair

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My reworked solution

By making loops on the new bi-pin bulb's pins I was able to solder them to 'goal posts' made from stiff wire. The wire was salvaged from CATV/CCTV coax, which typically has a copper-plated rigid centre wire (possibly made of steel)

Was it worth it? I think so. I don't expect to change these bulbs again for a long time.

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Reassembled and tested. (Instrument was turned upside down for access)

ReplyQuote
Posted : 21/06/2017 10:19 pm
Nuvistor
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Definitely not a bodge, when the item needs fixing and original parts are no longer available you do what's required as long as it is safe and functional.

Lots of old radios would have had different valves and bases fitted in a service workshop to keep the radios working. 

Nice work, much better with the graticule lit up.

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Posted : 21/06/2017 10:48 pm
Anonymous
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nuvistor said
Definitely not a bodge, when the item needs fixing and original parts are no longer available you do what's required as long as it is safe and functional.

Agree! Better to have it working than scraped.

Lots of old radios would have had different valves and bases fitted in a service workshop to keep the radios working. 

Nice work, much better with the graticule lit up.

Thanks, without those lights the CRT graticule is not visible.

BTW, I thought about substituting LEDs, which would run much cooler and last forever. A couple of challenges to overcome. Most troubling is the way the bulb lights the CRT face-plate from the side, but LEDs are typically top and not side looking. In this application the bulbs are driven from AC-coupled square waves, not sure what happens when an LED, which only conducts in one direction, is used? Possible solution is to add a reverse polarity silicon diode to the LED, and a series resistor to limit current. These new parts have to be mounted somewhere.

LEDs in Bi-pin shape are available, and I have these in another project (audio mixer control panel) However, they have six tiny LED 'chips' in the top, and emit very little light to the side. Funny thing, one has to be careful to put them in the right way. Bi-pin lamps are not polarized, and the only clue is that one lead of the LED is gold plated. As it is the bulbs (or LEDs) are quite a challenge to insert to the switch body after the key-cap is off. I'll take some PIX next time I'm replacing bulbs in control room switches.

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Posted : 22/06/2017 12:03 am