Technical Rotunda Tachometer Light Bulb Search

Greetings!
I have a 1963 Rotunda Faria 0-8000RPM tach whose tiny light bulb at 12 O’clock on the face needs replacement. The expert repairman, “Tachman”, found online, tells me it’s an AC bulb, but he cannot source them. Do any forum readers have an old broken Rotunda laying around with a good bulb? Perhaps someone has sourced this bulb, or has found an alternative replacement.

Many thanks for any help offered!
Bob

 

no pictures?

 

If there is no number on the bulb, get pictures and dimensions.

If I was guessing, I'd say it's a #53.

 

I converted my factory tach to an led only because there is less heat generated from the led style bulb. The heat is what makes the face stencil fail, especially on a dash mounted unit that is in the hot sun.

 

Is there no identification numbers on the lamp, or just one he can't find or cross into another? I ran into a similar deal years ago, had some burned-out lamps in some audio gear that none of the stereo repair places could identify. Finally figured out the lamps were industrially rated as opposed to 'consumer' rated. The difference? Consumer rated had a projected life of 2000 hours, the industrial ones were rated out to 20,000 hours. Had to buy them from an industrial supplier, some were special order and had minimum quantities. I still have some leftovers....

The problem these days is LEDs have taken over this market and I suppose many of the less-popular lamps have since gone obsolete. A LED may be the only choice by now.

 

Call this guy he can probably answer any of your questions or repair your tach. I also understand Rotunda 8000 RPM tachs can be very valuable.

Services provided
We restore early 50's to early 70's clocks. We also do quartz conversions on many of these, but you must contact us about this service. Only certain clocks can be converted to a quartz movement, make sure your clock is one of them before sending it in.

We restore dash clusters for many of the early Mustangs and Mopars. Please contact us for the models we do.

We also repair and restore Ford, Faria, Cobra, Shelby Mustang and Rotunda tachometers. We have one of the most extensive parts inventories for these tachometers in the U.S. If you have a Faria tachometer and want it converted to the 8000 RPM Rotunda Thunderbolt tach, we have all parts in stock to do this. We can convert the Shelby Mustang Cobra 6000 RPM tach to a 9000 RPM Cobra tach. We usually have all of these tachometers in stock for sale. Please contact us if you want a complete restored tachometer because we have many rare tachometers for sale.
Contact us
Please email me or phone me for your restoration needs. I am never too busy to speak with you about your restoration projects. If you have a question, please ask it; I am not so important that I can't speak to you about it.

Phidon Restorations
Phil Severance
283 Trotter Drive
Twin Falls, ID 83301

Phone: 208-734-4535
Email: philseverance9@gmail.com
Website: https://www.phidonrestorations.com/services.php

 

WOW!! All of you have been amazingly generous with your time and knowledge!! Thank you so much!! Kabinenroller’s comment about the heat fading the dial graphic was quite interesting! I’m going to pursue your suggestions and get back with what became of your efforts to help! Thank You!!!
Bob

 

I'm using the tiny led's for replacements also. They come in colors so if you're trying to do something from the 60's when red lights were used occasionally, there are options. I remember using colored cellophane to cover the dash lights for a different look.

 

Back when I used conventional bulbs in my dash lights I would “borrow” fingernail polish from my wife to tint the extra gauges bulbs to match the color of the factory dash bulbs.

 

Interesting how the light works...

 

https://www.bulbtronics.com/
https://www.1000bulbs.com/
These places may have something for you. The glowing red that kadillackid's instructions describe reminds me of old school (OLD SCHOOL) led's. I did a lighting store counter job for a while, (almost 40 years ago), and we sold them for Exit signs. They used almost no watts and could only muster a red glow. There were also indicator versions for lights in electronic equipment. They were for AC applications but, being diodes, actually ran on DC. They would have a polarity used in DC (only work one way around).
We need a picture. Mike

EDIT...
They may have been neon, now that I look around.

 

An AC bulb ???!!!
Not good-old DC?battery powered light blub??
I'd like to take a look at that drop cord you need to make it burn....
6sally6

 

Here is the rest of the page that @kadillackid shared above.

 

WOW WOW WOW!!!!! It REALLY is Christmas!! You guys have knocked this inquiry out of the park!!! I’m blown away with all the FANTASTIC information you have all offered! Those diagrams and description of how the light functions are unbelievable!! THANK YOU so much!!! Bob

 

When you fix the light issue please let us know how you resolved the problem. Good for everyone to know what you did.

 

I needs some ignition and electrickery schooling please (looks over to Squirrel and Crazy Steve with one eyebrow raised).

This thing takes the negative lead from the coil, and runs it through an "AC" bulb. The bulb output changes with ignition speed. My rudimentary understanding says that the coil output is 12V DC. It switches on and off when the points open and close, but is not an alternating current. Mebbe I am wrong though.

My mental picture is that the bulb is a normal incandescent one. At low revs, there is a fair amount of time that the points are open. No power flows to the bulb, and it would be dim(mer). At higher revs there is less time with the points open (?), plus the time that there is no power is so short that the bulb continues to glow (lag). This gives a brighter glow.

Have I got this right, or is there some AC electrical sorcery in the supply and bulb?

Cheers,
Harv

 

Harv, I’d bet there’s something inside the tach circuit that acts similar to a VFD for AC motors, it’s a rise and fall of converted AC via an SCR or Triac (forget which) that rises up to required voltage, then back to 0 volts. It’s a DC voltage that is rising and falling. Difference is AC rises and falls above and below zero volts. I’m speaking of a VFD that changes frequency to adjust motor speed, not what is used to convert single phase to 3 phase to just run a motor.

 

^^^^^Lol.
How about this instead…Google it

 

Just for fun some time, connect an oscilloscope to the primary side of the ignition coil, and see what happens. You'll see a voltage spike of around 200 volts when the points open.

I'd sure love to see a picture of that mysterious "AC" light bulb in the Rotunda tach, but so far no luck...he's keeping it secret. Is it a neon bulb? or what?



here's one I saw several years ago on a point ignition system...notice it does dip below 0V, and oscillates

 

If you ask my students it’s some sort of Voo Doo to get 200volts from a 12 volt system….

 

The secondary side of the coil gets over 10,000 volts. This is just the primary side!

It's the magic of a coil and a condenser, and a pulse of electricity.

 

Not voodoo, just plain induction.
I hooked the oscilloscope to the coil in a regular, cheap 12V relay a while back. I produced a -270V spike when turned off. Basically anything that produces electromagnetism has this effect, which can be very good to know as high voltage spikes easily can kill sensitive electronics that wasn't made to handle it. No worries in an old car, but you don't even want to think about what the wrong type of relay potentially can cause in a modern car. (Many relays have a resistor or diode parallell to the coil to reduce the peak voltage, cheap relays may not have that.)

 

Oh I know how it works, magnetism is a wonderful thing, convincing young people that it’s not just magic is another.

They don’t usually share my enthusiasm that we can get 40-50,000volts using twelve volts and some coils of wire.

 

Anything is magic if you don't understand it.

 

Might need a new flint.

That's all I got.

 

I agree with Jim, it's probably a neon lamp. You used to be able to get ones with screw/bayonet bases but no more. But you can still get solder-in neon lamps so some minor fab work could be done to install one. LEDs replaced them in most applications, who knows if one would work here.

Scant clues to what's going on inside the tach, a circuit diagram would help.

 

Thanks gents - appreciated.

Cheers,
Harv

 

1968 Allied Radio catalog....

 

Looks like you have it covered. But for other guys, if you need an oddball obsolete small bulb and can't find it check with pinball machine restoration guys. You'll need the number off the old bulb though.