[Ron Ramirez]

Thanks, Mondial. One more question - it appears that most LEDs I have seen have a forward voltage of 2 to 2.2V at a current of 20 mA. Does the 1.5K resistor drop the voltage from 23V to 2V, or from 15V to 2V? Ohm's Law indicates that it is more in the neighborhood of 23V to 2V, is this correct?

I'm just trying to make sure I understand all of this...as I plan to utilize what I learn in replacing the Stereo Beacon bulb with an LED in my 440-T in other vintage Fisher and other receivers later on.

[Mondial]

Yes, that's pretty much correct. You are starting out with a 23V supply. subtract from that the 2.2v LED drop and another .5 v for the saturation voltage drop of the switch transistor on the MPX board. That leaves you about 20v left. You also have the voltage drop across the 220 ohm series resistor. So neglecting the 10K, you have 1500 + 220 (or 1.72k) across a 20v supply which by ohms law results in a LED current of 11.6 mA.

So there is about 2.4v drop in voltage across the 220 ohm resistor compared to 8 v with the original bulb ( 23v down to 15v)

[Ron Ramirez]

A judicious adjustment of R401 will set your decoder to show a stereo signal only on high signal levels.

Thanks! I will do that once I make the LED Stereo Beacon replacement in the 440.

[Ron Ramirez]

Okay, I have ordered the remaining electrolytics for the 440, along with some LEDs to convert this - as well as my other Fisher receivers - to an LED Stereo Beacon.

This time I ordered from Digi-Key, mainly because they seemed to have a better selection of LEDs. It's my first order from them; we'll see if they are as efficient as Mouser.

These Fishers use a 47 bulb to illuminate the tuning meter. I also ordered some LED lamps with miniature bayonet bases from Pinball Life. These have the LED mounted to wires protruding from the bayonet base, allowing the LED itself to be easily positioned where you want the light.

[Ron Ramirez]

If anyone is still reading, I am expecting two CARE packages today from Digi-Key (electrolytics and LEDs) and Pinball Life (LED lamps with miniature bayonet bases). Last night, I prepared the 440-T for conversion to an LED Stereo Beacon by adding the resistors Mondial suggested and removing the old incandescent bulb from the plastic holder that fits into the metal tube to illuminate the Stereo Beacon. A new LED will be installed in that plastic holder. I've made notes on which color-coded wire to attach to the anode, and which to the cathode, of the LED. Keeping my fingers crossed...

[BrendaAnnD]

I'm sure you'll have no problems.

[morzh]

Good thing about an LED when the hookup is correct, all it does when reversed is simply not lighting up, but it won't go "poof". Then you simply reverse it and it works.
The current is too small to cause damage even if the reverse voltage is enough to cause the breakdown.

[Ron Ramirez]

SUCCESS!!!

Upon arriving home from work, I opened up the CARE packages from Digi-Key and Pinball Life. I went to the basement workbench, and installed one of the bayonet base LEDs with flexible leads in place of the #47 lamp behind the tuning meter. I also temporarily connected an LED to the wires which formerly connected to the incandescent lamp.

Yes - the LED Stereo Beacon does work!

But it needs some fine-tuning. I am reading only 0.5V across the LED, and while it is working, it is dim. It seems to me that a slightly smaller dropping resistor is in order. I'll measure the supply voltage and make sure it is 23V before proceeding.

Now, about the LED connected to a bayonet base via flexible leads - Yes, it illuminates the tuning meter very well indeed. A better choice might have been yellow, though, to more closely approximate the yellowish glow of the original #47. But white does look great, also.

Photos later.

[Mondial]

Ron, along with checking for the +23v, you might also check and see if the transistor on the MPX board is pulling the voltage across it to ground.

When there is a stereo signal, the voltage at the collector should be pretty close to zero. If the transistor is not turning on completely there may be significant voltage at the collector which will subtract from whats available to power the LED.

You can also try grounding the wire going to the MPX board directly and seeing how bright the LED lights. This eliminates the switching transistor from the circuit and should result in full brightness if all else is ok.

[Ron Ramirez]

I measured +24.8V where the schematic calls for +23V.

So I put a 1K and a 68 ohm resistor in series; the 1K measured slightly high and the 68 ohm slightly low; the combination was nearly exactly the 1130 ohms that Ohm's Law called for.

I tried it again. (This was before I saw your post, Mondial.) If anything, the LED seemed dimmer, barely lighting on a stereo signal but going out when tuned off signal.

I then read your post, Mondial, and measured the voltage going into the MPX board at point 4C. Sure enough, it is +24.3V.

Oh, and I tried your suggestion of grounding point 4C but it sparked when I placed the free end of an alligator clipped lead (the other end was already clipped to the chassis) so I immediately withdrew the lead. I think the LED went to full brilliance but I stopped right there as I did not want to do any harm to the MPX board.

I await your comments before proceeding.

[Mondial]

Yes, it appears that the problem is that the transistor is not conducting enough to fully turn on the LED. Don't worry about grounding the MPX connection. You are only doing what the transistor should do and it will not cause any damage.

The spark is caused by the charged 100 uF cap being discharged to ground. You can put the 68 ohm resistor in series with the cliplead to limit the spark.

How much does the voltage at the collector change when receiving stereo? It should go from about 24 to almost zero, so if it is not then either the transistor is bad or it is not getting enough drive from the MPX circuit.

[Ron Ramirez]

I decided to make an experiment, and lifted one leg of the added 10K resistor that was across the combination of the dropping resistors and the LED.

Result: I was still getting around 23V at point 4C. But sometimes while I was making the measurement, the voltage would suddenly drop to about 0.5V and the LED would illuminate at full brilliance. But when I removed the meter leads, the LED quickly dimmed again. Measuring the voltage showed it had risen to 23V again. If I left the leads on point 4C and ground long enough, the voltage would drop to 0.5V again and the LED would again illuminate at full brilliance.

It is as if it needs a shunt resistor, but perhaps higher than 10K?

[Ron Ramirez]

Mondial, I was writing my post as you were posting.

I'll check the collector voltage of Q404 tomorrow, when tuned to a strong local stereo signal and when tuned to no signal, and report back at that time.

[Mondial]

Ok, it seems that possibly you are not getting a strong enough stereo signal to turn on Q404, because when it did turn on the voltage at the collector dropped to .5 V as it should to turn on the LED.

The two diodes in Z403 rectify the 19 kHz stereo pilot signal and the resulting DC is fed to the base of Q404 turning it on and thereby lighting the LED. If there is not enough signal the Q404 will not turn on fully. The control voltage is also modified by the voltage from the stereo trigger control, which may need adjustment also to make the transistor switch properly.

Keep in mind that the LED is much more sensitive than the incandescent bulb, which would not light at all until Q404 turned on completely. So maybe the low level illumination you are getting does not actually indicate a proper stereo signal because it would never occur with the bulb.

[Ron Ramirez]

Some photos.

[Image: http://www.philcoradio.com/images/phorum...e00001.jpg]

First of all, I put a 1K resistor in series with a 68 ohm resistor under the chassis to use as a voltage dropping resistor for the LED.

[Image: http://www.philcoradio.com/images/phorum...0002-1.jpg]

Heat shrink tubing was placed over both resistors and both solder joints, and the free end was then soldered to the terminal strip where the white wire for the Stereo Beacon lamp formerly connected.

[Image: http://www.philcoradio.com/images/phorum...0003-2.jpg]

I used some hot glue to hold the new LED inside the old plastic shell of the incandescent bi-pin lamp.

[Image: http://www.philcoradio.com/images/phorum...0004-2.jpg]

After some careful soldering and more heat shrink, I now had an LED Stereo Beacon. Unfortunately, it is running dim, as you can see here...

[Image: http://www.philcoradio.com/images/phorum...0005-1.jpg]

...and here...

[Image: http://www.philcoradio.com/images/phorum...0006-1.jpg]

...and here. Notice the tuning meter is illuminated very well indeed.

[Image: http://www.philcoradio.com/images/phorum...0007-1.jpg]

Made by Pinball Life, this LED, connected to a miniature bayonet base, can be positioned where needed for best light.

[Image: http://www.philcoradio.com/images/phorum...e00008.jpg]

This design allows the LED itself to be placed close to the back of the meter, allowing much greater illumination than was possible with the 47 lamp.