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Old 25th October 2012, 06:45 PM   #11
Mooly is offline Mooly  United Kingdom
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It works out spot on for a 6 volt bulb.

You have 30 volts flowing through 400 ohm + 22 ohm + 15 ohm + x (the bulb). The bulb is 6 volt at 50 ma so that implies a resistance of 6/0.05 which is 120 ohm.

So we have 30 volts across 557 ohms which gives 53 ma. R503 (a 2K2) also shunts across the bulb + R212/227 chain.

So including that gives a 400 ohm and 147 ohm (equivalent) which gives a current of 55 ma total on 30 volts. That leaves 8 volts at the junction of the 400 and 2K2 resistors and 8 volts across the bulb and R212/227 is now 50.9 ma. Loose a few 10's of millivolts across the saturated transistor and you have 50 ma as near as makes no difference

So it works out. You 6 volts and 50ma flowing in the bulb. Now what if the mains goes up or down
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Old 25th October 2012, 06:56 PM   #12
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I lifted and checked all three resistors there...R210 (130ohm 1W)
R212 (15ohm 1/4W) and R227 (22ohm 1/4W) and they were all good!
The two transistors checked good as well.
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Old 25th October 2012, 07:03 PM   #13
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Your way quicker than I am at this...Haha!
Thank you Mooly...it looks like I need to get that lamp installed and all will be fine.

Now to find the same lamp like you listed here in the US.... 6V 50mA with an E10 screw base.
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Old 25th October 2012, 07:33 PM   #14
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I do have some #40 lamps here that are:
6.3 Volt 0.15 Amp 0.945 Watt T3-1/4 Miniature Screw (E10) Base, 0.52 MSCP C-2R Filament.
Could I use one of those for the indicator lamp without harm?

Another question.....could I also use those #40 6.3V .15A lamps for the dial lamp replacements which were originally 3.2V .16A bulbs.
Those dial lamps have a separate transformer winding which is listed as 2.8VAC.
Would that be too high of a voltage or damage that part of the transformer?
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Old 25th October 2012, 07:52 PM   #15
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A 0.15A stereo lamp won't glow very much but it probably won't do much harm. Because of the series resistor the lamp current rating is actually more important than the voltage, as it almost has a constant current supply.

Similarly for 6.3V lamp on a 2.8V supply, although here the voltage rating matters most.
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Old 25th October 2012, 09:00 PM   #16
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Quote:
Originally Posted by DF96 View Post
A 0.15A stereo lamp won't glow very much but it probably won't do much harm. Because of the series resistor the lamp current rating is actually more important than the voltage, as it almost has a constant current supply.

Similarly for 6.3V lamp on a 2.8V supply, although here the voltage rating matters most.
Thanks DF96, I just didn't want to ruin anything. I apologize for my ignorance with this, but substituting or changing things, I worry about burning something else up in the process. Getting these dial lamps and stereo indicator light all working isn't a big deal but it would be fantastic if I can replace them with what I have on hand and everything works!
I'll give them a try and see if the dimmer light output is OK visually.
The indicator lamp has a black tube sleeve around it and sits right behind a tiny red jewel on the face plate. So that might not be that noticeable.
There are 5 dial lamps that sit in front of the plastic dial face and then the 6th lamp backlights a little signal strength tuning meter (which works).
Again those might not be that noticeable being a little dimmer once I get the face plate all back together. Even if they are slightly dimmer it's certainly better than nothing at all.
When I opened this up to fix a bad channel 6 out of those 7 lamps were out.
Also the original 6 dial lamps were frosted and these #40 are clear so maybe that will help!
BTW, I found the filament resistance of those #40's which is 39.5 ohms if that helps.
Thanks again for all of you helping me with this!
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Old 26th October 2012, 04:20 AM   #17
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Quote:
Originally Posted by Mooly View Post
It works out spot on for a 6 volt bulb.

You have 30 volts flowing through 400 ohm + 22 ohm + 15 ohm + x (the bulb). The bulb is 6 volt at 50 ma so that implies a resistance of 6/0.05 which is 120 ohm.

So we have 30 volts across 557 ohms which gives 53 ma. R503 (a 2K2) also shunts across the bulb + R212/227 chain.

So including that gives a 400 ohm and 147 ohm (equivalent) which gives a current of 55 ma total on 30 volts. That leaves 8 volts at the junction of the 400 and 2K2 resistors and 8 volts across the bulb and R212/227 is now 50.9 ma. Loose a few 10's of millivolts across the saturated transistor and you have 50 ma as near as makes no difference

So it works out. You 6 volts and 50ma flowing in the bulb. Now what if the mains goes up or down
Just to see if I could actually do the math I followed your math and did the re-calculation using the 6.3V 159mA lamp which has a resistance of approximately 40ohms. (39.6)

So here goes:
400+22+15+40=477ohms @ 30V= 63mA.
22+15+40= 77 in parallel with the 2200=74ohms (I think that's correct)
The 400ohm would now have 25.2V leaving only 4.8V at the junction.
4.8V across the bulb + series resistors is 62mA.
minus a few 01th for transistor about 61ma.
Which would be only 2.4V at the bulb!!
That seems pretty low....hence the dimmer bulb, but nothing harmful!

Of course I don't know if I got that all correct.
I guess if it doesn't light up I'll have to find the correct 6V 50mA lamp.

Now on the dial lamps I have the same resistance of 39.6ohms x 6 in parallel = 6.6ohms with a 2.8VAC supply = 420mA
The original lamps were 3.2V 160mA, so that's 20ohms x 6 = 3.3ohms = 850mA.
So those will also be dimmer, but not harmful.

I'll just have to see if they work out and if not get all the right lamps!
Please correct me if you see errors in my math or conclusions.

This was a great learning session for me and actually a fun exercise....
Thanks for all the help!
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Old 26th October 2012, 06:57 AM   #18
Mooly is offline Mooly  United Kingdom
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You got it correct

The LED and resistors would still be a perfect solution for the stereo indicator apart from originality. Fitting a 120 ohm in place of the bulb would give the correct DC conditions. With 6 volts across 120 ohms we then add an LED and series resistor across that 120 ohm. The LED will have a volt drop of around 2 volts (give or take) leaving 4 volts to lose across its series resistor. So that would work out at around 4000 ohms allowing 1ma for the LED. We can't put an absolute value on it because it depends on the LED and how bright you want it. So you could end up with as high as 10K or as low as say 400 ohms.

The important thing with the dial lamps is that they don't draw more total current when on than the originals. LED's used here may not look right and it could be difficult to get the correct even illumination pattern. Another problem is these lamps running on AC. To use LED's you would need to provide protection from reverse voltage (by using a diode). Even then we have another problem... you would probably see the LED's flicker due to the AC. We can get around that one by arranging the 2.8 volt supply to become a DC rail and powering the LED's normally. Its easy but loses totally the originality.

(you probably know this but you can't measure filament resistance on a cold bulb as it will be very low indeed... just in case you found some old bulbs)
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Old 26th October 2012, 07:14 AM   #19
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I don't remember exactly, but I recall that the 60Hz flicker would be indistinguishable by the human eye. Not unlike the 60Hz refresh rate of the old NTSC standard(?). I seem to remember the threshold being something like 40Hz or 28Hz.
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Old 26th October 2012, 07:19 AM   #20
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by sofaspud View Post
I don't remember exactly, but I recall that the 60Hz flicker would be indistinguishable by the human eye. Not unlike the 60Hz refresh rate of the old NTSC standard(?). I seem to remember the threshold being something like 40Hz or 28Hz.
If I remember later today I'll try it with an LED and generator. If you look "out of the corner of your eye" rather than directly at such a light source then flicker is more obvious.
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