Replacing LED bars on Fostex Model 80

[fred s.]

A few of the LEDs don't light up on my Model 80 (closer to the top, +3 and +6 area).

Are these fairly easy to replace? Where can I get the matching parts?

 

[krhall]

Instructions for replaceing LED'S

These led's can be easily replaced. BUT these are polarity sensative.

Please read this very carefully.

Inside the plastic cap of the led you will see 2 poles, this is 1 of the easier ways to identify the polarity of the led.

This is important because i have seen some mistakes where people have replaced them and put them in the wrong way round.

I will give a description of the correct way to identify the polarity of an led.

1. Inside the plastic cap the SMALLER of the 2 poles indicates the + (Posative) pole. (the much larger pole is the - pole (negative) pole.

2. On some led's you will find that 1 leg is longer than the other, dont use this as a guideline as this can never be verified as a posative identification on the polarity ALWAYS go by the 2 poles inside the plastic cap for CLEAR indentification of polarity.

3. There are many different colors, shapes AND types of led's.

*WARNING* CHECK THE INPUT VOLTAGE BEFORE REPLACING LED.

Get a volt meter and check the rated current being supplied to the led before you replace it, most led's are regulated by a resistor and a capacitor. The standard voltage for a single phase 2 pole led is 3v DC 50Ma to 90Ma (note: Ma = Milliamps so 50ma = 0.05Amps and 90ma = 0.09Amps )

The type of led's in a fostex machine are single phase 2 pole led's rated at +3v DC (ie: they run on 3 volts - any voltage higher than 3 volts DC on a single LED and they could either burn out or EXPLODE).

The other types of led's are multi colored led's which comprises of the following.

1 negative pole, and multiple posative poles (1 for each color)

Example...

A 3 phase LED has 2 posative poles and 1 negative pole. The 2 colors are commonly controlled by a transistor circuit that switches the posative pole input to the other. so 1 color goes out and the other comes on.

On another note. There are led's that can use all 3 poles to combine 2 colors to make a completely different color, for example,

You have an led with the following poles

1 X Negative pole (-0V)
1 X Posative pole (color = Red (+3V)
1 X Posative Pole (Color = Yellow (+3V)

With this above configuration, i can input current to both poles for red and yellow to combine the colors to make an Orange color. Note that i am still using the same 3V input current, although the load is only marginaly higher, i can get away with increasing the voltage to a maximum of +3.5V BUT only when using both posative poles, if i wanted to revert to using just a single pole, i would then decrease the voltage back to +3V so i dont overload the LED when i am only supplying current to 1 pole.

Daisy chain circuit.

In this type of circuit i connect several LED'S in series, first i have to see how many led's i use, so lets say i use 5 led's connected in series.

First i would start with 3V for the first led then add 1 extra 0.5v of current for each additional led i connect.

so 5 leds starting at +3V which would total +5V total at 125ma.

This would light the 5 led's quite happily and wouldn't overload as the current completes a circuit through all leds connected. My personal prefference would be to round down to the nearest even value in volts. So what i would do for 5 led's is round it to 4.5 Volts at 100Ma and i would test it seperately before applying to a circuit board.

On your model 80 the led layout is similar but far more complex than described here.

The led's you seek are the oblong shaped led's rated at +3V DC.

They are easy to desolder and resolder, but for a good clean connection TIN the led poles first, (Ie: Tinning is a method of coating the led poles in solder to create a good clean connection, and also it will give a good strong solder point).

The 3 or 4 uppermost led's are red and the led's bellow are orange,

I have just recently sold my Fostex Model 80 as i have just replaced it with a Revox Machine.

I have found that it is not really practical to squeeze 8 tracks onto quarter inch tape as bleedthrough becomes a serious problem because the tracks are so close together. The ideal machine for 8 or 16 tracks would be a half inch machine as the tracks are better spaced and give better audio quality and run less risk to bleedthrough. I Have found that recording 2 tracks in only one direction gives astounding audio quality and eliminates bleadthrough completely and i now have 2 machines in this configuration for which i now use for recording my music on.

In the near future i do have a HUGE load of 7 Inch tape reels that are now surpluss to my requirements, so what i would like to do is possibly trade these in or sell them in exchange for 10.5 inch tape reels for my Revox machines.

At Present i have over 140 tape reels and all are boxed in either library cases or cardboard boxes.

I Hope all this information helps and that you manage to successfully repair your machine

My Electronics degree really did pay off as i would be lost without it.
When i was younger i used to make up loads of test circuits with relays and stuff just to see what i could do.

Hope this info helps

Keith

 

[fred s.]

WOW~Keith thanks for all the great info.

So i am assuming you did this on a Fostex 80? Where did you get the LEDs?

 

[Tim Gillett]

LED's are rated in terms of current, not voltage. So, a LED might be rated at 20mA and the resultant voltage drop across its terminals might be 2 volts.
But it's about current. Stick to the current rating of the LED and dont be too concerned about the voltage that develops across it.
If there's a series resistor connected to the LED, measure the voltage across that resistor (on an LED working correctly) and then convert that to a current value using ohm's law where current equals the voltage across the resistor divided by the resistor value.

My information is, common LED's have current values from 5mA to 30mA (Radiospares wall chart)

It's important to find out why the LED's arent working. LED's normally are pretty reliable if driven within their design limitations. Possibly the chip driving them is faulty or maybe poor connections on the circuit board or connectors.

Silly question but are you sure the signal is getting high enough to light the LED's? Just a thought since if it were the lower LED's it would be more obvious if the ones above them were glowing correctly.

Hope this helps. Tim

 

[fred s.]

The signal is definitely high enough

2 of the bars are fine (tracks 3 and 7), then from there there are random non working LEDs. For example track one all but one of them light up, and its below the +6 mark (I think it was, +2).

 

[fred s.]

Maybe I should just leave it alone...I just opened up the back and it kinda scared me off when I discovered I'm probably going to have to open up the whole thing to access the LED bars.

 

[krhall]

btw I orderd the led's from RS electronics in the Uk.

If your overseas then i am not sure, best to try an electrical repair store and see.

Keith

 

[Dr ZEE]

holli-moly... , what's goin' on here?

Are these fairly easy to replace?

I'd say the short answer to the question - NO.
But do-able, I'd imagine.
fred, here's basic info regarding LEDs in general, if you need it: http://www.kpsec.freeuk.com/components/led.htm
*****
Generally speaking replacing a dead LED is no big deal nor is a rocket'science work. Tim G gave you a good idea about how in theory it would be possible to figure out what led(s) you could use to replace the bad ones (that is if the leds are out (not functioning) due to the fact that the leds that are not functioning actually ARE bad , which may or may not be the case).
The problem is - how to do it practically in your specific unit. It may be not so hard or it maybe a major pain in the A$s even if you've done some electronics diagnositcs/repair in the past.
-Have you open the unit yet just to take a look how it is inside.? - it would give you an idea about how easy or how hard the task is (or may be) for you.
-SO you did it. If it looks like too much for you, then, I'd say, leave it alone. That is not to say, that you can't do it, though