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Old 13th October 2012, 07:35 AM   #1
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Default I think I majorly screwed up - Technics SU-7700

I was attempting to install LEDs to replace the lamp in my SU-7700. I was very careful in performing my calculations, however in my excitement I wired up two LEDS in parallel each with a 150 ohm 1/4 watt resistor when I should have used a single 68 ohm 1/4 watt resistor when wiring them in parallel.

Anyway, after about 5 minutes, the LEDs went out and I no longer hear the relay click when I turn the unit on. Additionally, I have verified that I am no longer getting the 6.3 volts at the lamp wires.

What have a screwed up and how can I track down what I need to replace? A link to the schematic for the unit is here: http://technicscatalogus.com/Manuals...e%20manual.pdf

Thank you kindly, and in advance!
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Old 13th October 2012, 08:49 AM   #2
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Have you checked the f4 fuse? That same 6.3V lamp voltage is half wave rectified to feed the relay drive circuitry.
I must question your LED calcs also. 4V divided by 68Ω is almost 60mA. Even 150Ω per LED is too much unless you're using special high power LEDs.
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Old 13th October 2012, 01:16 PM   #3
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I visually checked the fuses and all looked good. However, I'm going to put the meter on them and I will let you know what I find.

My LED calcs were (though I've decided just to stick with the original lamps):

(6.3v - 3.3v) / .025 = 120 ohm

I verified the calcs here: LED Calculators. As a learning experience, did I go wrong? Where?

Thanks!
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Old 13th October 2012, 01:23 PM   #4
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Yep! The F4 fuse visually appears ok, but it doesn't pass the continuity test. Will replace it and update the post with the results.

Please do advise on the LED calcs, I am not afraid to make a mistake in the name of advanced knowledge!

Regards
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Old 13th October 2012, 03:35 PM   #5
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Nor am I, batesman. I was making some incorrect assumptions about your mod; your calcs look fine (if the LEDs can take that 25mA @ 60Hz). I hope f4 gets your amp back in business.
I'm short of time right now, but I'm thinking the change affected the transistor operation. It's good to place the old lamp back in, though your mod may work with minor additional changes to the lamp/LED side and/or the muting/relay side.
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Old 13th October 2012, 06:10 PM   #6
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Also, led's don't have a spec for back voltage typically, and really old ones have a back voltage of spec of 7V. You need to put a 1n4001 to 1n4007 (any number in between), or a 1n4148 or some silicon diode in series with the led's to protect them from the back voltage. If the datasheet on your LED's shows a PIV of > 24 VDC I stand corrected.
Really old LED's can take 20 ma. 1.4x6.3v=9vdc peak. Led's drop 2 v. the silicon diode will drop .6v. so 9-2.6=7.4 v. 7.4v/.02a=370 ohm. For 2 leds in parallel, you halve it, 180 ohms is the standard 5% value. Since you used peak voltage instead of average, you could probably also get away with a 150 ohm standard resistor. Wattage is .072, any wattage resistor will do.
If your new LED is specified for more current than 20 ma, use a different denominator in the calculation.
I've found the LED's i've bought have the long lead as the line of the diode symbol. Putting a silicon diode in series, hook line to not-line ends. LED's in parallel, hook long one to long one (line end).
Two parts in series makes a vibration/bad solder joint hazard. I'd mount the silicon diode and the resistor on at least a 3 terminal solder strip screwed to something solid. Wrap the leads around the terminal so they hold mechanically before you add solder. Makes vibration resistant joints.
Make sure none of the terminals of the strip you use runs through to the mount screw. (I did use the grounded terminal once, the ST70 kept blowing the fuse?)
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Last edited by indianajo; 13th October 2012 at 06:26 PM.
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Old 13th October 2012, 09:47 PM   #7
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If thats 6.3v Ac the peak is closer to 9v. Reversed bias diodes in series (if I remember correctly) share the voltage according to size (like caps). So different diodes will take a different amount of back voltage. (I think).
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Old 14th October 2012, 07:08 AM   #8
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With LED's! just enough current to get the level of light you need. I remmeber from the 80's LED@s are the devils work! should only use ear of wheat tungsten lamps Apparently less noise .
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Old 14th October 2012, 07:13 AM   #9
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I'm guessing the OP had his LEDs connected in parallel... reverse parallel that is. So neither LED will see a reverse voltage greater than the Vf of the other.
To each his own, but IMO a terminal strip is not at all necessary to connect two components in series. Bend each lead into a U, hook them together, pinch each lead onto the other with pliers. That is a strong mechanical connection that also takes solder well.
batesman, my approach to your mod would be to take a couple of measurements of the original lamp circuit, then attempt to replicate those same conditions with the LED replacement. Do you have a DMM available? It's not absolutely necessary but would provide actual versus calculated conditions.
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Old 15th October 2012, 06:23 AM   #10
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Hi all! First, I just wanted to say thanks for the great response to my post! I really appreciate the enthusiastic help!

Update:
I replaced the fuse and all is right with my receiver again. I did manage to find the correct incandescent lamp for the amp, so that's working just fine.

For the sake of getting our learn on, the supply voltage is 6.3 volts (checked with a DMM) and two wires are at the lamp socket. The LEDs have a forward voltage of 3.3 and are 25 milliamp. Based on my understanding I could put in a single LED with a 1/8 watt, 150 ohm resistor soldered to the anode. However, that doesn't put out quite enough light - or the light just isn't diffused well. Perhaps due to the narrower (30 degree) viewing angle of the LED?

This is where I went wrong. I tied two 150 ohm resistors to the supply voltage wire, soldered a single LED to each resistor (1:1), then tied the cathode ends to the ground / neutral.

I think in this parallel configuration, I needed a single 1/4 watt, 68 ohm resistor in order for the circuit to work correctly.

This is still relevant for me as I may have to perform the LED swap out on my ST-7300. Low voltage festoon lamps are hard to find! :\ I think I kind of like the "old school" incandescent look though.
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