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/Amplifiers/Technics SU-7700 Service manual.pdf
Thank you kindly, and in advance!
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/Amplifiers/Technics SU-7700 Service manual.pdf
Thank you kindly, and in advance!
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!
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!
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.
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.
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?)
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?)
Last edited:
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.
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.
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.
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.
An LED, like other diodes, is a one-way device - it allows current flow in one direction only. You have an AC power source - current flow in two directions. And the lamp circuit is in parallel with the relay circuit - a change in one will affect the other. This latter is what caused the problem.
Well, I just connected everything together and I have found that I have no sound coming from the left channel. All of the fuses check out. I don't know if what I did caused this issue, but I am inclined to think so, as both channels were operational prior to my stint with the LEDs.
Using the wiring diagram (which I'm still learning to read), how should I start troubleshooting this issue?
Again, my thanks in advance.
Using the wiring diagram (which I'm still learning to read), how should I start troubleshooting this issue?
Again, my thanks in advance.
Update:
I did some additional research tonight and found a number of Technics receivers similar to mine that had this problem. Turns out that the "TAPE REC" and "TAPE MON" switches affect the audio path (though I'm still not sure why). I isolated the problem to the TAP MON switch and after a little switch jiggling, the audio returned to the left channel. So now I know this is a job for DeoxIT.
Just out of curiosity, has anyone successfully used other electrical contact cleaner instead of DeoxIT? <--That stuff isn't cheap.
Thanks!
I did some additional research tonight and found a number of Technics receivers similar to mine that had this problem. Turns out that the "TAPE REC" and "TAPE MON" switches affect the audio path (though I'm still not sure why). I isolated the problem to the TAP MON switch and after a little switch jiggling, the audio returned to the left channel. So now I know this is a job for DeoxIT.
Just out of curiosity, has anyone successfully used other electrical contact cleaner instead of DeoxIT? <--That stuff isn't cheap.
Thanks!
i use the aliphatic hydrocarbon spray CRC cleaner from the lumberyard. $.99 a can sometimes. Most problems are household dust caused by skin flakes, smoking, etc.
Warning, it is highly flammable, no smoking, open flame, pilot lights, electricity on or off within 10 M. Set up a fan before hand to dissipate the fumes.
Don't use the $25 a can brominated hydrocarbon CRC cleaner from the electrical supply. It dissolves plastics like PVC and styrene.
Deoxit is chemically active, I don't want to use it. If it eats the oxide off, it probably promotes corrosion in the future so you'll need it again in a couple of years. The active ingredient is proprietary, so I can't do research on it.
Ethanol is fine but doesn't spray.
For actual corrosion, abrasive rubbing with an eraser is my preferred method. Requires access, though. I give up on copper or tin switches after too much time, they need to be gold or rhodium plated for audio preamp use.
Warning, it is highly flammable, no smoking, open flame, pilot lights, electricity on or off within 10 M. Set up a fan before hand to dissipate the fumes.
Don't use the $25 a can brominated hydrocarbon CRC cleaner from the electrical supply. It dissolves plastics like PVC and styrene.
Deoxit is chemically active, I don't want to use it. If it eats the oxide off, it probably promotes corrosion in the future so you'll need it again in a couple of years. The active ingredient is proprietary, so I can't do research on it.
Ethanol is fine but doesn't spray.
For actual corrosion, abrasive rubbing with an eraser is my preferred method. Requires access, though. I give up on copper or tin switches after too much time, they need to be gold or rhodium plated for audio preamp use.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.