Hi all,
I have been working on this amplifier for quite some time and I'm close to have it all restored. In my learning journey (I am quite a newbie in electronic), I have learned a few things but I'm still struggling at understanding some concepts.
To give you some background, I ran the amp in over voltage for a while until Moolie helped me out figuring that I had a broken regulator (23V instead of 15V - See this thread). This may have damaged the MPX circuit. I had to replace Q302, Q303 and Q304.
I have also stupidly broken the leads of a polystyrene capacitor (C320) that I replaced too (for those interested see this thread).
The manual calls for a 4.5V 40mA lamp. It was burned out. I replaced it with a 5V 60mA lamp (perhaps 50mA - I'm not sure).
Basically, the stereo light circuit is supposed to work like this:
And it is working but I get a very low voltage for the light (2.5V instead of 4.7V shown in the service manual) and therefore a very dimmed light.
My questions:
I have been working on this amplifier for quite some time and I'm close to have it all restored. In my learning journey (I am quite a newbie in electronic), I have learned a few things but I'm still struggling at understanding some concepts.
To give you some background, I ran the amp in over voltage for a while until Moolie helped me out figuring that I had a broken regulator (23V instead of 15V - See this thread). This may have damaged the MPX circuit. I had to replace Q302, Q303 and Q304.
I have also stupidly broken the leads of a polystyrene capacitor (C320) that I replaced too (for those interested see this thread).
The manual calls for a 4.5V 40mA lamp. It was burned out. I replaced it with a 5V 60mA lamp (perhaps 50mA - I'm not sure).
Basically, the stereo light circuit is supposed to work like this:
And it is working but I get a very low voltage for the light (2.5V instead of 4.7V shown in the service manual) and therefore a very dimmed light.
My questions:
- First, the basic question: How come when stereo operation is not detected the voltage is supposed to be 14.2V instead of 4.7V at the lamp and it is not burning out the lamp?
- I have 1.1V instead of 3.3V at the emitter of Q302 (freshly replaced 2SC710 with 2SC458), what could cause the lower voltage?
- I assume the 2.5V at the lamp is caused by the previous observation but could it be because I did not use the exact specifications for this lamp?
- What else would you recommend to verify?
Thank you Extreme_boky. That makes sense.
Strangely, this is what the service manual calls for and I can see many posts on the internet for this value:
But you guys are pointing me in the right direction. The bulb I used is 5V 60mA. I thought it would be close enough but it looks it is not.
I will try the 110 ohms resistor test suggested by Rayma and I will let you know.
Strangely, this is what the service manual calls for and I can see many posts on the internet for this value:
But you guys are pointing me in the right direction. The bulb I used is 5V 60mA. I thought it would be close enough but it looks it is not.
I will try the 110 ohms resistor test suggested by Rayma and I will let you know.
The mfr can use any light bulb they want in the design, there's no right or wrong.
But because of the series and parallel resistors connected to the light bulb,
the circuit is very sensitive to the exact characteristics of the actual bulb type used.
So you have to get the exact right one. I'd buy several of them.
But because of the series and parallel resistors connected to the light bulb,
the circuit is very sensitive to the exact characteristics of the actual bulb type used.
So you have to get the exact right one. I'd buy several of them.
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Yes, it is good with value of 4.65 kohms.
I did replace the bulb with a 109 ohms resistor and you were right, I get 4 volts now. So definitely, the current rating of the bulb has a big influence on the voltage.
Given the 4 volts I got, I think 4.5V was just right and not so dim.
Now, I also did a test with a 5mm led and a 50 ohms resistors (that's what I got on hand). Voltage is 5 Volts and the led is a bit brighter than the fm-am light indicator. Also it won't shut off completely.
V-Vf=RI
4.5V-3.3V=R*0.02A
R=60 ohms
Is there anyway to tame it down a bit and to have it shut off? I think I have seen someone somewhere saying that by adding a resistor somewhere, it will shut off but I can retrieve it.
As "rayma" says modern LEDs will light with almost no current. In your case current is leaking to earth via R318 - R323 - R322 when Q303 is switched off. You could increase R318, its there to discharge C318 so the light turns off quickly when stereo disappears. If you are planning to stick with the LED you will need to increase R318 by quite a bit for it to turn fully off which will mean C318 not discharging immediately the signal goes to mono so there will be a delay in the LED turning off. Try and see, its matter of balance.
The resistor R1 must not be shorted out by the transistor. Correct placement is in post #15.
LEDs have limitations for the current they can handle. The absolute maximum current is usually 20mA.
So you need to keep the LED current when "on" between 1mA and 10mA. This current is equal to ( V1 - 2 ) / R1.
The 1mA will be rather dim, and the 10mA will be rather bright. So choose around 3mA, which for V1=5V, gives R1 = 1k.
The left end of R2 is connected to a control DC voltage, which turns the transistor on and off.
If the control DC voltage is a few DC volts positive, say +5V, the transistor will be on, and the LED will be off.
If the control DC voltage is 0VDC, or ground, the transistor will be off, and the LED will be on.
You have to supply the control DC voltage.
LEDs have limitations for the current they can handle. The absolute maximum current is usually 20mA.
So you need to keep the LED current when "on" between 1mA and 10mA. This current is equal to ( V1 - 2 ) / R1.
The 1mA will be rather dim, and the 10mA will be rather bright. So choose around 3mA, which for V1=5V, gives R1 = 1k.
The left end of R2 is connected to a control DC voltage, which turns the transistor on and off.
If the control DC voltage is a few DC volts positive, say +5V, the transistor will be on, and the LED will be off.
If the control DC voltage is 0VDC, or ground, the transistor will be off, and the LED will be on.
You have to supply the control DC voltage.
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I wanted to do the opposite when I had +5V, I needed the led to be on and when I had less (it can't get to 0V), I wanted the led to be off. So I re-arranged a bit my circuit.
But no matter how high I would increase R2 (I even tried with 1M ohms), it seemed that Q1 would always be in conduction and the LED would not turn off. I started to be desperate when I found a simple solution on the web that worked like a charm!
If you just add a zener diode in series with resistor, it blocks the current until the zener voltage is reached:
As you can see, this way, I get about 2mA in the led (My actual values are a bit different from LTSPICE).
Assembly before wrapping it up in heat shrinkable tubing (please forgive my poor soldering skills):
1K ohms resistor - 8.2V Zener - 5 mm LED
And the final look (orange fm light is incandescent, red is the stereo indicator):
Your advice, Rayma, to set it to about 3mA was spot on. Same glow as the other one. A big thanks to you (and to Extreme_boky and Totally Analogue) for all the help and the precious advises.
But no matter how high I would increase R2 (I even tried with 1M ohms), it seemed that Q1 would always be in conduction and the LED would not turn off. I started to be desperate when I found a simple solution on the web that worked like a charm!
If you just add a zener diode in series with resistor, it blocks the current until the zener voltage is reached:
As you can see, this way, I get about 2mA in the led (My actual values are a bit different from LTSPICE).
Assembly before wrapping it up in heat shrinkable tubing (please forgive my poor soldering skills):
1K ohms resistor - 8.2V Zener - 5 mm LED
And the final look (orange fm light is incandescent, red is the stereo indicator):
Your advice, Rayma, to set it to about 3mA was spot on. Same glow as the other one. A big thanks to you (and to Extreme_boky and Totally Analogue) for all the help and the precious advises.