Kenwood KA-8006 integrated amp power indicator lamp burnt out. Service manual calls for 8v 50ma wired pilot lamp. But I'm getting 18 volts across the connecting pins. I have very little electronics knowledge.
Before replacing it I measured voltage across pins 11 and 8 in the attached schematic. Expecting to get about 8 volts I was surprised to get a reading of 18volts. The resistor in series is 390 ohm 1 watt.
I attempted to back into voltage drop to see if the resistor is the problem, but I'm not making sense out of that. So before I start replacing lamps to have them burn out what should I go after -These components are on the power supply board, which will be a PIA to get at, sits vertically with the business side of the components between the board and transformer.
Other than this problem the amp is working very well, at least to my ears.
Thanks for any help.
Before replacing it I measured voltage across pins 11 and 8 in the attached schematic. Expecting to get about 8 volts I was surprised to get a reading of 18volts. The resistor in series is 390 ohm 1 watt.
I attempted to back into voltage drop to see if the resistor is the problem, but I'm not making sense out of that. So before I start replacing lamps to have them burn out what should I go after -These components are on the power supply board, which will be a PIA to get at, sits vertically with the business side of the components between the board and transformer.
Other than this problem the amp is working very well, at least to my ears.
Thanks for any help.
Last edited:
There will be no problem with the measured voltages. If the bulb is supposed to be 8 volt 50ma then that is what is needed. The higher voltage reading is simply because no current is being drawn. The resistor drops the excess voltage but that volt drop occurs only when current is drawn.
The bulb needs to be correct for it work though, the 50ma bit is critical. If needed you could fit an LED but you would need to greatly increase the resistor value.
(your circuit is slightly incorrect as drawn. The bulb won't couple through a 0.01uf cap. No current could flow if it did)
The bulb needs to be correct for it work though, the 50ma bit is critical. If needed you could fit an LED but you would need to greatly increase the resistor value.
(your circuit is slightly incorrect as drawn. The bulb won't couple through a 0.01uf cap. No current could flow if it did)
Correct Schematic
Thanks for the reply, the closest I can find locally is a 6v 50ma bulb at RadioShack. Will that work instead of the specified 8v lamp?
Here is the full schematic of that area from the service manual:
Thanks for the reply, the closest I can find locally is a 6v 50ma bulb at RadioShack. Will that work instead of the specified 8v lamp?
Here is the full schematic of that area from the service manual:
A 6 volt one would burn out more quickly. If the bulb looks like it will fit then all you need to do is increase the value of the 390 ohm resistor. Your next common value up is 470 ohm. I would try that. Make sure it is a 1 watt (or more) type. If the bulb looked to bright on that then go up to a 560 ohm.
That's what I was thinking, however I will have to dismantle the front end of the amp to get at the 390 ohm resistor. Can I add a resistor in series between pin 11 and the lamp to accomplish the drop from 8v to 6v? I calculated 40 ohm .1 watt, or something close to that.
That's fine doing that. 39 ohm is the closest preferred value or 47 ohm for the next above. I'd go for at least a 0.25 or 0.5 watt to avoid failure.
(if you run a bulb at a little lower than its rated voltage then the life of the bulb rises dramatically. For example a 8 volt bulb might be rated at 2000 hours, run it on 7.5v and it could be nearer 3500 hours. Drop that to 7v and it could go to 6000 hours. It makes a massive difference)
(if you run a bulb at a little lower than its rated voltage then the life of the bulb rises dramatically. For example a 8 volt bulb might be rated at 2000 hours, run it on 7.5v and it could be nearer 3500 hours. Drop that to 7v and it could go to 6000 hours. It makes a massive difference)
You could add a couple of forward-biased 1N400x diodes as voltage droppers.
Or, put a 3.3V zener in series. The lamp brightness may be reduced a little but lamp life will be extended.
Or, put a 3.3V zener in series. The lamp brightness may be reduced a little but lamp life will be extended.
Mooly, Thanks so much. So if I go with a 47 ohm 1/4 watt resistor in series, then the 6v bulb would be seeing about 5.7 volts. I think I will go that route. If or when I find the correct 8v bulb, all I will have to do is take the 47 ohm resistor out of the path.
- Status
- Not open for further replies.