The power indicator lamp burnt out on my vintage amplifier. The 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 is 390 ohm 1 watt.

I attempted to back into voltage drop to see if the 390 ohm 1 watt 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 a 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.

 

If the Lamp is not connected, you might be getting open voltage value.
With the lamp connected the voltage will drop to the required value.

 

The resistor doesn't do anything to the voltage when you are measuring across a burned out bulb. So, the source is 18 volts. Assuming the bulb will drop 8 volts if fed 50 mA per the bulb spec, you can limit the current with a 180 ohm resistor. You already have a 390 ohm resistor that will limit an 8 ohm bulb to 25 mA. That much resistance will actually not let the bulb drop 8 volts so you will end up with something less (maybe 6 volts across the bulb) and something slightly higher than the calculated 25 mA across the resistor because the resistor will have to drop more than 10 volts if the bulb drops less than 8 V.

Anyhow, you are safe with an 8V 50 mA bulb if you have an 18V power supply and a 390 ohm resistor in series.

 

The reactance of the capacitors will be 265 ohms @60hz, plus the resistor will limit the voltage across the bulb.

 

Thanks for the reply, not sure I understand everything that GopherT is telling me though. My next problem is finding the right bulb. The closest I have found locally is a 6V 50MA. Will I need a higher ohm resistor than the 390 ohm that is there? My problem is not being able to get at the 390 ohm resistor without dismantling a major portion of the amp. Will adding a resistor in series between pin 11 and the lamp allow me to use a 6v 50ma bulb instead of the specified 8v 50ma bulb. I would need to drop an additional 2v at the same 50ma = 40 ohm resistor .1 watt.

 

That's a fine plan. But use a 1/4 watt resistor. You don't want to be right at the max rating for the resistor. A 2X safety factor or more is fine.

 

I think that's what I will do. Then if/when I find the correct 8v bulb, all I will have to do is pull the additional little resistor out of the path. Thanks all for your advice, I really appreciate it.

 

You might also consider replacing the lamp with an LED. If you are interested, we can help you figure out the little subcircuit that would work (it would involve a resistor, the LED, and another diode and are all things you can probably solder together and put in the same place the old lamp was. But if this is vintage equipment then you might not be interested in something like that.

 

I think that's what I will do. Then if/when I find the correct 8v bulb, all I will have to do is pull the additional little resistor out of the path. Thanks all for your advice, I really appreciate it.

You probably don't need the additional resistor at all. The diode is going to half-wave rectify the current which is going to cut the effective voltage and current seen by the bulb in half.

 

A 6v bulb will be fine,it won't matter, go for it

 

You might also consider replacing the lamp with an LED. If you are interested, we can help you figure out the little subcircuit that would work (it would involve a resistor, the LED, and another diode and are all things you can probably solder together and put in the same place the old lamp was. But if this is vintage equipment then you might not be interested in something like that.

That's an interesting solution. On the contrary there are many collectors of vintage audio gear that upgrade the lighting with LED's especially for tuner dials. I thought about it but felt it was beyond my knowledge. I can solder, I did replace the 15000uF 63V filter capacitors on this amp. Thanks for your help.

 

A white led will drop 3.5V, the total current in the circuit is 27mA, with the resistor and capacitors, you can put your led across the bulb contacts, anode to 11, cathode to 8.

 

The power indicator lamp burnt out on my vintage amplifier. The 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 is 390 ohm

.

It seems to me to be rather peculiar to put a half-wave rectified bulb across half of a capacitive voltage divider!!!

AFAICR: you can get telephone switchboard bulbs about that current rating at much nearer the actual voltage, 28V bulbs are probably a certainty, so only a small resistor would be needed to drop the excess.

Personally; I'd just convert it to white LED - on an isolated low voltage secondary its perfectly safe to use a wattless dropper, you have to include some resistance because the current peaks at a higher value either side of the zero-crossing point where the rate of change is greatest, you also have to protect yje LED from excessive reverse voltage - you can use an inverse parallel pair of LEDs, or one of those LEDs can be an ordinary silicon diode - you just get half the light.

 

A 6v bulb will be fine,it won't matter, go for it

OK, picked up the 6v 50ma bulb at the Rshack. They didn't have any 47ohm resistors in stock. So I decided to take your advice and hand twisted the 6v bulb in place. Measured across the connection and got 3.7 volts. The 6v bulb doesn't seem any brighter than the original 8v, so I soldered it in place. I think I'm good to go. Thanks all for your advice.

 

OK, picked up the 6v 50ma bulb at the Rshack. They didn't have any 47ohm resistors in stock. So I decided to take your advice and hand twisted the 6v bulb in place. Measured across the connection and got 3.7 volts. The 6v bulb doesn't seem any brighter than the original 8v, so I soldered it in place. I think I'm good to go. Thanks all for your advice.

I should have added a photo of the finished job. The amp is on the bottom, Kenwood KA-8006, power indicator lamp is on the left side of the amp. Thanks again.