Al-811 meter lamps

[WB4CTX]

I started to replace my blown meter lamps with LEDs. Before I replaced them I decided to check the voltage across the lamps. To my surprise instead of 12v or so expected my meter is showing 24v across each meter. I decided not to replace the lamps until I figured out what is going on. Anyone who can help me? This is an older al-811.

[K4RVN]

You need to check the lamp voltage under load in case there is a dropping resistor in the meter supply line. Wire a few 12 volt leds in series and connect them then check the voltage if you have them on hand. Call Ameritron and ask them is another choice.  I have an 811 but cannot make any sense out of the schematic to see how the meter lamps are supplied. I just could not find them but I'm not that good at being a tech. If you have one good  spare bulb you can use that to check the voltage under load. You can calculate a dropping resistor if you know the milliamps your leds pull, or use a suitable pot if you have one in the junk box. The leds I bought are rated at 12 volts.
Maybe someone who has done it and remembers what they did will post. I will be anxious to take notes for my 811 amp.

Frank

[WB4CTX]

You need to check the lamp voltage under load in case there is a dropping resistor in the meter supply line. Wire a few 12 volt leds in series and connect them then check the voltage if you have them on hand. Call Ameritron and ask them is another choice.  I have an 811 but cannot make any sense out of the schematic to see how the meter lamps are supplied. I just could not find them but I'm not that good at being a tech. If you have one good  spare bulb you can use that to check the voltage under load. You can calculate a dropping resistor if you know the milliamps your leds pull, or use a suitable pot if you have one in the junk box. The leds I bought are rated at 12 volts.
Maybe someone who has done it and remembers what they did will post. I will be anxious to take notes for my 811 amp.

Thanks for the reply. I would guess the LEDs draw no more than 20-25 ma current.


Jim

[K4RVN]

Jim,
The meter lamps could also be 12 volts in series to explain the 24 volts. I checked my Al80 A schematic and parts list but they have a 12 volt supply. I could not find anything in my AL811 parts list. That does not mean it's not there, but I could not see it.

Frank

[WB4CTX]

Thanks I will look at that.
Jim

[N3QE]

I started to replace my blown meter lamps with LEDs. Before I replaced them I decided to check the voltage across the lamps. To my surprise instead of 12v or so expected my meter is showing 24v across each meter. I decided not to replace the lamps until I figured out what is going on. Anyone who can help me? This is an older al-811.

The voltage driving the meter lamps is unregulated. The DC supply uses a voltage doubler circuit and shows a very high drop between no load and regular load. It can be as high as you observed when the amp is not keyed and the lamps are not drawing current. It will drop substantially when the lamps are drawing current and a little more past that when the amp is keyed and then modulated.

The 12V bulbs suggested here will have a long life, longer than the nominal 8V bulbs also commonly available: Suitable meter lamp replacement suggested in a different thread: http://www.mcmelectronics.com/product/25-455

[WB4CTX]

Thank you. I think that solves my question. I was measuring with no load since the lamps are blown. I have a three led strip for each meter so I will just put them in as I intended. Thanks so much for the explanation.
73
Jim
Wb4ctx