Yamaha CR-800 bulb voltage 18,5 Volts?

[Mr. Yamaha]

Hi there, scored a nice mint CR-800 receiver yesterday. Sold as broken. Came with the original manual, which is also mint.

Vacuum cleaned, De-oxit in the pots and switches: it works like a charm. Nice.

All lamps are out, so I thought about a LED upgrade as I did for my M-4 and C-4. For determining the resistor value I measured the voltage which is coming from the points where the lamp wires are connected and I read 18,5 V.

That seems a bit much isn't it? Is it adjustable or is it the way it should be?

The service manual gives me nothing about this.

Thanks!

 

[eizner23]

I am not familiar with that model. Is that 18.5VAC? And how many LEDs do you plan to drive?

 

[ConradH]

I'm guessing based on other units, but they probably ran the bulbs in series, so 8 V bulbs, and the load of all the bulbs will drop the voltage closer to 16 V.

 

[Mr. Yamaha]

I am not familiar with that model. Is that 18.5VAC? And how many LEDs do you plan to drive?

Hi there, no, it's DC. I'm going to use 4 LED's (not in series).

I'm guessing based on other units, but they probably ran the bulbs in series, so 8 V bulbs, and the load of all the bulbs will drop the voltage closer to 16 V.

The Yamaha's I have / know mostly have 12V or 14V on the bulbs. In this CR-800 they are not in series, they all have an own + and - lead from the same points on a board. Between those points I read 18,5V.

 

[Mr. Yamaha]

I overlooked something in the service manual. It looks lik it should be 12V. Maybe resistor R943 or R945 is defective? I'm not really an expert on resistors. Can de be 'broken' and resist less?

 

[petehall347]

r945 looks like its there to limit the current
when all lamps installed the voltage should drop .

 

[Mr. Yamaha]

All the lamps are installed, but they're all broken and have a black fog inside the glass.

How would the voltage drop when the lamps are installed, can you explain? I thought the supply voltage remains what it is, no matter what kind of 'users' are connected.

 

[ConradH]

Resistors don't often go down in value. That R945 in the ground lead will lower the voltage on the bulbs, which I assume are 12V bulbs all in parallel. If you measure the voltage with no bulbs present, you'll get 12V, but with bulbs installed there'll be a voltage drop across R945 and you'll see less. For an LED conversion there are several ways to go about it. The dial pointer should have its own resistor to limit the current and set the desired brightness. The other three could be wired in series and use one resistor. R945 could be jumpered, or be part of the resistance, which would provide some short protection.

 

[avionic]

The tuning and signal meter lamps are 12v/ 60ma X three paralleled. The dial pointer is a single 12v/60ma paralleled with the other 3 lamps. Supply voltage is 12vdc.

 

[avionic]

If you measure the voltage with no bulbs present, you'll get 12V,

Probably higher without the lamp load.

 

[ConradH]

True, unless we suspend ohms law, but I figure the 12 is regulated and will stay somewhere close. The unregulated AC bulb supplies I see get loaded down a lot more.

 

[Mr. Yamaha]

But that still doesn't explain why I read 18,5V on the board with defective lamps installed ;-) I'm going for LED's all parallel with each an own resistor.

But should I calculate the resistance with 18,5V minus forwarding voltage or 12V minus forwarding voltage?

 

[ConradH]

It looks like the "+12R" supply is just an unregulated half-wave rectified supply, and a 3.9 ohm protective resistor. Without the bulb load it probably rises to the 18 you're seeing. I'd replace the 470 uF cap if you haven't, as they tend to get hammered on half-wave supplies. Since the LEDs will use far less current, I'd calculate everything based on maybe 17 volts, but being completely anal, I'd actually clip lead to the supply and dummy up the LEDs on a plug-in proto board to find the resistor values for the brightness and current I wanted. A sane person would just subtract the fwd LED voltage from 17 and call it a day.

 

[petehall347]

does that 12v supply other stuff too ? i tried to follow the schematic but it is hard to follow . it made me wonder if its important or not to have the correct lamps to drop the voltage .

 

[ConradH]

It looks like the only other thing that supply runs is the relay, which hopefully isn't too fussy. See if you can find out what voltage the relay is- it might be desirable to load it down a bit more than just the LEDs. (relays often have the voltage printed on them somewhere, if you can see it, or sometimes encoded in the part number.)

 

[Mr. Yamaha]

Thx guys!

 

[Mr. Yamaha]

It turned out well:



More info:

Thread

 

[ConradH]

Nice!

 

[sssboa]

So you guys use leds for 12V? I also read around 18V with my multimeter on either pair of lamp lugs in CR-800. I don't understand how it could drop under load like it was coming from a battery, especially on the pointer lugs where only 1 lamp is connected.

I will only use leds for the 3 meter lamps to protect them from heat. I installed leds already but they're very weak in comparison to incandescent lamps, barely visible with ambient light on. Not sure what voltage these leds are for, but guess it's 18V as I had looked for such.

Just ordered 12V leds and will see, I use warm white light leds, which give really greenish old copper effect on the silver reflection surface of the meters. I was thinking of yellow leds to have an effect closer to incandescent lamps, what do you think?

In the picture, leds in meters (barely visible light unless in darkness) and incandescent lamp in the pointer.

 

[avionic]

does that 12v supply other stuff too ? i tried to follow the schematic but it is hard to follow .

Warning Zombie thread...



12 volt DC supply is just for the lamps,3 red LED's and the protect relay.