resistor (Electronics) help here.. (1 Viewer)

[Jay H]

Here's your chance to help out a CompSci guy fix a tuner (an HK Citation 23 tuner) who's backlit LCD screen died because the little light bulb burned out. I opened up the whole unit and found the dead lightbulb and it's attached to two poles on the main PCB, one of them has a resistor inline witht he wire that leads to the bulb, the other side of the bulb is just a straight wire. I'm trying to figure out what kind of lightbulb (volts and wattage) to replace it with. An email to HK hasn't come up yet and I'd figure I'd try here before I try to pry this info out of an authorized service center who I'm afraid will try to get me to bring it in.
Anyway, I used an A/C voltmeter and measures about 32v AC across the terminals but INCLUDING the resistor. Will that affect the voltage reading? All I remember from 8th grade electronics class or Physics is Ohm's law V=IR
The resistor, at first looked to me to be
brown green black red but then I found that tolerances are only gold or silver which probably means the brown I'm seeing is probably gold. That makes it a 2,000,000ohm resister if it is Red Black Green Gold, does that sound reasonable? or am I missing a bar or something? I'm told to look for the gold/silver tolerance bar and read from the other end. The one side of the resistor is covered with some shielding but it doesn't look to be covering another bar though..
any ideas of the voltage, I can probably guestimate the watts needed by replacing the bulb (not in a socket) and seeing if the LCD is readable...
Jay

 

[Bill_Weinreich]

Four bars indicate that a tolerance is used. You normally read them from the end closest to a lead.

Have you tried reading the voltage before the resistor? The voltage will read lower after the resistor.

BILL

 

[John Miles]

Couple of points:
1) 32V is an odd (high) voltage for a small pilot light. Usually this would be from 6 to 24V. It's possible that they're running a 24V bulb or even a 12V bulb with a resistor in series to drop the voltage.
2) What color is the resistor's body? If it's a light tan, then yes, there would ordinarily be a gold or silver tolerance marking at one end or the other. If it's a metal-film resistor with a blue body, then the procedure for reading its value is different.
3) It's not a 2 megohm resistor if it's in series with a light bulb. By the same token, the voltage will NOT read any lower on either side of the resistor, because your meter won't present enough of a load resistance to cause the resistor to drop any significant voltage. V (voltage drop across a resistor) = I (current through the resistor) times R (the resistor's value in ohms). A light bulb will draw a significant fraction of an amp, but your meter will likely draw only a few microamps, so regardless of the resistor's value, the V drop won't be very high when you measure it with the meter.
4) Finally, because this appears to be a really weird pilot-light circuit, I wouldn't substitute a non-OEM-spec light bulb without careful consideration. The resistor may be there only to improve bulb life by reducing its cold inrush current, or it may be needed to actually reduce the supply voltage to meet a particular bulb's spec. In the latter case, its value will be intimately related to the light bulb's current-consumption spec, which we don't know. Using the wrong bulb could smoke the resistor. Sight-unseen, I wouldn't recommend making this type of repair without a service manual at hand (which you can order by calling the manufacturer's parts department).

 

[Jay H]

Bill W, I would but the lead closest to the bulb is shielded by some shrink wrap. Both leads are soldered to two posts connected to the main board so I convieniently used those.

John, yup, it's a tan resistor kind of about 1.5-2mm in diameter. I do happen to have a cheap RS multimeter set to the 50v AC scale.. I did it a couple times but perhaps the meter isn't calibrated right. In either case, I'm still awaiting HK's response and failing that will try to see if I can order the part from my local authorized service center. I presume they must have or have available the service manual and can simply order the part by phone and I can come pick it up. Too bad it isn't socketed, I guess they don't expect to replace it for some reason.

Thanks guys!

Jay

 

[Holadem]

Jay said

A light bulb will draw a significant fraction of an amp, but your meter will likely draw only a few microamps, so regardless of the resistor's value, the V drop won't be very high when you measure it with the meter.

Correct me if I am wrong, but I believe he was wondering if the voltage across the bulb was the same as the one accross the bulb AND the resistor.

The answer is NO, it's not. You need to measure directly accross either the bulb or the R (you need to read its value correctly). Use Ohm law to find the current through the bulb, that might help with the specs...

32 VAC (why AC?!!) is really strange. Perhaps there is more to it than just a burnt out bulb.

--
Holadem

 

[John Miles]

Well, he did say the bulb was burned out. So yes, right now, the voltage across the bulb is the same as the voltage across the bulb and resistor.

 

[Jay H]

Well, after talking to an authorized service center, a new light bulb (official HK part) will be $4.95 + shipping, however it would seem silly to me to pay shipping that is probably more than the cost of the unit. So I decided to call HK since the service center couldn't tell me the specs on the bulb. I call Harmon International in Long Island (owners of JBL, Harmon Kardon, Infinity...) and they tell me it's a 14V bulb and also give me a parts number just in case. I am gonna go to my local electronics store and see if I can find a 14v bulb with enough wattage to work. Anybody see a problem with this? I was recommended by some people to see if any automotive lightbulb might work if autos use 14v bulbs.. But anyway, wish me luck and I'm open to any guesses or suggestions of what wattage to try to find!

Jay

 

[John Miles]

Again, assuming there's a resistor in series with the bulb, the current drain (or the wattage, assuming you're comparing two 14V bulbs) does need to match. Matching the physical size of the bulb may not be enough.

I think you made a bad call by not taking them up on the $4.95+shipping. How much is your time worth that you'll spend combing the shelves at RS and various other stores for a part that may not be correct?