Kenwood kt-5500 & ka-5700 bulbs out but test ok?

markmark

Active Member
Morning all,
I've picked up a Kenwood ka-5700 amp & kt-5500 tuner from 2 different places. The tuner has 3 bulbs & none wiil light. 2 of those bulbs test ok with an ohm meter. Are they like old christmas tree lights? If one goes out, none of them will work?
The amp only has only one bulb (not lighting) but it ohms out good.
I don't want to start buying bulbs or cutting wires & swapping bulbs around to test without knowing it's the bulbs & not something else.
Thanks for any advice,
Mark
 
Hi Mark,

I'm not familiar with the KT-5500, but for the KA-5700, there's not much there. You're looking at the power transformer, a fuse and the bulb.

You need to break the circuit to check the resistance of the bulb. Either unsolder it or cut a wire. Not sure if you did that or not.

Try the following:

Check the 500mA fuse. That's easy to remove and test.

If that's good, check the voltage across the bulb. You should see about 8VAC. If you do, the bulb needs to be replaced.

--Jeff
 
Can you measure voltage or only resistance? As said, the amp bulb comes straight off the transformer with the 500ma fuse in series, so only a couple things to go wrong there.

Not sure about the tuner, but if you follow the voltage you should be able to sort out whether they are in series and need three good bulbs to work.
 
When you measured the bulb, did you remove it from the circuit first? If you didn't, the bulb could be an open and you're actually seeing the 8V secondary tap on the power transformer.

It is possible to measure the bulb while it's still connected if you remove the fuse and pick the correct spots for your test leads.

I'd still encourage you to measure the voltage. I recently had an intermittent bulb. The resistance looked good when I removed it for testing. When I put it back in I could turn it off and on by tapping on it.

That bulb is over 30 years old and odds are it just needs replacing.

--Jeff
 
If you have current to the bulb and it is not lighting, I assume it is burned out. I would replace them all for even lighting. Should only cost you $1-2 per bulb. I got a close replacement for my receiver at Radioshack and soldered it in.
 
Christmas in July

Morning all,
The tuner has 3 bulbs & none wiil light. 2 of those bulbs test ok with an ohm meter. Are they like old Christmas tree lights? If one goes out, none of them will work?
KT-7500 and KT-6500 tuners have dial lamps in series (like old Christmas tree lights); my guess is the KT-5500 also has series lamps.
 

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dial lamps in the Kenwood KT5500 are in series as well

KT-7500 and KT-6500 tuners have dial lamps in series (like old Christmas tree lights); my guess is the KT-5500 also has series lamps.

dial lamps in the Kenwood KT5500 are in series as well
 
Bulbs are wired in series across the low voltage AC

My Kenwood KT-6550 has three, 8V 0.3A bulbs. They are wired in series and all are in series with an upstream resistor, then that goes to the low voltage secondary winding on the transformer. So if one is burned out, the others will extinguish as a result.

I think the total voltage is around 24VAC but it will drift higher with no load.

You can use LEDs nowadays but you must know what you are doing or you will burn them out.

I may go with LEDs as mine (the Stanley incandescent bulbs) are out too. Though I have a KT-5500 that I may borrow one or two from if the bulbs are the same. I think they are.

An engineering note: Wiring bulbs in series has its pluses and minuses. It reduces parts count and reduces the load on the transformer so that can be a lower power (VA) unit.

One thing to consider is that the differences/tolerances in the filaments will cause unequal voltages on each bulb. This causes premature death of the bulb that has the highest voltage across it (since the current is the same through all the bulbs). Plus the empty feeling of having no lights working when one burns out.

I have had some Technics receivers whereby if a bulb went out, the others remained lit. I guess those are in parallel and not in series. One can rewire the tuner lights and use three 24VAC bulbs in parallel if one desired.

Interesting point is that if there are other circuits using the 24VAC supply, they will be affected by improper loading of that winding. Right now, my DC is running a few volts higher than normal because there is not loading from the lights to lower it.

I wonder if the incandescent lights remaining out or not replaced can cause other components to be over stressed due to the higher DC voltage on them.
 
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Kenwood KT-6550 Display lights Upgrade - NICE!

Today I had a few hours to replace the 3 blown Stanley 8V incandescent bulbs. These are normally wired in series. But I wanted to do something better than that. I rewired the circuit so that each LED is fed independently with 28VAC. So basically each LED circuit is in parallel with 28VAC power supply after the big 10 Ohm resistor. This is not necessary for an LED circuit but it would have been useful for the failure-prone 8V incandescent original bulbs. This way if one failed, the other two would stay lit. But it makes it easier to run the LEDs with different current if needed.

1) I used bright White LEDs from Radio Shack - two 5mm and one 10mm but you can really use whatever color you want as long as they are not dim lights.

For the main display I used two high brightness 5mm white LEDs. Radio Shack P/N 276-0017.
For the meters I used one ultra high brightness white 10mm LED. Radio Shack P/N 276-0005.

People may tell you to use 10mm LEDs for this project. But the bright 5mm LEDs work well too. The 10mm may fit better in the rubber holders but it turns out the angle was wrong and I did not want to bend the metal. Using the two 5mm for the main display lighting allows play to angle them perfectly.

Also, don't use LEDs with diffused lenses. They throw the light all around. Use ones with clear-like lenses that focus the light. You want the light focused toward the acrylic light carrying material.

I reused the three rubber holders as these came in handy to keep the leads separated so they don't short out and to hold the LEDs. And a hot glue gun to secure the rubber holders in place.

2) Coming off of the transformer is around 28VAC for the front panel lights. A 10 Ohm resistor was in series with the original three, series connected incandescent bulbs.

3) I left the 10 Ohm resistor in the circuit, then connected two 1N4007 diodes in series with the resistor (yes, I am anal as one would do fine). Then I added three 2k,2W resistors. Each 2k resistor feeds each LED's anode (+). The 3 cathodes (-) I tied all together and connected to the other side of the 28VAC.

4) The result is a nice, well-lit display and gauges. This circuit will outlast all of the circuits of this tuner. One thing I noticed is the meters (which are using the 10mm LED) are a little bit dimmer than I would like but still nice. I think it is because the 10mm LED focuses the light two sharply on the concave "light pipe" of the meters. So maybe two 5mm LEDs there in series facing each side of the concave section would do the trick.

See the pictures. It really looks good.

I guaranty you that your tuner will sound better after making this modification. Okay I am just kidding with that part folks. Maybe it is just mental.

:D

See pictures attached.
 

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