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I'm new to Electronics. I have recently received a vintage radio Pioneer TR-305. I am trying to replace the panel light bulb inside this radio. I opened the radio panel and found the bulb soldered into the wires. I turned on the radio and tried to measure the voltage using the multimeter from the 2 wires soldered to the bulb. It reads very intermittent and small voltage. I then measured 2 ends of a 1.5V battery and it reads 1.3V - which indicates the multimeter is working fine with the same settings.

Do you know how to measure voltage points inside a radio board? I thought I was the same as on the breadboard but definitely something else is going on here...

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  • \$\begingroup\$ It sounds like something's broken in there besides just the bulb, to me. Or maybe the bulb is one of those type that fail short. \$\endgroup\$ – Hearth Mar 17 '19 at 3:44
  • \$\begingroup\$ Your meter is probably fine (although 1.3 V means your 1.5 V battery is probably nearing its end of life.) A light bulb has a lot of resistance (by design) and therefore most of the voltage drop should appear across its two leads. The fact that you are reading almost no voltage (intermittent might be more of a matter of how well you hold your meter leads against the two contact points, so I'm not making anything of that just yet) across the bulb suggests either that the bulb is internally shorted (rare) or that what's supplying the bulb itself isn't working. That suggests you have more work. \$\endgroup\$ – jonk Mar 17 '19 at 3:52
  • \$\begingroup\$ Some old tube radios had the pilot light wired in series with tube filaments and other parts of the circuit. If a filament is open, the bulb won't light, but a higher voltage should be across it. If the bulb is bad, the radio won't work. You need the exact replacement bulb if it is series with other circuits. You should ask this question on an antique radio forum. \$\endgroup\$ – Charles Cowie Mar 17 '19 at 14:47
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Perhaps (probably) the lamp is being run on AC. Try switching your multi-meter to an AC volts range. You will not see any voltage indicated on an AC range from the battery (on most low-end meters) but it should show the lamp voltage.

6.3VAC would be a common voltage for a North American designed product, not sure about Japanese radios of that vintage. Depending on the type of bulb the operating voltage with the lamp connected may be considerably less than the lamp rated voltage, to get more than the lamp rated life.

The once-ubiquitous #47 bulb had a life at full voltage of 3,000 hours or just a few months 24/7.

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    \$\begingroup\$ Thank you so much Spehro! That was it. I switched to AC mode and it read 6.3v out from across the bulb. I looked up on eBay and saw that they are selling 6.3v AC bulbs for the vintage radios / receivers so that's a thing. It's a Japanese made Pioneer 1970s radio \$\endgroup\$ – Vintage Radios Lover Mar 18 '19 at 5:37

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