Cheap & quick power supply tester

[Paul Dietenberger]

Before you fire up a radio for the first time, you need to see what condition the power transformer and other power supply components are in. See here for more details:
http://www.antiqueradio.org/dimbulb.htm

Many restorers will buy and use a metered variac for this purpose. But if you're not sure if you want to pony up the money to buy one, here is a metered dimbulb tester. You can build it with a couple of bucks worth of hardware store parts and two multimeters (which you should have anyway.) You don't even need to know how to solder.

Schematic (links below photos go to larger versions):

http://personalpages.tds.net/~pdieten/schematic.jpg

Outside:

http://personalpages.tds.net/~pdieten/dimbulb-case.jpg

The case is a $4 wooden cigar box from the craft store, with a coat of Polystain slapped on in case anything gets spilled on it (of course, the stuff is so hard to work with it looks like I spilled the Polystain...)

Anyway. In the case, top center is a lightbulb socket. Lower left is a single-pole, dual throw switch. The top setting goes through the bulb, then through the ammeter, and on to the AC socket. The bottom setting of the switch goes directly to the ammeter, bypassing the bulb, and on to the socket. This lets me read the current a healthy set is drawing. Center position, of course, turns the unit off.

Bottom center are jacks to plug in a meter set to read AC amps. Lower right is where the radio under test is plugged. Right side are jacks in parallel with the outlet; a meter set to read AC volts can be attached here to read the voltage at the socket.

Inside:

http://personalpages.tds.net/~pdieten/dimbulb-inside.jpg

In use, with a 40W bulb in the socket and a chassis plugged in, with only the rectifier tube in place. The meter in front is in the ammeter jack, set to read AC amps. The one in the back is in the voltmeter jack, set to read AC volts.

http://personalpages.tds.net/~pdieten/dimbulb-in-use.jpg

Questions/suggestions are welcome. It's just something quick-and-dirty I use to check new radios coming in the house.

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Paul

http://personalpages.tds.net/~pdieten

[Norm Leal]

Hi Paul

An AC amp meter may not be available. Most of us have DC meters even if they are low current.

A low current DC meter can be used to read AC current. With a silicon diode bridge (5 amp or more) wired in series with the radio connect the meter between + and - terminals on the bridge. If the meter is in MA rather than amps add a shunt across its terminals. Use a low value resistor, fraction of an ohm or even wire, as a shunt. Accuracy isn't important.

Meter reading is relative. A radio with shorted transformer will read very high. I adjust a shunt so an operating radio reads less than half scale on the meter.

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Norm

[Paul Dietenberger]

Thanks Norm. In my case I found that Sears Craftsman meters with 10amp AC ammeter functions are available for $20 - an irresistible bargain IMO. The one in front in the last picture above is the $30 model which autoranges and goes up to 20A. Not true RMS, but I think they'll get the job done.

Is something like Radio Shack's 276-1181 bridge rectifier suitable for what you describe? (6A, 200PIV)

I know that just for power-supply-problem checking the actual meter reading isn't really important; just that if the bulb lights the power supply is no good. But I think it's kind of nice to see just how much current a nominally functioning radio is pulling.

It's easy enough to figure out what the meter should read when the tester is running in ammeter-only mode. Find the label or the note on the schematic where it is stated how many watts the set requires. Divide by 120 (or whatever your AC line voltage is) and there's your amps. For example, I had a 60W radio plugged into the box. 60/120 = 0.5 amps. If the radio read much higher than this, I'd have to look for the reason why.

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Paul

http://personalpages.tds.net/~pdieten

[Norm Leal]

Hi Paul

That's a good solution.. Nice price for AC amp meter.

A 6 amp @ 200 PIV bridge will work. This is a case where very little voltage is across the bridge.

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Norm

[Dennis Daly]

Quite an elegant homebrew setup there, Paul.

Actually?- If I used ramp-up methods for "triage" purposes on a new find, I would opt for the "dim-bulb" method rather than a variable autoformer alone.
At least with the "dim-bulb," there is an additional apparent visual indication and a "buffering" effect of sorts, when using the "dim-bulb," that one wouldn't have with just the variable autoformer (variac) and ammeter alone.

Den

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[Paul Dietenberger]

Thanks, I guess it turned out pretty well for something I threw together in an hour and a half, much of which was spent searching the shop for my hole saw and spade bits. It's compact, easy to handle and does exactly what needs to be done. I'm pleased with it.....

[Leigh]

[Paul Dietenberger]

Hmmm. They also have a 4A 400PIV bridge, but if that won't work either then you'd have to mail order a part (unless you happen to have access to an electronics house), which kind of kills the whole quick-and-easy factor. Not that it matters if you have a meter that reads AC amps like I do....

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Paul

http://personalpages.tds.net/~pdieten

[Norm Leal]

Hi Paul

A 6 amp @ 200 PIV bridge will work when making an AC amp meter from DC meter. Only a few volts will be across the bridge. 50 PIV would be enough.

This is very handy if you don't have an AC amp meter. Movement reading is relative. A radio drawing proper current reads 10 - 30% on my meter. Shorted quickly goes off scale allowing a person to turn a radio off long before smoke.

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Norm