Radio Shack sells a mini amplifier speaker for small project work. Would that be useful as an audio signal tracer in antique radio servicing? Does anyone sell a modern device suitable for such work at a low cost?

If you decide to use it, put a capacitor on the hot test lead to protect the circuit.

Alternately you can locate old test equipment on Ebay, or Ham fests in your area, got to arrl.org to find the listings.

:Radio Shack sells a mini amplifier speaker for small project work. Would that be useful as an audio signal tracer in antique radio servicing? Does anyone sell a modern device suitable for such work at a low cost?

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An amplified speaker for computer use works fine,with a properly shielded interfacing wiring interconnect. It can then be either battery powered for total isolation or the isolation provided thru a wall wart power pack. I have picked up units for as low as 1-2 bucks at ham fests, swap meets or thrift stores.( Even seeing a matched set for $4 bucks and wheedling an 'unknowledgeable' vendor into only selling one unit, "I only need one speaker" ...and U know which unit I select!) Also an add on diode detector and its HF filter cap can be built into a clip on probe and provide further use as an additional detected RF accessory.
73's de Edd
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:Radio Shack sells a mini amplifier speaker for small project work. Would that be useful as an audio signal tracer in antique radio servicing? Does anyone sell a modern device suitable for such work at a low cost?

When using a modern low impedance speaker (modern referring to most speakers produced since the mid-1930s) for signal tracing, some form of impedance matching must be provided just as that provided in the output circuit of most tube type amplifiers. A transformer must be used. The high impedance side, when driven by the feable high voltage, low amperage currents found in audio circuits, will give you leverage to drive the low impedance side which drives the speaker. Driving a low impedance speaker directly off of a high impedance audio circuit leaves much to be desired. When using a matching transformer, you will be able to trace incredibly weak signals. The only substitute that proves itself far superior is the high impedance headset, usually running around 2000 ohms.

That said, Radio Shack also sells matching transformers for public address work. They may or may not have these in stock now that they have been discontinuing a lot of their speaker sales. If you find one, it will have multiple taps on it. One side will list wattages, and the other will list resistances for various types of speakers (3, 4, 8 ohms, etc.). Purchase this transformer for use with your speaker. Wire the appropriate low impedance tap to the speaker. Use the highest impedance winding (should be listed as lowest wattage rating--measure resistance to be certain...several hundred ohms) for your test work. You may wire your choice of condensers and such to the transformer for various isolation work. Normally, in order to trace the plate section of varioius circuits, the other side of the transformer must be wired to B+. By using a 500 volt .01 or .05 MFD condenser (also sold at Radio Shack) on your test lead side, the other end of the transformer may be grounded. In most circuits a signal may be detected through the condenser. There are certain circuits in which this will not work, and I will not go into detail about these.

Thomas