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Speaker Test Replacement - Printable Version

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Speaker Test Replacement - Robbie Roberts - 02-18-2006

I just acquired a Heathkit Signal Tracer (T-3), and part of it is a speaker which can be connected for test purposes: Is this OK to use with my 37-650?

I think it might be easier than dragging around the factory one, but I thought I'd check...

Scott

PS- No new progress this week with the 37-650, other things have taken precedence, unfortunately. But I've gotten a few more test equipment pieces to work with...

- Terry Burns - 02-18-2006

Hi Scott,

You will have to use your factory speaker. Your speaker has a "field coil" on it that serves two purposes. It has about 300 or more volts going through it to act as a magnet for your speaker that works with your voice coil to move the speaker cone and it also is used as a choke in the power supply for your radio known as a "PI" configuration. Look on your schematic and see part number 56 it ties in with positive ends of your two filter caps. Thus the "PI" name. Without this field coil you would not have any B+ for your radio.

Hope this helps

Terry

- Robbie Roberts - 02-18-2006

Cool- I tried it for a couple minutes- the connections on the Heathkit are for the speaker, but there is also a voice coil, and this does have a center tap on the transformer, which looks like the one on the speaker in the schematics. The center tap is marked B+, and it looks like I could connect the show with the HK unpowered, Blue/white tr and Green/white tr into the speaker, and the white and green into the output transformer, jumpering from the green/white tr to the B+ connection, and come out the same. Check out the schematic for the AG-10 on page 20 of the partial HK manual at BAMA: ftp://bama.sbc.edu/downloads/heath/t3/pages/20.gif
or: ftp://bama.sbc.edu/downloads/heath/t3/t3.djvu and go to page 12.

It looks like if I make the connections above, with no power to the Heathkit, I should be ok.

I have not been able to make that work yet, but I will retry with the B+ hooked in. If nothing still, then I'll have to grab out the speaker.

Please, Take a look, let me know what you think...
Thanks!
Scott

- Terry Burns - 02-19-2006

Hi Scott,

I think that speaker connection on your Hk is for a permanent magnet speaker. The one in your radio is an electrodynamic speaker. The field coil located on the back of your speaker is an intregal part of your radio power supply. It is used to to smooth out the rectified dc voltage. It works in conjuction with your filter caps. I think I would use my original speaker to constantly monitor the sound quality and to make sure the voltages when measured for test purposes are correct. I always use the original speaker when trouble shooting a radio.
Maybe Chuck S. or someone else could explain it better.

Hope this helps
Terry

- Robbie Roberts - 02-19-2006

I gave it a quick try last night, with no positive results, so I put the factory speaker back in the mix... Guess I just have to listen to the noise while I try to set up the alignment...

Scott

- Poston Drake - 02-23-2006

Hi Scott,

Your Heathkit signal tracer has a PM speaker inside, and also a universal output transformer. The various jacks (such as the one labelled B+) serve to allow you to connect any combination of the following:

1) The internal PM speaker only
2) The PM speaker and the full primary winding of the output transformer for a push/pull audio output
3) The PM speaker and one-half of the primary winding of the output transformer for single-ended audio output.

There will also be a rotary selector switch to allow you to select proper impedance match for the output transformer.

Another switch allows you to use the output transformer in the Heathkit in combination with the speaker in the radio.

These signal tracers are handy to have on the workbench, since they permit easy substitution of a speaker, an output transformer, or both.

As has been mentioned, you must leave your electrodynamic speaker connected due to the field coil which is an integral part of the power supply circuit. But there is nothing to stop you from disconnecting the two leads to the voice coil and substituting a PM speaker, or if you don't want to hear any noise at all then substitute a resistor instead of the voice coil. Don't just disconnect the voice coil without substituting a temporary load (either a PM speaker or a resistor).

Poston

- Robbie Roberts - 02-23-2006

Cool- an alternate method! So I could theorhetically get just the field coil as a spare, disconnect the speaker entirely, put the spare field coil in the circuit, and attach the speaker leads to the HK tracer, or put resistance in its place, and use the meter to measure the strength of the output, instead of hurting my ears?

Scott

- Poston Drake - 02-25-2006

Scott,

Yes, that would be correct. If you choose the resistor route, a good choice would be a 10 ohm/10 watt.

But if your only concern is volume from the tone of the signal generator in the speaker during alignment, then an easier route would be to simply turn down the volume control of the radio. Yes, the Philco alignment instructions say to set volume control at maximum, and so do those of most other manufacturers, but this is not necessary. Turn it down to a comfortable level for your ears, or to a point where you can get some meaningful reading on your output meter.

On modern superhet receivers, you don't need to set the volume control at maximum as it says in the alignment instructions. These instructions are a holdover from the "old days" of early radio, where the volume control changed the operating point of the RF amplifier tubes.

You will have no problems in aligning your 37-650 with its volume control set at mid-point, or even lower. But regardless of where you have the volume control set, definitely adjust the attenuator of your signal generator for the lowest possible output sufficient to give a meaningful reading on your output meter.

Poston

Re: Speaker Test Replacement - dcastor - 08-06-2011

Terry,

Thanks for the advice...any suggestions on a specific model signal tracer? It appears there is some variation in the features offered.

Thanks again,
Dave