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ERROR in Philco Model 38-690
Tweeter Wiring The Philco Model 38-690 is a high fidelity receiver and worthy of special consideration, especially since it is designed with a 14 inch woofer (low-frequency speaker) and two 6 inch tweeters (high-frequency speakers), along with four 8 inch passive radiators! The 14 inch woofer is actually a full-range speaker, handling most of the audio load; yet it can easily shake the room with its thundering bass. The tweeters are there to provide added high frequency "presence." Properly wired, they can do a good job at this. However, your 38-690 tweeters may NOT be properly wired! If you have to put your ear against one of the tweeters to hear it, then it suffers this factory wiring error. Fortunately, this is very easy to correct. The schematic diagram of the Model 38-690 shows that its two tweeters are wired in series, driven by the set's driver tube using a special winding in the audio interstage transformer. (See the partial schematic below; click to enlarge it.)
Why aren't the tweeters
connected to the audio output circuit? The original schematic shows that the two tweeters are wired OUT OF PHASE! This closeup will help illustrate our point:
See how the two WHITE wires are connected together? This effectively places the tweeters out of phase with each other. When speakers are out of phase, they cancel each other out, allowing very little sound to be actually heard. I had noticed this effect before on Model 38-690 receivers, and had wondered why the tweeters were so quiet when Philco went to the trouble to include them in their top-of-the-line 1938 receiver. But, until I recently had occasion to service another 38-690, I hadn't paid attention to this wiring discrepancy. Examining the amplifier/power supply chassis, sure enough, the tweeters were wired exactly as the schematic shows - white wires connected together, both green ends connecting to the low-impedance secondary of the interstage transformer with a 1 uF electrolytic in series with one green lead. More about that electrolytic later. Changing the wiring as shown below resulted in improved performance - the tweeters made their presence known once this change was made.
I increased the size of the electrolytic capacitor to 2.2 uF, using a new 50 volt non-polarised unit. This first order filter reduced the crossover frequency to 9000 cycles, assuming a series impedance of 8 ohms as the two tweeters are connected in series. A 1 uF capacitor yields a crossover frequency of over 19000 cycles, near the upper limit of average human hearing and certainly above the design capability of the 38-690, as it contains a 10 kilocycle filter to reduce audio response above 10 kc! A better crossover point would be around 4000 cycles, which would require a capacitor of 4.7 uF. Just for fun, I tried hooking the bottom white wire direct to the yellow-blue tracer lead of the interstage transformer, removing the ground and the crossover capacitor. This resulted in even more volume from the tweeters - not overpowering, but a great improvement in volume. However, as the capacitor blocks lower frequencies from the tweeters, the 2.2 uF unit was left in place with the ground per factory specs; after all, this was not my radio, it is quite valuable, and I didn't want to do a lot of "tinkering" with it. However, if I had it to do over again, I would try a 4.7 or even 10 uF capacitor instead. I did leave the circuit wired like the picture just above, however, with the tweeters wired green-white-green-white as shown, so that they would be in phase with each other. For
further reading on the subject of crossover capacitors for
high-frequency speakers, read more
at the Basic Car Audio Electronics site. Return to the Tech Section Index. © 1997-2006, Ron Ramirez. All
rights reserved. Unauthorized duplication prohibited.
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