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38-690 -- silent tweeters - Printable Version

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38-690 -- silent tweeters - Guest - 08-06-2008

I got my 38-690 up and running but there's no sound at all coming out of the tweeters. (The woofer sounds awesome.) It has the 4.7 uF cap in place of the stock 1 uF, and I played with the phasing. I troubleshooted all the connections and they seem OK. The 4.7 uF cap tests OK on my Fluke meter. The field coils and voice coils of both tweeters seem to measure fine with an ohmmeter. The resistance of the secondary of the output transformer to the tweeters measures 2.8 ohms. Does that sound like it's in the ballpark?

If the tweeter secondary is shorted, I'm assuming I'll never find a replacement transformer since it's an unusual design that also feeds the woofer. I could try to wire a separate output transformer to the tweeters in parallel with the original, but I wonder how that would work without messing up the feed to the woofer. Anyway, I hope the transformer is OK.

Does the 2.8 ohm measurement seem correct for the tweeter secondary? I'm kind of at a loss as to why there's no sound from the tweeters.

Thanks,

Bob

- Texasrocker - 08-06-2008

Whew,Im not familiar with that model Philco. Check the schematic for your set. I remember I had the same type prob with a German made Grundig radio before. Someone doing previous repair on the grundig before, didnt use polarized type caps in the crossover between woofer & tweets, and it silenced the tweets in that set until I changed them out.
Ron or Chuck will know more than I on the Philco sets for sure. Just trying to help with suggestion. I just bet your orig tweets are still ok!?

- Ron Ramirez - 08-06-2008

Hi

The resistance of the secondary sounds about right, if memory serves...

As a quick test, try running the tweeters direct just for a minute or so, without the capacitor. If they work, then the new cap is your culprit - could be defective?

Is the cap a non-polarized cap?

How is the resistance of the individual tweeters? And their field coils?

If all else fails, try 2.2 uF, non-polarized, and PLEASE let me know if you have to do this.

- Guest - 08-06-2008

Hi Ron --

That's good to know that the transformer is probably OK. The cap is non-polarized, and I'll have to check to see if it's the culprit when I get back from work tonight. One thing I just noticed is that the restorer swapped the leads from the transformer secondary, with the blue one going to cap 151 and ground and then to the tweeter pair, and the yellow-blue-tr going straight to the tweeter pair. I wonder if this would make a difference (I'm not sure of the purpose of the connection to ground in this circuit.) If so, then whoever had the radio before me never heard a thing out of the tweeters.

Bob

- Guest - 08-06-2008

Well, now things are really weird. I happened to connect the woofer only part of the way, so that the field coil was energized but not the voice coil. With the woofer voice coil disconnected, the tweeters worked fine regardless of whether they were grounded. (I think they sounded better with a 10uF crossover cap compared to a 4.7uF, but this was without the woofer connected.) When I connected the woofer voice coil, then the tweeters went completely silent. If I disconnect the tweeters from the circuit, then the woofer gets louder. If I reconnect the tweeters, then the woofer is slightly less loud, but I can't hear any sound from the tweeters. Arrgh, what on earth is going on here?! Icon_mad

Bob

- Ron Ramirez - 08-06-2008

I suggest you recheck your wiring in the audio driver and output circuits. Reverse those wires going to the tweeters; whereas your blue wire is now grounded, lift it from ground. Do not ground the yellow-blue tracer wire yet. Make sure blue goes to green; yellow with blue tracer to white.

If properly phased, the tweeters should be wired, in series, green-white-green-white, as shown in the Tech Section of my website:

http://www.philcoradio.com/tech/tweeters.htm

Try the set out.

If it makes a difference, you may have found your problem. Re-ground the return white wire from the tweeters, and the grounded end of cap (151), and try it again.

If it does not make a difference...I would begin to suspect the driver transformer (152). You may want to do a resistance check of all of the windings; making sure that none of them have any shorts to one another.

Edit: There should be no connection whatsoever between the woofer voice coil and the voice coils of the tweeters. This is beginning to sound to me like someone in the past got a little creative with the wiring.

- Guest - 08-07-2008

It doesn't seem to matter how I wire the tweeters (ground, no ground, various combinations of white/green). They don't work when the woofer is connected, but they do work when its voice coil is disconnected. I guess I better check the output transformer 152. This is so frustrating...

Bob

- Guest - 08-07-2008

Well, it looks like the problem was a weak 6F6 driver tube. Swapping it seemed to fixed it. Playing with the crossover cap, I think that a 10uF gives a better sound than a 4.7uF, and either of them are *way* better than the stock 1uF, which practically cuts out all the sound out of the tweeters. Phase is also very important. Not only do you have to connect the speakers white-green-white-green, you have to also check whether green-white-green-white works better. Either of these puts the tweeters in phase with each other, but only one puts them in phase with the woofer, and it makes a big difference. All of this is easy to accomplish by swapping the cables to the voice coil pins on the tweeters. I'll have to color code mine for future reference. The sound from this thing is pretty incredible once it's operating on all cylinders. Icon_eek

Bob