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 Post subject: Motorboating
PostPosted: Feb Sat 21, 2004 2:00 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 117
Location: Brookeland, Tx USA
I'm getting desperate and need some big words from the Philco Gurus. I've got the old 41-629 BC and SW aligned and working nicely. Tone, Volume and quality of audio are near perfect. When I switch to the phono position and move the volume control to approx 50% or better, I get a real speaker thumping motorboat. The system uses a 7B5 oscillator that supports both the radio portion and the phono. When switched into phono, the signal is sent from the pickup (BoL) to the grid of a 7C6<BR>Preamp. The plate of the Preamp is sent thru the volume control which is tapped at 1 meg on to the tone control. A 400cps tone from the audio gen is clean all the way thru the 41 outputs. I suspect my problem may be in the oscillator, but I don't understand why it is okay in BC and SW but not on Phono.<P>Jesse <P>------------------<BR>If it Ain't Broke; don't fix it!


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 Post subject: Motorboating
PostPosted: Feb Sat 21, 2004 2:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 252
Location: cave city ky 42127
hi jesseca5 is there a cap conneted to the volume cont if it is bad it will cause motorboating or bad ground so replace the caps with orange and clean the volume cont with contact clean from (http;//www.radioshack.com<BR>and replace that 7B5 WITH A NEW ONE<BR>AVALIBLE from htt;//www.tubesandmore.com<P>good luck jakeSW23<P>------------------<BR>visit http:www.cavecity.com you will find antique radios in the antique distric of cave city kentucky 42127


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 Post subject: Motorboating
PostPosted: Feb Sat 21, 2004 2:43 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 117
Location: Brookeland, Tx USA
Thanks, Jake....as the old gal on the tube used to say, "Been there and done that". I replaced the 7B5 with a 'NOS', and the volume control was just recently replaced. There is a 100mmf mica between the hotend of the vc and the 1meg tap. I also replaced that.<P>Jesse<P>------------------<BR>If it Ain't Broke; don't fix it!


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 Post subject: Motorboating
PostPosted: Feb Sat 21, 2004 2:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1873
Location: Redmond, WA USA
Hello Jesse,<P>Have the filter capacitors been replaced? Try putting another electrolytic in parallel with either (#21) the 8 uF (decoupling) cap, or (#21a) the 16 uF second filter cap.<P>The 8uF cap is my bet.<P>If the .001uF cap (#45) between the 7C6 plate and the phono/radio switch has been increased in value, that could also cause motorboating. The decoupling cap. (8 uF) might also have to be increased.<P>Good luck,<BR>Izzy<P>(Safety note: Remember to discharge the filter capacitors before working on a radio. When replacing (or substituting) electrolytic capacitors, double check the polarity.)<BR>


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 Post subject: Motorboating
PostPosted: Feb Sat 21, 2004 3:23 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37194
Location: Livermore, CA
Hi Jesse<P> Believe you've been working on this for some time? Usually motorboating is caused by bad filter caps but you replaced these.<P> Motorboating can be caused by too great of low frequency response. Has the coupling cap between 7C6 & 41's been increased in value. The original cap was .01 mfd. Try a lower value such as .0047 or .0068 mfd.<P> When your Philco is on phono there is an additional preamp. This additional gain can cause motorboating. As Izzy mentioned, be sure cap #45 isn't larger than .001 mfd. Try a shield on your 7C6 preamp.<P> <P>------------------<BR>Norm


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 Post subject: Motorboating
PostPosted: Feb Sat 21, 2004 4:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 117
Location: Brookeland, Tx USA
Thanks for the input guys; there are some good points to investigate to see where my problem might be. I'll start with the Filter Caps since they are about six years old now. They could be breaking down. Also, on the coupling cap #45; it is still a .001uF so I'll put in a new one to see if that helps. As for shielding the 7C6 Preamp, it is a loktal but some aluminum foil and a tie wrap should do the job. Norm the cap between the 7C6 and the 41 output tubes is still a .01; I'll look in my junk box and see if I've got a lower value.<P>Jesse<P>------------------<BR>If it Ain't Broke; don't fix it!


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 Post subject: Motorboating
PostPosted: Feb Mon 23, 2004 2:22 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1452
Location: Cambridge MN USA
Something else that extends the low end of the freq. response is an increase in the grid resistor value. Before you cut out that .01, be sure the value of the grid resistor didn't go sky high.<P>Scott Todd<BR><P>------------------<BR>


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 Post subject: Motorboating
PostPosted: Feb Mon 23, 2004 4:42 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1873
Location: Redmond, WA USA
Hi All,<P>Any of the resistors in the output section could change LF response. You might check part #s 68, 69, 70, 72. Negative feedback from #69 (1.5 Meg) probably lowers the grid circuit impedance more than one might first expect.<P>Izzy


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