Replaced the capacitors but all I get is a hum - Printable Version
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+--- Thread: Replaced the capacitors but all I get is a hum (/showthread.php?tid=7826)
Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-01-2013
Hello everyone,
I am very new to radio restoration and I have run into a problem. I picked up an old Philco 42-390 at an auction recently and decided to try and make it work again. The radio itself is in great condition, but it did have what appeared to me to be the original capacitors. I ordered a capacitor replacement kit for this model and soldered in all of the new capacitors to include the electrolytic. I just plugged the radio in and all I can hear from the speaker is a hum. I am about 99 percent sure I installed the new capacitors correctly. Assuming this is true what should my next steps be? I apologize if something similar has been posted before. Thanks in advance for any help.
Andrew
RE: Replaced the capacitors but all I get is a hum - Warren - 11-01-2013
Do you have a service manual/schematic for it? You should use it to check voltages at the various test points. Also, make sure your resistors are within tolerance. If all that checks out someone can help you with more.
RE: Replaced the capacitors but all I get is a hum - morzh - 11-02-2013
Andrew: good news is, if you hear hum, most important parts of your set are OK. Your sound, your power and such.
Now, replacing the caps is great and is a necessary step but it is not often that a radio totally stops working due to a bad paper cap. Not unheard of, but not the most common problem.
1. Do you know how to read schemartics, do you know the basics of electronics?
2. Do you know what's different about tubes and what a tube radio can do to you if you are not careful?
3. Do you know how to measure voltages and such?
RE: Replaced the capacitors but all I get is a hum - klondike98 - 11-02-2013
Welcome to the Phorum!
You can get some schematic info here: http://www.nostalgiaair.org/Resources/535/M0013535.htm or here: http://www.audiophool.com/Philco.html
If you are not familiar with working on vintage radios you should read this first: http://www.antiqueradio.org/begin.htm
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-02-2013
Thank you for the replies. I do have the schematics already and I would rate my understanding of them at marginal. As far as the danger associated with tube radios, my understanding is that once energized the capacitors can be deadly due to high voltage. This is true even when the radio is unplugged and there is a proper way to discharge capacitors prior to removal. I removed the old ones since the radio had not been plugged in for years. I do understand how to measure voltage. At least on household circuits. Is it safe to plug a chassis in upside down to check voltage?
I don't understand what you mean about testing the resistors to ensure they are within tolerance. How do I do this?
One thing that I failed to mention is that the speaker can produce some static if I move the volume and tuner at the same time. I'm not sure why that is. I also replaced the power cord because the old cloth one was fraying badly. I did this by installing a new bar with new line capacitors that totally bypassed the old ones.
Andrew
RE: Replaced the capacitors but all I get is a hum - jerryhawthorne - 11-02-2013
Andrew, welcome and please respond to Mortz's questions as they are key to helping you.
Jerry
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-02-2013
Thanks Jerry,
I am trying to respond but I think since I am new here my posts have to be reviewed first.
Andrew
RE: Replaced the capacitors but all I get is a hum - Ron Ramirez - 11-02-2013
That is correct.
http://www.philcoradio.com/phorum/showthread.php?tid=5651
As you have now made more than two posts, Andrew, your posts no longer have to be approved. Welcome!
RE: Replaced the capacitors but all I get is a hum - Warren - 11-02-2013
Most meters to test voltage can also test resistance. You can check your schematic to find what the resistance of a particular resistor should be. You would put one probe on one side of the resistor and one on the other and it should tell you the measured resistance. To make sure it is an accurate reading you should disconnect one end of the resistor before taking a measurement. The measurement of the resistor should be within apprx 10 to 20 percent of what the schematic says it is. Anything more than that should be replaced.
RE: Replaced the capacitors but all I get is a hum - klondike98 - 11-02-2013
Something to watch for on a 1942 set is the rubber coated wiring, it can be in bad shape and cause shorts. See Ron's discussion of this here: http://www.philcoradio.com/tech/wiring.htm
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-02-2013
Thanks Bob. I did run into some of that. I went with shrink tube on the bad ones. Surprisingly most of the rubber wires are in really good shape on this radio.
Andrew
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-02-2013
Warren,
Unfortunately my tester will not measure resistance, but I will run out tomorrow and pick one up. Thanks for the tip. I will check them all once I get a better tester.
Andrew
RE: Replaced the capacitors but all I get is a hum - morzh - 11-02-2013
I wonder what kind of tester sold in the second half of 20th century (or today) does not measure resistance. Are you sure? or are you sure it is a tester (or multimeter) and not a simple voltmeter?
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-02-2013
I'm sorry let me clarify. It is a Fluke T+. It just measures voltage and checks for continuity. Since we are talking about electrical testers, does anybody have any recommendations on a decent tester. I would prefer to not keep buying these. I would rather spend more on features I might not need. I went cheap with the T+ because I thought it would be all I needed.
Andrew
RE: Replaced the capacitors but all I get is a hum - klondike98 - 11-02-2013
A digital multimeter will fill the bill. Here's one on Amazon for $25
http://www.amazon.com/Mastech-MS8268-Digital-Manual-Multimeter/dp/B0050LVFS0/ref=sr_1_2?s=hi&ie=UTF8&qid=1383449190&sr=1-2&keywords=multimeter and there are many others to choose from.