Looking for help with first chassis restore.

[BostonDave]

I acquired my first antique radio, a Philco 39-25, a few weeks back. In my inexperience I plugged it into an ac outlet and although the radio didn't tune any stations, I was able to plug in a CD player and a guitar through an RCA jack on the back and get good sound out of the speaker without any hum.

So after learning a little more about good chassis restoration practices I ordered all new caps to replace the paper and wax including some safety caps for the line-filter capacitors bakelite block. While waiting for my caps to arrive I built a dim bulb tester.

Now I wish I had taken the time to test the radio after replacing each cap, but I didn't. Nor did I test each new cap. I just replaced them all, rebuilt the bakelite block and replaced the power cord. I don't have a tube tester, but the tubes were working when I got the radio (at least enough to be amplifying and outputing whatever I fed through its inputs).

So after the recap I plugged it into my dim bulb tester and the bulb glows brightly. I've double checked all the connections and don't see any place where a short would occur. I've verified that my rebuild of the bakelite block in the power circuit is correct.

I read somewhere that a bright bulb right away will indicate a short in the transformer.

How do I go about troubleshooting from this point? I'm worried that a bad cap went in. Can I test the caps without taking a lead off or do I have to begin desoldering those and testing? Should I be looking at the power stage/circuit first or could a cap anywhere cause this condition? Do I need to start checking resistors - if so where might be a good place to start?

In a previous service of this radio two of the electrolyte cans were bypassed, but the third was not. I bypassed the third (part 18) by putting a new electrolyte cap in its place. There was only one wire coming out of the center of that can. I connected the replacement cap's positive lead to the same place that the wire was connected and because the can's base was not insulated from the chassis I assumed that the caps inside were grounded to the can and connected the negative end of my new cap to the base of the can.

The only other descrepancy I'm aware of is that the .006mF cap (part 36) was replaced by a .05mF cap. I didn think that could that cause a short though.

I've attached the schematic for convenience/reference.

Any suggestions, tips, advice is welcome and much appreciated.

[Tom Bavis]

Pull the tubes and recheck - if the dim-bulb (40W or greater) glows brightly, the transformer is shorted or AC is connected incorrectly. You could have crossed supply and return wires on the line-to-line cap block. There COULD be short in the block, but new caps are seldom bad... by the way, 'Y' capacitors - safety-rated for lin-connection - are the ones to use in that location. Regardless of the original value, I use .01 or .015 uF capacitors here to limit the amount of "leakage current" to well under 1 mA.

Also, cap item 36 shouldn't be increased - it will cause loss of high frequencies (there's already a 'Low" position on the tone switch - do you want "Low" and "Lower"?). Use .005, not .05.

[sean]

If this set is like other 1939 Philcos I have worked on, the leads to the power transformer are probably rubber insulated. If so, you might want to check the condition of the insulation to see if any of the leads have shorted. It is recommended that the rubber insulation either be sleeved with some heat-shrink tubing or the wires be completely replaced.

[BostonDave]

Originally Posted by Tom Bavis

Pull the tubes and recheck - if the dim-bulb (40W or greater) glows brightly, the transformer is shorted or AC is connected incorrectly. You could have crossed supply and return wires on the line-to-line cap block. There COULD be short in the block, but new caps are seldom bad... by the way, 'Y' capacitors - safety-rated for lin-connection - are the ones to use in that location. Regardless of the original value, I use .01 or .015 uF capacitors here to limit the amount of "leakage current" to well under 1 mA.

Also, cap item 36 shouldn't be increased - it will cause loss of high frequencies (there's already a 'Low" position on the tone switch - do you want "Low" and "Lower"?). Use .005, not .05.

I did use .01uF 'Y' caps in the block. I will surely double check again tonight to make sure I didn't wire them incorrectly in there before opening up the transformer to look for trouble.

The .006 cap for part 36 has been on order. The other cap was already snipped before I realized I didn't have a proper replacement so the .05 went in for now. I'll be sure to size that cap properly when the one on order arrives.

[BostonDave]

Originally Posted by sean

If this set is like other 1939 Philcos I have worked on, the leads to the power transformer are probably rubber insulated. If so, you might want to check the condition of the insulation to see if any of the leads have shorted. It is recommended that the rubber insulation either be sleeved with some heat-shrink tubing or the wires be completely replaced.

I had expected to find a lot of rubber insulated wire in there (my 1936 Rogers-Majestic is full of it), but the 39-25 had almost none. All cloth-covered and it all appears to be in very good condition. All of the leads going into the transformer are cloth insulated and they all look good. If there is no short the bakelite block I'll open the transformer tonight and see what's happening in there.

[AUdubon5425]

Did you test the windings with an ohmmeter? Pull the rectifier. Then disconnect the primary lead from the capacitor block, turn the power switch off and see if there's 13 ohms across the primary winding. Do the same for the high voltage secondary, disconnecting the center tap lead (green).

Also, check the rectifier tube socket for signs of arcing. Double check the electrolytics to make sure one wasn't installed the wrong way.

Also, one of those electrolytics may be leaky. To test them, disconnect the positive lead from the capacitor. Set your ohmmeter to the highest range, then test for resistance across the capacitor. If it is leaky it will show a low resistance. A good cap will deflect then read on the high end of the scale - usually 500,000 ohms or better.

[BostonDave]

I found a short in the bakelite block! The leads from the two new caps in there were touching. Corrected this and things are working nicely. Better volume compared to when I originally brought it home.

Thanks for everyone's input!

Got some AM reception as the night wore on after hooking up a small wire antenna, but not very loud and tended to drift in and out. Going to have to see about getting something placed up in the attic.

[AUdubon5425]

Search around - the Philco antenna issue has been discussed. I think a 3-6 foot antenna run under a rug or around the baseboard would do the job. I'm going to have to do the same thing when I get to work on my 38-7.

[BostonDave]

I built an indoor loop antenna like the one pictured here:

http://www.audioasylum.com/forums/tu...ages/7246.html

Not much AM reception yet (I can get two AM stations from a more modern receiver in the same room), and I seem to get one short wave station consistently but it comes in very faintly. I may try taking the radio and antenna upstairs (shop is in the basement) to see if that helps.

One thing I did notice is that while touching the wire that connects to the top of the 6A8G (first detector/oscillator) tube reception is almost perfect, but still getting only that one station. What would this indicate... (Other than the fact that I'm a better antenna than the homemade loop apparatus)?

[StarMover]

Have you tried cleaning the tuner gang? I've found that really helps with many of the SS recievers I'v worked on.

[BostonDave]

I'll look into giving that some more attention. I think I basically just brushed it out and used some compressed air on it. Should I attempt anything more radical? I've heard of people putting these things in the dishwasher, but have never had the courage to try that myself.