I first want to thank you all for help with the electrolytic re-stuffing and that stand alone value for the .5mf metal box I had asked about several weeks ago. Now I was lucky with that AA5 and Philco 38-93 I did about 6 months ago. After a quick parts swap both radio's started singing and still do. Well I guess I am not so luck with this one. I double checked all my values that replaced the old parts (Capacitors and Electrolytics). The Resistors are within tolerance. I know I have everything wired right from what I can tell, but I can't seem to get the radio to play.
Let me give you a little history in past trouble shooting for this set: The first time I turned on the set before restoration I got nothing after several on and off switching I did receive several stations (I thought I reformed the restored electrolytics (Sometime in the 80's) that caused it to come to life. So it did work before restoration and I did take several of the voltage readings and they seemed to be good on the RF, Det Osc, and IF stages. After restoration I got nothing on a first fire up and then after turning it on the second time I did get several stations but they would disappear and then come back every once in a while. Now I don't get anything but static and crackles as well as pops.
I did test all the tubes and replaced several tubes that had slow cathode heating. I know the audio output is working because I get feed back when I put my finger over the grip cap. But I do not get any sort of station when I put my finger over the other grid caps. Right after restoration if I put my finger over the Det Osc. I did receive stations, but when I took my finger off it dissipated to nothing again.
I did several Tube socket data readings specifically on the R.F., Det OSC, and I.F. The filament voltages on AC for all three were right around 6.20 when the chart on the schematic calls for 6.3v. The Plate Volts were all in the ball park of 245.8v DC plus or minus a few. The chart calls for 245 to 255v DC.
Now what is strange: the Screen Grid volts for the R.F and Det Osc. measure at 79.5 and 75.1 Volts DC and the chart calls for 90v. The I.F. Screen grid volts reading with the tube in the socket reads -78.9 and calls for 90v on the chart. Could this be my problem? I am quite lost and this is my first time trouble shooting a radio at this depth. I know the coils in these early Philco's are known to go open. Any help and guidance would be great, I really would like to get this set going. It is a Code 121 single speaker cathedral model.
Thanks guys, the chart I am talking about is here on nostalgia air.
Looking at the schematic, the screen grids of the RF, Det/Osc and IF are all connected together so the voltages on them should be identical. In itself, the voltage reading of 75 to 80 V is not too significant as long as all three are the same. This voltage is somewhat dependent on the grid bias of the tubes, so if you are not receiving a signal, the screen voltage may be a bit lower than 90 v. With a signal, an AVC voltage is generated which increases the negative grid bias in the tubes, which in turn will raise the screen voltage.
In any case, with a 75 to 80 v screen voltage you should still be receiving signals, so you may have to look elsewhere for the problem. You might check and see if the oscillator is operating by tuning a nearby AM radio to the receive freq plus the IF of 260 KHz and see if you can hear a carrier (for example, with 71B tuned to 1000 KHz, you should hear the osc at 1260 on a nearby radio).
If there are no miswiring errors then a check for open coils is probably in order.
I did test the Osc. Coil for continuity and here is a diagram of my findings. Could I also test the other coils? I know theses early Philco's are notorious for having shorted winding in the coils. Also if I hook up a test signal off a signal generator and connect one side to chassis ground and the other onto the antenna lug could I check to see if I get a signal that way?
Hi;
My best guess, is that You are missing or have an open on one of the Coils.. The only way to know for sure is to ohm from each pin on the coil to where it is connected to and find that on the schematic or follow the wires and find where all the connections go.. Make Your own mini schematic and compare that to what the Schematic shows.. That should then make it perfectly clear where you should have a coil connection.. It looks like the lower coil connection is broken.. As you should have three coils and you only have two..
THANK YOU Marty
Well I know that I am not missing a coil. At first, right after restoration, I was able to pick up several stations that would come in, then they would disappear and then come back in again. When I put my finger on the grid cap of the 36 tube (Det. Osc.) stations would come in loud and clear. Now if I put my finger on that very same place I don't get anything. The radio only produces a bunch of loud crackles and pops. If I put my finger over the R.F. grid cap or I.F. I don't get anything were'as right after restoration I was able to pick up stations when doing such a gesture.
I also tested the 1st I.F. Transformer coil and here is what I got.
If any of you have worked on a 71 and can help me out that would be great. I have done everything I know how to, and that is not a lot for being an amateur restorer.
Here is a schematic off Nostalgia air. Hope it helps. Thank you all for the help.
Hi ;
I am sorry that I mis-led you.. I was not implying that you were missing a coil, but that the coil had a possible break in it.. The Two upper coils with a common connection which as near as I can tell, connect to C-15 and C-16 .. Seems to be OK.. but the Lower coil, that is attached to the Cathode of the 36 Tube and it looks like R-18 and C -19 is not there, IE, not shown as a third coil.. According to the ohming of the coil form that you have done.. You have two unknown connections on the Coil form.. Check to see If they go to the Cathode on one side and the Capacitor Resistor on the other side, between those two point, You should get an ohm meter reading.. And IF you do not then there is most likely a break in the coil.. And that should be investigated..
THANK YOU Marty
Yes I have changed the 36 tube twice, the one that was in the radio set tested good but I do not know about shorts, I don't think there are any, as crazy as it sounds I have to restore my tube tester as well its about 75 years old and I don't really trust the short light on it, but for all I know it could be great. I just finished scrubbing each of the tube contact sockets with de-oxit as the set is dirty, I am going to wait for it to dry and then try it out again (most likely this is not the problem but it never hurts to try), after it was suggested by a fellow hobby member to pull each tube in the string leaving the 42 in place to try and pin point what section the static is in. Hopefully this helps, I will try here soon.
Any other suggestions would be great, Ron if you are browsing hopefully you will stumble across this post, I am also looking for the ohm readouts of each coil if anybody has them, there are not any on the schematic.
I think I found a break in one of the winding on the OSC coil after tracing the 36 K to ones section of the coil. According to the schematic that lug section that the K goes to ends up coming out of the center bottom lug that goes to R18 and C19. The only problem is that I get no continuity between both and when I Ohm it out the reading is in the Meg Ohm where as the other winding in the coil that have continuity test in the 3 or 4 ohm range.
Here is a diagram below to show what I mean. Thanks
Does anybody out there rewind coils, particularity this Osc one.
Most of the time the winding that is open it is the small winding on the outside of the osc coil. This is the feedback winding. If this is the case you can rewind it yourself. Two thing you will need to know. How many turns is it? Usually 20-25t. And the direction it is wound.
Remove the old winding. Wind new winding in the same direction as the old. The original wire was 38G this is not very critical. Any thing from 32 to 40G will work fine.
Terry
Terry, you nailed it, it is on the outside of the osc coil. There are actually two paper tubes. The lug I am getting no continuity on is the lug on the outer tube.
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