Philco 38-12 radio

[Electronic M]

10'+ is better. 100'+ is ideal.

[timmy]

I'm pulling my hair out , this simple radio should work, well unless a tube is bad then I would have to get 4 of them since the voltages look good so then rectifier tube is good.

[jr_tech]

It doesn't look like you have many strong AM choices.

https://radio-locator.com/cgi-bin/lo...req=&sort=freq

Have you tried peaking the if transformers? Longer antenna wire?
Is the dial calibration fairly close, or does the set need alignment?

jr

[timmy]

Originally Posted by jr_tech

It doesn't look like you have many strong AM choices.

https://radio-locator.com/cgi-bin/lo...req=&sort=freq

Have you tried peaking the if transformers? Longer antenna wire?
Is the dial calibration fairly close, or does the set need alignment?

jr

I have one strong local am station and it comes in but no volume only max can I here it. I even tried the radio outside still no volume very low. The ant wire is really long. While listening to the station I tried tweaking the screws but it's all right on the money , I marked them befor touching them and it's right there. I went over my work and can't find anything. But if a tube is bad I have no way to tell which one it is since I don't have an older tester to test them nor do I have an idea which one it might be instead of buying them all. If I had an idea of what to try as far as testing to maybe see what tube it is that would be good.

[dieseljeep]

Originally Posted by timmy

I have one strong local am station and it comes in but no volume only max can I here it. I even tried the radio outside still no volume very low. The ant wire is really long. While listening to the station I tried tweaking the screws but it's all right on the money , I marked them befor touching them and it's right there. I went over my work and can't find anything. But if a tube is bad I have no way to tell which one it is since I don't have an older tester to test them nor do I have an idea which one it might be instead of buying them all. If I had an idea of what to try as far as testing to maybe see what tube it is that would be good.

What kind of reception do you get on another AM radio in the same location, especially the station you are receiving?
That Philco radio is just as hot as any other AA5 with similar circuitry. It's a standard AVC equipped radio, not one of those non-AVC circuits, where the volume control is located in the antenna and bias circuit.
Make sure the bias voltages are on the money as they affect the first and second audio stages.

[Electronic M]

You may be able to rule out audio by injecting audio past the detector and seeing if volume is adequate.

[WISCOJIM]

Originally Posted by Electronic M

You may be able to rule out audio by injecting audio past the detector and seeing if volume is adequate.

I was just about to ask what happened to standard troubleshooting? An inexpensive signal generator/tracer would make simple work of this, and one should be in every TV/radio hobbyist's possession.

.

[jr_tech]

Originally Posted by Electronic M

You may be able to rule out audio by injecting audio past the detector and seeing if volume is adequate.

If you don't have a signal generator, just touch the grid cap of the 75 tube... if a fairly loud hum/buzz is heard through the speaker, the audio section is likely working ok. Try the "one finger signal injection" both with the grid cap connector on and off the tube.

jr

[timmy]

Ok I have a cheap am radio and it works fine and loud strong signal, since the station is near me. The 75 tube cap off there is a buzz touching the cap and not all that loud and cap on less buzz touching it. The 6a7 wet my finger and touch the cap and the station comes in Alittle stronger. I will check the voltages on the 78 and 75 tubes again.

[timmy]

Ok the voltages, the 210 v is 211 v where there is 95 v I'm getting 97 v ?and the 2.5 v on tube 75 I'm getting 3.3 v and on the same tube the 2.3 v I'm getting 3.0 v .the 290 v on the rec tube I get 293 v. Minus 8 v I'm getting minus 14 v. The 90 v on 78 tube im getting 78 v . I would think the only one that's off a bit would be the output tube where the -8 v is -14 v and the voltage don't change with the volume being turned. I did check the volume pot which is .5 meg where it tests at .9 meg.

[jr_tech]

Did you check the 70 ohm and 250 ohm bias resistors? They set the -8 volts on the output tube.

jr

[timmy]

Originally Posted by jr_tech

Did you check the 70 ohm and 250 ohm bias resistors? They set the -8 volts on the output tube.

jr

Yes I checked those, the 70 ohm is 69 ohms and the 250 is 265 ohms. If it's a weak signal causing the low volume I went and checked the ohms on all the coils except the can on top, they were good.

[dieseljeep]

Now, I guess it's time to try some new tubes! Every thing else seems to be reasonably in order.

[jr_tech]

I guess that I am somewhat concerned about the high resistance of the volume control... perhaps disconnect the wire to the center leg and jumper it to the high end of the control and see if volume improves.

jr

[timmy]

No difference with the volume control. Is it possible that one of 2 Micas could be bad or shorted , 6 and 17 on the schematic. More so 17 right off the audio output tube.