[Patrick]

You know, one of these days I won't have such a problem tracing down voltage problems. But I've found that the older you get (at least me) the shorter the retention span. This is the third voltage problem in the last few months that I haven't been able to solve.

Anyway, re-capped the unit. replaced all the dogbones as they all drifted 25% or more higher. Checked the choke and the field coil, and all the RF coils. Traced the schematic 4 times and found no errors, cold solder joints, broken wires, etc. Modified the input transformer. Re-checked all the tubes. Can't find a problem anywhere.

And these are the voltage readings I get (in red). Measured with both a VTVM and a digital meter (which was slightly higher than the VTVM). Power transformer voltages are very close.

[Image: https://i.imgur.com/XILAnCv.jpg]

[Image: https://i.imgur.com/t9vqzqy.jpg]

[Radioroslyn]

I'll mention a couple of things. The difference between the vtvm and the dvm is negligible in comparison to what was use in '32. A 1000 ohms per volt would what the reading are based on.

Check the 250 ohm side of the volume control. It controls the amount current the 24A's in the rf amp circuit. If it's bad it can screw up the voltages. On some of these old Philco schematics the voltage readings are not measured from chassis ground but from odd places like across the tube in question or from the rectifier plate. Check the service notes to see that your measuring at the proper place.

GL

[Patrick]

Both sides of volume control are good. Can't find any measurement specifics in service notes.

[Radioroslyn]

Just from a cursory look your voltages it would seem that something is wrong at the cathode of the detector. I would think that with 50k to ground you would have more than 1.4v. Check for bad tube, socket, wrong value resistor, damaged cap by soldering iron, ect. Grid to cathode leakage or short could do it.

[RodB]

Yes, Terry is on it. Another clue is the high plate voltage on the detector telling you very small current through the tube.

[462ron]

Just my 2 cents worth, but I wonder if some of the Philco printed voltages are a misprint. Concerning the grid voltages on the 27 1st audio and the grid voltages on the 71A outputs, Philco’s posted value of 50V can’t be right. Both 1st audio and output grids are traced back to test point “B”, and that point goes to the center tap of the power transformer which would be the most negative point in the circuit developed across the 187 ohm resistor to chassis ground. Plus consider having 50V on the grid of the 27 while the cathode is grounded doesn’t make sense unless the .01mfd cap #19 is leaky. I’m thinking your -8V makes more sense.

Ron

[rfeenstra]

I have my restored 20 still on the bench. I never did a voltage check. Maybe I'll power it up and do some checking. I'll let you know what I find.

[Radioroslyn]

RCA sez -40v on the grid w/180v on the plate class A.

[462ron]

Terry, Philco service info has +50V on the grids, probably should say -50V especially considering the grid circuits return to the center tap of the power transformer via point “B”. Yes/no?

Ron

[Patrick]

I found another post that said the grids of the 71's are measured to the filaments. I get 42VDC that way. I did manage to pick up a VERY weak station, But my concern now, I guess, is the detector voltages. I've checked everything. Tried 2 different tubes, re-checked all caps and resistors. Tube socket, grid cap. etc. etc. Nothing.

[RodB]

There's three resistors in series with the 24 detector tube, 12 (50k), 17 (500k) & 18 (100k). Make sure they are correct. If we're good with the resistors then one of the bypass caps may be defective. You could disconnect them one at a time. Hang in there!!

[RodB]

I noticed also that all the grid voltages in the rf section are not right. Maybe something in the power supply is miss wired. But you triple checked the wiring twice.

[Patrick]

I re-checked those three resistors, and checked the caps with my LCR meter. All good.

[radio1]

Upfront check, measure the resistance of the 24a tube's plate (before the 27) to ground with the unit powered off. Trace it out to ground on the schematic, what do you see? Concentrate on that weak signal, have you checked the aircap's picofarads and grounds?

[rfeenstra]

I took the time to fire up my 20 and take some measurements.  All voltages are referenced to the center tap of the A filament supply, Volume on full with the antenna disconnected.  Line Voltage is at about 121 volts.  It seems obvious that the voltages on the 71A's have to be listed incorrectly. If nothing else, they forgot a - sign.  My 24 voltages also have the plate much higher than the listed voltage.  The radio works great for a TRF with no AVC.  I did have to rewind 2 of the RF coils, one of which was the detector coil.

Looking at the given detector voltages, the factory gives a net grid voltage of -7 volts.  My 20 voltages give a relative grid voltage of -2.6 volts.  The tube in my radio should be closer to saturation than the factory numbers so the factory plate voltage should be higher than mine.  Obviously they are not.  Perhaps if I tried a NOS tube it would change a bit. But, again I think the factory numbers are off.  FYI, all resistors are spot on.

   


Oh I forgot! I have a bucking transformer in this thing that brings the line voltage down to about 107 volts.