[MrFixr55]

Hi Roy,

OK, so the sound is good on the audio injected? Sound should be quite good. There will be some "color" to the sound based on the single end output that some people find pleasant. (there is a whole cohort of audiophiles that build "single ended" amps as opposed to "push-pull amps. BTW, if you get two 100 Ohm resistors, tie one end of each resistor together, tie the other ends to the 2 signal wires, and connect the "junction" of the 2 resistors to Terminal 2 of the phono board, and the shield to Terminal 3 of the phono board (remove the jumper between terminals 1 and 2), you will now have both channels and the sound should be quite good. I do this often when I play big band music through one of my radios from the 1930s, as the sound is much better than having been modulated / demodulated and the noise so inherent from interference today.

Re your voltage and resistance measurements, your measurements for the speaker are good. First thing I would worry about for the poor performance of the radio circuit is the voltage on Pin 4 of the Mixer (6A8) and IF (6K7. This is the Screen voltage and too low a screen voltage will affect the gain of the tubes. While the service diagram specifies a meter whose loading of the circuit is at 1000 Ohm per Volt, using a DVM is not causing an erroneously low voltage. If there was an error due to use of a DVM, the error would be a higher voltage than expected. The problem is in the radio itself. What the voltage drop is across a resistor depends on both the value of the resistor and the current drawn. In a series circuit, current at any point in the circuit is the same. The voltage across any component in the series circuit depends on the current and the resistance of the individual component. For this reason, if the only load across the resistor is C15 (The tubes are pulled from their sockets) and if C15 is good and you are measuring with a DVM which has an almost infinite input resistance, then there will be almost no voltage drop across the resistor.

Try pulling the Oscillator Mixer tube (6A8), leave the IF tube (6K7) in, powering up the set and measuring pin 4 of the IF tube. If still not correct, remove the IF tube. It has to be correct now. IF not, check C15. If good, leave the IF tube out and reinstall the Osc Mixer tube. If the voltage is good or slightly high, then the IF tube is bad. If the voltage with both tubes removed is good but goes bad with either tube inserted, then the resistor may be bad. It may measure correctly at the minute current drawn by a DVM but may exhibit a much higher resistance as the current demand of the tubes increase.

I am not sure what you mean by Pin 6 of the IF tube (6K7. Per the RCA Receiving tube Manual Version RC14 (1940), there is no connection on pin 6. Pin 5 is the suppressor; Pin 8 is the cathode. Both of these are at ground potential.

If AVC voltage is not sufficiently negative, I would not worry about that. The more strong the signal, the more negative the AVC voltage. The gain issue will cause low output from the detector and AVC circuit.

As you can see, there are 2 schools of thought regarding replacing all the caps. The term "shotgunning" or "loading the parts cannon" is often used to describe a technique that if you replace the whole thing, you will fix it. Like a shotgun which will more likely hit a target than a single bullet. However, the admonition to replace all the caps is because the paper - foil caps do in fact deteriorate with age even if not used. The danger here is that paper - foil caps and the old wet electrolytic caps fail by shorting and when a cap that is across the output of a power supply shorts, then bad things happen to the rectifier, power transformer or both. In a similar manner, if a cap that couples the plate of the 1st AF amp to the grid of the output tube shorts, this will cause the tube to draw excessive current, causing the plate of the output tube to glow red from being overdriven. This usually destroys the tube and possibly burns out the output transformer. If an output trans former is open, then the current of the output tube is dissipated through the screen grid which will glow incandescent, burn up and short to the control grid or suppressor grid. Of course, this is also bad.
Hence, the reason to replace the caps and check the coils, transformers and resisters prior to powerup.

Please also note, that for some of us, myself included, it is very hard to convey "tone" in writing. All advice given is advice only. Please don't think that anyone is scolding. You are now a valued member of our fraternity, and we are very happy to have you. Ask any question that you want without fear of anyone thinking poorly of you! There are no dumb questions.

Congratulations on the work done so far! Keep plugging at it and you will have restored a 91 year old radio to its "as new" performance and learned some electronic theory in the process, making yourself ready for the next project!!

Hope all of this helps!