[MrFixr55]

Hi Jeff,

If you have worked on any "post-war" transformer powered radios, you will notice some differences. In post war radios, the chassis is "B-" and tubes requiring bias are "cathode biased" by individual resistors between cathode and chassis.

In this set as in many pre-war sets, they referenced the cathodes of the tubes to chassis ground, but the chassis is positive with respect the most negative of the power supply, the HV secondary center tap. This is the reason that I wanted you to verify resistors #23 and 24 as they partially complete the circuit between the cathode of the output and, in fact, of all tubes and the power supply. If these resistors are bad, all bets are off. I would replace these with 2 Watt metal film resistors, whether good or bad, as they carry all of the current of the B supply. Since the voltage drop across these resistors is fairly low, the power dissipation is low, but pretty close to 1/2 watt.

The other "biggie" is a rather unusual feedback arrangement where the secondary of the audio output transformer is between ground and the output tube cathode. This provides a feedback path (I would assume negative feedback, but it could be positive feedback to enhance bass response). If this connection is not made, no current will flow through the output tube and there will be no audio. If someone modified this and connected the output tube cathode (pin 5) to chassis ground, this would work, but if connected to the output transformer secondary but the other secondary lead is not grounded, than there will be no audio. I have never seen this method of providing feedback, but I also never saw the output tube plate to 1st audio tube screen and input feedback used in the Crosley 5C2.

[Jeff Lepore]

I have now grounded #26 cap to resistor #24. The other day that resistor started smoking so I replaced it with 470 ohm. That’s all I had. So I’m now reading 470 to chassis ground.

[morzh]

You did not ground it, you connected it. Grounding only means connecting to GND (chassis).
Now, 470 Ohms is a bit too much, this is your negative bias for the 41's grid, and it will be about twice as negative, which might affect the output.
As for smoking, if it smoked for teh reason of too much current, 470 ohm will have to smoke almost twice as bad as the power dissipation at the same current is proportional to the resistance (I-squared * R).

Have you found out why it smoked?
What else, oter than the #19 1M resistor on one side and what the sch has on the other side (#23, 15m K of 75 tube and the vol pot) was connected to it?

[Jeff Lepore]

I meant I connected it"…. And maybe it smoked because 26 cap was grounded..Resistor #19 is 1.2 meg. And 23 is 49.ohms. So now when I power it up I have a humming sound that can be heard from 50v up to 110. Speaker coil shows 6.8v with set powered to 75v AC. I finally have sound. is that good or bad???

[Jeff Lepore]

Btw, I forgot to mention when I got the set, in the tube socket 75 there was a 7B6. The 7B6 was plugged into an adapter so it would fit into the 75 socket. The set didn’t power up anything so I put back in a 75. And also the hum is there whether the tuning push button is in or out. Maybe that’s why no wires are going to the push button assembly. it was modified to bypass the push buttons all together. Am I wasting my time on trying to get this mess working again?

[Jeff Lepore]

I checked the voltages on cap 25 and 26. Both were within range of your numbers. Also checked tube 41 pin 2 and 5 were good. And good news touching grid cap on 75 produced a buzzing sound along with hum. Making progress…. I’m getting excited bringing this thing back to life. You guys are good….

[Jeff Lepore]

After  connecting cap 26 to the correct location. I checked the voltages on cap 25 and 26 and they were in range of the numbers you suggested. I then started getting a slight hum. I touched grid cap on tube 75 and got a buzzing sound . So now the audio section is working. The hum is there whether the tuner push button is pushed in or out. I’m guessing previous owner modified set and bypassed the push buttons.
    Btw, I forgot to mention, when I got the set there was no 75 tube. In its place was a 7B6 tube with an adapter to fit the 75 socket. I removed it and replaced with 75. Someone was messing with it. So I’m making progress. you guys are good. Also I had to replace resistor 24 with a 470. That’s all I had. hasn’t burned up yet. I’ll put a 300 in on Monday. Thanks for all the help. I also wrote back a couple of times last night but couldn’t find them in this discussion.

[MrFixr55]

Jeff, NO!! you are NOT wasting your time! Not only are you restoring history, you are gaining troubleshooting skills, so keep at it, and you will get success. Worst case scenario, my usual statement, install a "Radio / Aux Input switch" and jack and you can pipe old time radio through it!

You stated that you moved Cap # 26 negative lead to R24, the important thing is to ensure that you connect the negative lead to the JUNCTION of Resistor #26, Resistor #19 AND the center tap of the transformer HV secondary.

This should not have caused the resistor to burn. Having Cap #26 connected to the chassis would have increased the draw through Resistor #24.

Definitely replace both Resistors 23 and 24 with a minimum of 2 watt resistors.

Next step is to touch the grid cap of the IF tube and determine if there is noise from doing so, especially hum or continued static. If so, that tells us that stage is working. If not, troubleshoot in a similar manner as the Audio stage. Measure Plate, "Grid Plate" voltage on the convertor (1st detector) tube, the 6A7.

If you do not have one, obtain a copy of the RCA Receiving Tube Manual RC13 from 1937. Here is a link to the World Radio History website, a great resource!

https://www.worldradiohistory.com/BOOKSH...13-OCR.pdf.

For some reason, many schematics of this era are published without pin numbers for the tubes. In addition, the RCA Receiving tube has some great descriptions of circuits that these tubes are designed for, and a good description of the "Pentagrid Convertor tube", introduced around 1933 or so.

I will post more later, when I get snowed in.

[RodB]

Did you guys see that? You're good! Better get it running before he changes his mind.

You can probably connect an antenna to the radio so it can sing.

[morzh]

A radio with RF intact, even if not aligned, will receive a strong local station with no additional antenna.
Mine do it in a basement, of all places.

[RodB]

Yes Mike, but do they sing?

[Jeff Lepore]

IF tube 78 doesn’t make any buzzing sound when touched.

[morzh]

Any sound whatsoever? It is not supposed to actually buzz/hum, but may introduce some sounds. This is an RF stage.

Actually, short of trying to put an IF frequency through, I'd suggest hook up a wire to an antenna input and see if anything comes through.
Or maybe get it near another radio (transistor?) and see if they howl together like a pair of wolves at the Moon?

[morzh]

Rod

Absolutely! Like Enrico f... Caruso!

Speaking of Caruso:

Two Odessa (Ukraine) guys meet.
One says:
- You know, Enrico Caruso comes to Odessa! Can you believe it? I am so excited!
The other says:
- Caruso? He so sucks!
- Why do you say so? Have you ever heard him singing one of his repertoir songs?
- No, but Rabinovitz hummed one for me.

[RodB]

mmm hmmmmm