I’ve consulted a couple of the books which I got from Jim Koehler, Elements of Radio Servicing (Marcus and Levy, 1947) and Elements of Radio, Second Edition (Marcus and Horton, 1948). It looks to me that my Philco 46-480 has the AVC bus connected to the screen grid, the output of the preceding IF transformer connecting to the control grid, and the suppressor grid grounded. It seems a close design to this textbook example of a 1st IF section, except that this example only connects the screen grid to a fixed 100V rather than to an AVC bus.
And, just as I discovered and corrected on my own schematic, the control and suppressor grids are no connected together.
Wasn't that the Suppressor grid that we were talking about, that got connected with the AVC?
Anywho, the initial question was, could the Control Grid be connected to Suppressor Grid?
To me the answer is, no it shouldn't.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
I agree, the two should not be connected. Due to poor image quality of the original Philco schematic, it was unclear if the design intended to connect the two. Had I thought about it more deeply, I would have realized that they shouldn’t be connected. But, at the time, I suppose I was more focused on trying to interpret the original schematic so that I could reproduce it.
Good morning, folks! As of two days ago, I am finally finished with repainting the exterior of my house. Now I can finally return to more important matters, such as this restoration!
I am almost finished with this restoration. All which remains is the power transformer. I cleaned it up some time ago, but never did purchase the sleeves to cover the cracked insulation of the high side wires. I had tested it using normal heat shrink (rated for 600V), but the high side is 640V nominal. Yesterday morning I ordered some 11 AWG fiberglass spaghetti style sleeving, rated up to 7kV, from Antique Electronic Supply. It should arrive on Wednesday. I had two choices--either 20 AWG sleeving or 11 AWG. I have some of the 20 AWG on hand, but it's too small. Even if I were to strip the original insulation off of the wires, I think they are 18 AWG. My thought is to leave the insulation intact, and cover it with the 11 AWG sleeve. Should fit just fine.
Here's my progress from the last week. I finished the push-button assembly, I wired in the 32-4074 IF transformer I had received from Mark Oppat, and completed the audio output section. I replaced the brittle wires on the cord for the speaker, and installed a new AC cable with a polarized plug. I reworked the power supply to include a 1A fuse on the hot side, moved the on/off switch to the hot side, and moved the X1Y1 line filtering capacitors downstream of the switch so that they are de-energized when the radio is powered off. This is all depicted in the Rev 1 version of my schematic.
Speaking of the push-button assembly, I discovered some damage to the right-most LC circuit. One of the wires had been severed (probably due to mice chewing on it, based upon the rest of the damage I've seen). I was able to repair it, but left a minor burn mark on the cardboard tube from my soldering iron.
Aside from that, I discovered one minor error on my schematic. When I created the Rev 1 version to depict the changes to the power section and resistance/capacitance substitutions I have made, I did not move a wire type call-out for one of the cables passing from the underside of the chassis to the push-button assembly for the power switch. This error only affects Rev 1, so I've fixed it and reuploaded here.
Here's the radio as it looks right now, waiting on the power transformer and the tubes to be inserted.
The sleeving for the transformer arrived early, so I went ahead and finished reassembling. The chassis looks far better than I expected! I’m very pleased with the outcome.
I plugged it in using an isolation transformer and powered it on. All of the tubes on the main chassis lit up, but the two on the RF chassis did not. The first in line on the RF chassis to receive power is the 7H7 used for the 1st IF transformer. I tested pins 1 and 8, and I believe there must be a miswire somewhere. My multimeter alternates between seeing 6.3V and OL when measuring AC voltage. Looks like I’ll have some hunting to do. But I am excited to see the chassis powering on and the dial lamps illuminated. That’s probably the first time in decades that this radio has had any power flow through it.
After sleeping on it, I believe the issue is that I may have forgotten to install a ground connection between the RF chassis and the main chassis. The screws connecting the two may have completed the circuit intermittently, but I need to make sure I installed a rigid connection. I will investigate this evening.
Very nice job. A real contrast between how easy the main chassis is to work on, vs how tightly the RF chassis is packed. Cleaned up very nicely and excellent workmanship! Hope grounding the RF chassis (braid strip?) rewards you with sound!
PS- Have a funny feeling that that shield does not belong on Detector-Audio tube . No way to ground it. Maybe belongs on the IF tube? the base shells of Loctal (Loktail?) tubes are grounded at the "tail".
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Thanks for pointing out that shield! I will try to find out where else it may belong. I simply installed it back where it once was when I bought the radio. There is a metal bracket which slips over a pin on the underside of the tube and then comes out and up the side wall of the tube. Additionally, there’s a metal brad between one of the socket rivets and the chassis. I installed a fresh rivet when reinstalling the tube socket. I figured that the shield must connect the two contacts.
Let me looks for some photos online of other 46-480’s to compare.
if there is a "flag" running up the side of the tube that touches the shield, then yes, a shield belongs there. Could be that the set was designed for a wafer and metal based tube instead of a full bakelite based tube. the shield looks too "spread out. Is that shield on the 6H6 or is it on the 6SQ7?
n a previous post a question was raised whether the AVC was connected to the screen grid off the IF tube. I don't think that it does. In some early "Screen Grid" sets with tetrode RF or IF amp sand no AVC, "Local - Distant" switches or even volume controls were used to vary the screen voltage to vary the gain of a stage. In these "modern" sets (any superhet set since about 1935 so using diode detection), the AVC voltage is generated off the detector and is used to bias the IF amp and sometimes the Convertor and RF amps through the return of the IF or RF transformer secondaries. In other words, instead of being between the grid and ground, the secondary winding is between the grid and the AVC bus. There should be a fairly large size cap (0.05 - 0.1 uF) between this AVC bus and ground. If this cap is missing or bad, severe "motorboating" will occur.
On your set, the AVC is generated from the diode section of the 1st AF Amp and the Detector / discriminator circuit, is filtered by a 0.05uF cap and controls the grid bias of the 1st IF and Convertor stages, but not the 2nd IF stage
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
It was, in fact, a missing jumper to connect the main chassis ground to the RF chassis ground. Just like that, all of the tubes lit up. But still no sound. I did a complete test of the audio output section, and it works great. The quality of sound reproduction is amazing if I feed a signal into the J200 port used for audio input testing. I then began testing the IF section. I could initially inject a 20mV IF signal into the input of the second IF amplifier, and heard an output tone. I used a 500 Hz signal on a 455 kHz carrier. The next step in the documentation is to input an IF signal into the input of the middle IF can by connecting the generator probe to the plate of the first IF can. I tried that, and immediately heard a loud pop. After that I was unable to get an IF signal to pass through, even if I went back to the first step which worked. I see no signs of damage, but something obviously happened.
I am wondering if I got lucky the first time when I tried feeding a signal into the grid of the second IF can. The documentation says to use a “moderate” level signal. I’m wondering if 20 mV is actually too low.
I can still inject audio to the audio section and hear it through the speaker. So that’s good news.
Joseph
Philco 46-480
Philco 49-906
(This post was last modified: 04-08-2025, 10:59 PM by jrblasde.)
If you didn’t put a DC blocking cap between the signal generator probe and the plate of the 1st IF it’s possible you blew something out in the generator as it got hit with a high DC voltage. That pop might have been a resistor or two in the generator output attenuator network or possibly worse if the generator is a solid state unit!
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