[mister_big77]

I posted a quick reply, but it seemed to disappear. Today, I attached my signal generator (RCA WR-50B) to the radio. I set the frequency to 900 KC. Brought up the radio and tuned up the middle of the dial, and was able to receive a station very clearly. When I tuned up the band slightly, I could hear the modulation from the SG. When I moved the frequency on the SG I lost the station. I attached my external antenna and could not pick up any stations. Since this is a six tube radio with a 12SK7 before the oscillator (12SA7), could my problem be in the first RF amplifier?

[MrFixr55]

Very strange!

This radio has a tuned RF stage, but the Convertor is not tuned. Only the local oscillator is.

From what you describe, there may be an issue with the RF, but I think that the bigger possibility is the oscillator. It may be that your generator provided the frequency that heterodyned with the station that you picked up. If the station was loud, the RF Amp may be good.
If you get a strong station and change the SG freq what happens? I wander if U got the strong station due to a beat freq from that station's freq vs the SG Freq. Do this again with the modulator off. Is the frequency of the station read out correctly on the Radio's dial when the station is received under this condition?

Ordinarily, I would say to touch a long wire to the signal grid of the 12SA7 but since the stage is not tuned, that may not tell you anything.

How accurate is the SG? Check against a known station on a good radio, loose coupled to that radio's antenna.

Connect via a capacitor to the 12SA7 side of C5, set for 455 KHZ and rock the SG dial till you get a loud tone from the SG's modulator. That should tell you what the IF Freq is.

Either your IF is off, your local oscillator is off, or the local oscillator is dead.

Take a transistor radio, tune to a station about 455KHz above a known good station, turn the chassis upside down and hold the radio near the oscillator coil. Tune the CoRadio to the known good station and rock the tuner around that frequency. The other radio should squeal or get quiet when the known station is tuned. If there is no response from the test radio, the oscillator is bad.

If you get the squeal but at the wrong freq, you could try the trimmer on the oscillator section of the tuner cap. mark the current location and rock the trimmer. If no improvement, return to the marked position.

said earlier in this thread that there was little difference between the various AA6 sets. This is me eating my words, as I have never seen a unit where the RF and oscillator was tuned but the convertor was not. I have seen AA5s with 3 section tuning caps but never saw an AA6 with a 2 section cap and the convertor input not tuned.

The one thing is that when you get this working, you (and I) will have learned a lot.

[mister_big77]

Thanks, I'll try as much as I can tomorrow.

[Arran]

I have a Fada 1001 with an extra RF amp, but I do not think it is tuned, the usual approach was for the output of the RF amp to feed into the grid of the mixer section of the convertor tube via a capacitor, sometimes with a choke in series. I will have to see what they did in the Fada, it's been a while since I overhauled it.
Regards
Arran

[MrFixr55]

Any AA6 or similar set that I had or now have, has a 3 section tuning cap, one section for the RF, one section for the mixer and one section for the local oscillator. Granted, my Radiola 16, 7 and 18 sets have an untuned RF stage, but these are TRF sets (Except that first RF stage is not "T"). Examples include my friends Philco 89 from #&!!, My 46-350, my Zenith 8G005 TO, 6G001 portable and 4 AM/FM sets. My 6S52 does not have an RF Amp but does have a 3 section cap because it has a rather fancy 2 stage antenna coil.

I think that the scheme that CoRadio and Fada used were to save money, possibly at a loss of some selectivity but the gain of having an RF Amp for more sensitivity and the important protection against "broadcasting" of the local oscillator, especially if the CoRadio "hotel" radios were connected to common antennas.

[Arran]

Here is a link to the Fada 1001 schematic, it looks very similar to the Coradio http://www.nostalgiaair.org/PagesByModel...005623.pdf

[mister_big77]

I have attached a diagram I made of the front end up to the first IF transformer (and a bit more). I have also attached a picture of the bottom. I haven't had time to do more testing yet, as I replaced my water heater this morning.

Bob

[MrFixr55]

Hi Mr. Big,

Nice job wiring!! Nice job on the drawing also. I note that your drawing shows a tuning Cap (capacitor) having a large section and a smaller section. The smaller section should connect to the local oscillator circuit. The Large section should be connected to the RF circuit. In your drawing, the green wire (antenna coil) appears to connect to the smaller (oscillator) section. It should connect to the larger (RF) section.

If that fixes it, great. BTW, when you get it working, you can replace the antenna coil with a loop or loopstick antenna.

Good luck

[mister_big77]

Thanks!! I'll check on the variable capacitor connections.

Bob

[mister_big77]

Voltage Marked Up Schematic

[MrFixr55]

Hi Mr. Big,

Read your marked up schematic. I don/t think that it makes too much difference which side of R15 that the B+ to the output stage connects to. It should probably be on the other side of R15 but 2 bigger issues:

I compared this schematic to a FADA 1000, also a 6 tube set. Amazingly, except for Screen and B+ for all tubes other than the Output, Plate, the FADA having a loop antenna vs the CoRadio having a transformer for a long line antenna, and the presence of a wave trap, the FADA 1000 is remarkably similar to the CoRadio. A link to the FADA 1000 Schematic from Nostalgia Air is below:

http://www.nostalgiaair.org/PagesByModel...005622.pdf

The wave trap may be there if this radio was popular in the Great Lakes area or other maritime areas, as believe it or not, there was long wave marine communication via Morse Code (I remember hearing this on radios as a kid in the 60s, especially on fall nights.)

The most glaring issue is the 1st RF (12SK7) Screen which should be the B+ Value of 69V that the Output tube screen and IF Plate and screen should be, instead of almost zero V Check the wiring from the 1st RF Screen to the Convertor (12SA7) Screen and B+ to the Convertor side of the 1st IF transformer. Start out by ohming it. You may have a bad connection despite your great workmanship. Reheat them/ The lack of screen voltage is likely killing the RF Amp.

The other issue is the value of coupling cap C5 between the RF Amp plate and the Convertor grid and the coupling cap for the oscillator coil. You have a "?" for their values. For the FADA 1000, they are both 100pF

Also verify that you have the Antenna coil connected to the RF Stator of the tuning cap (larger plates) instead of the Oscillator (smaller plates). This will detune the Oscillator, and there would be no input to the RF like there should be.

Hope this helps.

[mister_big77]

I'll check what I can later today. When I replaced the filter caps, I cut the wire at the cap and attached the new ones to their respective wires. So, wherever they were attached to, they still are.

Bob

[mister_big77]

It keeps getting more strange. I was checking the first IF transformer to see what voltages I got going in and out, and now they are the same. So I am now able to receive two stations on top of each other. One is around 960 and the other around 1400 (hmmm, 456 KC apart?). I can only receive them if I attach the long wire antenna, and they come in better if I have my hand near the chassis. the transformer antenna seems to do nothing, but I haven't checked it for continuity and proper connections yet.

[MrFixr55]

Yeah, I think you want to check the antenna coil. The primary should show continuity from where the 0.01uF cap connects to it to the "B- Mecca Point" (as opposed to the chassis). Ohm across the RF side of the tuning cap (the section with the larger plates. You should have continuity. There should be a wire going from the cap stator connection to the secondary of the antenna coil. the other end of the secondary goes to the Cap Frame (rotor) or to B-Mecca. There may be a center tap connected to the AVC circuit, but the important connections for the secondary are between the rotor and stator of the RF Section of the Tuning Cap.

Voltage on the plate side of the IF transformers vs the B+ Side will likely not show great differences as the plate resistance for IF tubes is around 1 Megohm as opposed to <50K hm for Audio Power Output tubes.

You can try to connect the long wire directly to the RF Tuning Cap section stator connection. You may get many more stations, but if the coil is open, you will get images. You can replace the antenna coil with a loopstick or loop antenna from a similar vintage radio. In a suburban area, you should not need an external antenna if you install a loop antenna in place of the coil.

Your pictorial leads me to believe that the secondary of the antenna coil may be connected to the oscillator section of the tuning cap and not the RF section.

Good luck!

[mister_big77]

Thanks. I had a busy weekend, so I hope to get to work on some of it tomorrow. I also have a Philco 90 that a "radio guru" (literally he's a guru. He's over 90 years old and been in electronics all of his life) is helping me with. Tomorrow afternoon we start testing the 90 :-}