[mister_big77]

Good Evening All:
I recently bought a CoRadio (looks like around 1948) at a flea market. It came with both original keys, and the coin rejector is in it and works. Overall, it is good shape. I replaced all of the caps (except the micas), and any resistors that were out of tolerance. All of the tubes are good (35Z5, 35L6, 2 x 12SK7, 12SA7, and a 12SQ7 - six tubes total). I brought it up through the isolation transformer, variac, and dim bulb tester, and it appears to come up normally, but all I get is hum when I turn up the volume. I don't receive any stations. I have attached an external antenna, and when I get close to the antenna, the hum/static gets loader.

I would give the model number, but I haven't been able to find hardly any documentation. This is the box like version painted in textured grey, and you get one hour of play for a quarter. I understand that it is probably a pretty much standard AA6, but without a schematic/parts list, and my limited experience, I am afraid of doing something that will make things worse. Anyone have any ideas on what I should be looking at?  Thanks

[Radioroslyn]

So how do you know the tubes are good? Tested them?

[Radioroslyn]

Just as general troubleshooting advise. Sound like the local osc isn't working. Check to see if the 12SA7 is in the proper socket. Bent plate on the tuning cap or weak 12SA7. Check to see if you have hv on the plates and screen grids of all the tube except the 35Z5 & 12SQ7 just plate. No sg. - lead on your voltmeter connects to the - side of your filter caps.

[mister_big77]

Sorry about not addressing the tubes. Yes, I tested them, and they all tested good. No shorts or leakage, either. Since I haven't been able to find any documentation that fits the radio, I have guessed on the tube location (not exactly guessed, but not 100% sure either). I will go through the radio again, checking against other known six tube radios. Thanks for the quick replies.

[RodB]

Try checking voltage at pin 5 of the 12SA7 with a DMM. You should measure -2 to -4 volts if the oscillator is running. If it's zero there's the problem. Start by re-checking the oscillator solder connections, coil continuity, clean tube socket pins, pin 4 should measure B+. The capacitor connected to pin 5 is usually a mica and sometimes it can fail. There's usually a 22K resistor connected to pin 5. If it's more than 24K, replace it.

[mister_big77]

I have attached a diagram of what is on the chassis, and my guess as to what the tube locations are. I am also attaching the schematic that I have been using that is considered "close" to what I have. Does this look normal for an AA6 radio?

[RodB]

Just for reference, C5, C7, C17 and C18 are typically 250pf mica caps (red, green, brown dots) and C12 looks like a tone conditioner .001mfd to .01mfd selected to your taste. Tube layout looks good, you can pretty much verify that from the schematic vs wiring (eg. The oscillator coil will be connected to the 12SA7 socket). Looks like all you need is a running oscillator. If you have access to an rf generator you should be able to send the IF (455 kHz?) into the antenna and see the voltage at the volume control change with signal strength.

[mister_big77]

I went through and replaced the 22K resistor and checked one of the caps (and it was within tolerance). After that, the static was mostly when the tuning capacitor is closed. I was able to pick up one station near the middle of the band using a long wire antenna. I'm not sure what the B+ voltage should be, but I was measuring 80 to 90 volts. I will do more investigating and testing tomorrow.

[MrFixr55]

Hi Mr. Big,

There is only slight variation between manufacturers on the "AA5 (or AA6) circuit. Similar AC-DC radios of the AA5 or AA6 type have about 100 - 110V at the rectifier cathode, about 100V at the Audio Output plate and about 80-90V at the Output tube screen (the B+ to all the other circuits. B+ on the 1st Audio plate is typically low, in the 60V range or lower.

I am a believer in "divide and conquer troubleshooting.

Safety first!. Use an isolation transformer or make doubly sure that the chassis is at neutral potential when testing. Most folks move the power switch to the "hot" side of the circuit and installl plarized line cords to ensure that the chassis does not become hot.

After taking the above precautions, inject a battery powered audio source (CD Player, etc.) at earphone volume level between the 2 legs of the volume control. You should be able to get strong, clear audio.

If you can receive an over the air station, your oscillator works. If you get static while turning the tuning control, inspect the capacitor carefully for shorting. Plates can bend and aluminum corrosion between the plates may cause intermittent shorting.

This radio should be able to pick up whatever any other radio in your house can pick up, even with a relatively short wire, due to the RF Amplifier. However, do try an antenna of at least 20 feet, dropped out the window.

The plethora of computer driven devices, CFLs, LED bulbs, etc. make the average house a hostile environment for AM radio reception, particularly for those radios that do not use a ferrite antenna.

[mister_big77]

Thanks for the advice. I use an isolation transformer to a variac to a dim bulb tester. I have also rewired the power with a fuse and the return wired to the switch. I prefer to be safe than sorry. I'll check those voltages you listed. Currently, I have no negative voltage on pin 5 of the 12SA7 tube. I need to find out where that is going. I'll try some deoxit on the variable capacitor as all of the rotors look to be straight. I told myself I wanted to learn when I started this adventure. I will get this working.

Bob

[MrFixr55]

Hi Mr. Big,

If you replaced the electrolytic caps, you can take the dim bulb out of the circuit.

It is good to fuse the hot side of the power to the radio. However, it is not as necessary as fusing a radio containing a power transformer. The heater tap of the 35Z5 serves as the fuse. Since the plate of the rectifier is connected to the tap in the heater, any short in the tube or the B+ circuit will cause the entire current load of the short to present itself between the line and the tap, causing the heater to burn out, stopping the current flow. However, a fuse is usually cheaper than a 35Z5, so good on you for installing the fuse.

You definitely want to have the power switch on the "hot" side rather than the return side for safety. I don't know why the manufacturers insisted on putting the switch "on the low side" of the line. Maybe to lessen hum, but in that mode, if the switch is on the low side and the power is off, then the chassis becomes "hot" through the tube heaters which have a low resistance when cold. Even though there is a cap and resistor to limit shock, it is still possible.

Did you measure the 12SA7 Pin 5 voltage while actually receiving a signal? If so, did you lose the signal when attempting the measurement?

Do you have another AA5 type radio. Locate it next to this one and attempt to tune stations. If you receive more stations on this "test" radio, then yes, you have issues with the one you are repairing.
Try running a feeler gauge between the plates of the capacitor when they are fully meshed (Radio set for 530KHz).

At this point, I would avoid getting the tuning capacitor all wet bathing it in DeOxIt. The stuff is expensive anyway. concentrate on the bearings and the spring that grounds the rotor of the tuning cap.

Try to disconnect the leads between the tuning cap and the rest of the circuit, then use an ohmmeter and check resistance between rotor and stator for each section of the tuning cap while sweeping the band. The cap must read "open" at all times. Seeing anything other than infinite resistance indicates that the cap is intermittently shorting, from any surface corrosion "fuzz" between the rotor and stator plates. It is important to disconnect the connections at the stator otherwise you will be reading the resistance of the coils that parallel the capacitor.

Best of luck with this. There are many experts willing to help you conquer this and preserve a piece of history.

[mister_big77]

I checked the variable capacitor, and it looked OK. I replaced the old square mica capacitor with a new one, and got the oscillator working. I still have a lot of static and can only pick up one station (in a major metropolitan area), so it looks like I move on with my troubleshooting. It's a start :-)

Bob

[MrFixr55]

Does the static occur only when turning he tuning knob or is the static constant?

Does another radio located in the same location and plugged into the same outlet do the same thing?

Ohm the antenna coil (primary and secondary).

Another trick is to touch the stator terminal of the RF Amp section of the tuning cap with a long wire.
The section with smaller plates is the oscillator section. Don't touch that one. If you get a clearer signal, then troubleshoot the RF Section. You may have an open RF transformer coil (antenna coil). You may have an open secondary in the 1st IF transformer. I had that happen to me on a 1940 Mantola battery portable.


I previously said that most AA6 radios have very similar schematics. This one has a glaring difference, in the capacitive coupling between the converter and the RF amplifier, and the lack of a tuned circuit between the two.
You likely have the correct tubes in the correct sockets, otherwise, the radio wouldn't play at all.

[mister_big77]

I seem to go one step forward and two steps back. I was able to pick up station (barely) two days ago, and now I cannot. I checked and resoldered the first IF transformer connections, but it did no good. I got out a transistor radio and confirmed the oscillator is working, and at different frequencies, but I still get a lot of static at the lower end of the dial and no stations with a long wire antenna attached. Does anyone know where the first place I should be able to find a station with an RF/IF probe?

[MrFixr55]

Hi Mr. Big,

One thing I missed your answer to. Is there static as you turn the tuner, and the static stops after you turn the tuner? (Kinda like a scratchy volume control?)

I had a similar set that drove me crazy until I cleaned the cap by using a, air gun and feeler gauge between the meshed plates of the variable cap. I could not see the crystallized aluminum fragments, but they must have been there. I am suspecting that if the problem is the variable tuning cap, then the issue is in the "RF" section and not the oscillator section.

You can try signal injection. Is the static loud? Does it vary with the position of the volume control? If so, and the volume control is not the cause of the static, then the AF section is good. Confirm by injecting an audio signal.

There are 2 "classic" methods of troubleshooting, signal injection and signal tracing. Signal injection involves a signal generator that is tunable across both the broadcast band and the IF Frequency. This device is usually modulated by a 400 Hz oscillator or externally modulated. You inject the signal, usually at IF frequency, starting at the detector. If the tone from the generator is heard, then inject at the grid of the IF Amp tube. if the tone is heard, then inject at the plate of the Convertor. If heard, then inject at the grid. Then inject RF at the grid. If you have access to a signal generator, this is your best bet, as this tool is also used to align the radio.

In signal tracing, the tracing rig or probe must have a detector in circuit and an audio amp after the detector. One starts at the front end of the radio and attempts to detect a signal. If the signal tracer is sensitive, tunable and there is a very strong station, start at the antenna, then the grid circuit of the convertor (in your radio at C5). Then go to the IF Frequency and check at the plate circuit of the convertor. If a signal is detected, then go to the grid of the IF until the signal is lost.

Another trick:

Do you have an all wave receiver that has a "long wave" (about 100 - 500 KHz) band? If so, tune around 455 KHz to pick up the IF and place near the radio, then attempt to tune the radio in question.


Is there a lot of static and noise at the upper end of the dial?

One other possibility. This radio may be "transitional" between pre-war and post-war. Pre-war IF transformers were tuned by "trimmer" caps. Post-war radios were tuned by slugs. The transformers had mica caps across the coil. These caps suffer from what radio enthusiasts call "silver mica disease" where the cap intermittently shorts out. However, when this happens, the noise is across the board.

A pic of the top side and underside or the radio would help.

Good luck