What would cause very low volume on AM. It has normal volume on Shortwave and police band. The radio I am working on has been fully recapped. The radio is a Silvertone 4784.

Here is the schematic:
http://www.nostalgiaair.org/resources/224/M0017224.htm

What section under the chassis should I start looking around for a problem.

Thanks for the help in advance.
Kris

Initial speculation would be a wiring error or, a wrong component. Normally the high bands die, when tubes get tired.

You are going to have to go over everything associated / common to the faulty band only. On BC band that will incude the AGC / AVC.

Marc

An examination of the schematic reveals that the 1st 6K7G RF stage is used only for the broadcast band.
For the shortwave bands, the signal is routed from the antenna through coils L2 and directly in to the 6A8G mixer, bypassing the 6K7G RF stage.

So there is a very good possibility the trouble is in or near that 6K7G RF stage.
Could be:
a bad tube
a bad L1 antenna coil
a bad broadcast section of coil L2
a faulty bandswitch
a miswire
a need of alignment, perhaps just tweaking antenna trimmer C1 at 1400 kHz or so(on top of rearmost section of variable capacitor)

Thank you both for the help. I will check the 6K7G tube first and hopefully that is the culprit.

Edit: The 6K7G tubes test good but the 6A8 tube is in the questionable section of the emission test.

Could the 6A8 tube be the problem with low volume only on BC band?

Kris

It would be unusual for the 6A8 to play up at the low frequency even that emission is low. As I have stated it is more likely for it to cease oscillation at higher frequency.

I do find it odd to have a TRF stage on BC band. Normally you would have it on the SW bands as 6A8 is not a SW tube and was superceeded by 6J8 & other Triode Heptodes in SW. The RF pre-amp was often used to assist the 6A8 on SW.

Check that the AGC line is correct. it is also applied on SW, which is not common, as it can destabilise the 6A8. Too much AGC on BC band can shut the 6A8 down.

Marc

I know this sounds stupid but the schematic is basically unreadable by me and I can not see any section marked AGC. What is the AGC hooked too? Which tube and pins? Thank you for the help.
Kris

Pictures under the chassis. I am working on testing the coils.

The big 20K dogbone in 060 looks a little like it has been hot. I do not like the look of the bare wire near the hole adjacent the pot to the left 059.

AGC is not marked, it requires circuit knowledge to identify it. AGC Voltage is developed on the diode plates of the 6B6 and is fed to the 6A8 & 6K7 via the voltage divider r10 r11 then r9 which also supplies grid bias to the eye tube. R1 is also part of the AGC / AVC. The stronger the signal, the more is the Neg voltage sent to the control grids of the tubes that AGC is applied too 6K7, 6A8. It is the change in AGC volts that open & shut the eye of the tuning indicator & that can be used to guage if there is too much AGC voltage.

I note that c20 can also feed positive into the AGC, which would impinge on the eye tube & AGC. Tubes do not like positive on thier grids. The eye tube would not function.

Marc

The 20K dogbone resistor measures around 20K so it is still good but I will replace it just to be sure it is not causing the problem. What wattage should I use for that one?

I checked all of the bare wires and they are not touching the chassis or any terminals or wires.

I will go ahead and replace R10, R11, R9, R1 & cap C20. I will let you know how things are working after that.

The eye tube does not open and close as it should. It is wide open and barely moves.

Marc, Thank you for the explanation of AGC, I think I get it but I don't know if I would be able to spot it in another radio yet. I really appreciate your help.

Kris

I am not a great believer in replacing parts "willy nilly" the wattage of the resistor is normally in the specs. If it tests 20K I would leave it alone. I prefer to measure the resistors (not aways posible in circuit) however it is not inconceivable that there is an open resistor in the AGC rail.

For a shadow angle of Zero degrees 6U5 requires -22V on the grid (pin 3, 6pin tube). This will only be obtained on a very strong signal.

If you measure the grid voltage of the eye tube (Grid to chassis / cathode) use a very high impedance meter. There are some articles on eye tubes, if you wish to study there operation, to get a feel for their behavior. ARC did have a very detailed article on them recently & would be worth following up on.

The very fact that the eye tube is doing naught, does tend to point at at an issue in, or affecting, the AGC.

Marc

swap the two 6K7G tubes. see if that improves AM reception. re-seat tubes. burnish switch contacts. ask yourself 'where do I look'. AM circuit because it has lower volume than SW, right ? look at the diagram. it shows the rotary switches in the AM mode. tracing the lines all the way through you can see the 'circuit' that has to do with AM from the antenna through the first tube to the last and through everything in between and finally through the speaker. these components should be familiar to you. what does each component do along the way ? if it is not doing it thats where the problem is