My Astor is up and running, however I've found that I need the volume up to practically full to hear anything at all. I thought it was just bad radio reception until I found the record player was the same.

The volume control goes from min to max at about 3/4 of a turn.

I put a ohmmeter on the volume pot (500k ohm) and the resistance shows 0 ohms at min, 650k ohm at max, and oddly enough, about 45 ohms at 3/4.

Is this something that can be repaired? or do I just have a bad pot that should be replaced? If I should replace it, anyone know any good sources?

It looks like a standard modern 3 terminal, single gang 'D key' long shaft pot, but the key only goes partway down the shaft.

Thanks in advance.

Mawdryn my email is open send me a hello. I may be able to help. send me your phone no.

Normal swing for a pot is around 135 degrees. Whilst that is a very common off the shelf pot. Double check that it in not a tapped one & its tapped 40K.

I am suspicious that that is not the entire problem check capacitor values especially coupling ones.

Marc

What I am offering if you need one is a New Old Stock 500 k Pot if you need one that is.

What happens if you temporarily connect the middle wire to the non-grounded outer terminal? Should be VERY loud. If not, it's not the pot.

Thanks very much for the replies everyone.

Taller, Thanks for the offer. I'll see what I can do with the other fine suggestions here first.

Marc, this is after a full recap. The pot is centre tapped as you suspected. The centre tap terminal was hidden from sight before I removed the pot. - apologies.
If I'm measuring resistance between the the wiper and either end of the resistance strip, the centre tap shouldn't affect that should it?

Ian, I'm not able to do so presently, as I've removed the pot from the radio for a better look / clean.

I know I'm probably showing my ignorance with some of these posts and I apologise, but I am trying to gain a better understanding.

I noticed the following things with this specific pot which may or may not be relevant - I don't have any other vintage pots to compare it with...

I'm getting the same uneven resistance around the turn when the pot is completely removed.
It's also curious that I'm getting 650k ohms+ when the schematic says it's a 500k pot? (and that's with the pot removed)

The interior looks clean and has a vertical resistance strip around the inner wall with a copper spring arm on a disc attached to the shaft that rubs against it....

I can't help but suspect this isn't the original pot... it looks too modern with it's black plastic and metal base. (radio was built in '48)

For reference, it looks a lot like the circled pot shown in the image here except with a longer shaft:

https://i.stack.imgur.com/aD7nN.png

Sounds like the pot will be OK. 650k is close to 20% tolerance which for a C or "log" taper pot (usually used for volume controls) is about what you'd expect for the period.

Pot sounds like an "Aerostat" brand from the description. Might be original.

The non-linear resistance as you turn it is intentional. It's needed to get a smooth volume control. Signal level to sound pressure level is a logarithmic relationship.

With the pot out, try the "burp" test. Hold the wire that goes to the centre pin in your fingers (radio on, don't worry, it's safe). You should get a very loud hum. If not, you have a fault after the volume control. Bad or wrong value resistor maybe?

Some designs implement negative feedback by connecting the output stage cathode circuit to the speaker. If the speaker transformer primary is open circuit, the stage functions as a triode cathode follower with G2 as the anode. It will still work at low volume and low output power, not good for the output valve though. So check the anode voltage on the output stage.

Reading through the posts this almost sounds like a man made fault after a”total recap”.
A common problem and one that can waste a lot of time correcting.
Potentiometers are not a common failing component, and generally would go open circuit, probably causing the radio to go flat out at the stop.
Again a case of observation, repair to reasonable condition as far as performance goes then considering experience with side cutters and soldering iron, recap one at a time. Turning on between work.
Obviously some caps need to be changed almost immediately in the repair processes.
But this comes with experience.
JJ

Thanks everyone.

This is a lot to take in, but is certainly helping my understanding of how this all works together.

Ian, I'm getting about 226VDC on the plate/anode (guide states should be 235) , and about 244VDC on the screen/control (guide says 245) on the 6V6. I'll run the burp test when I get home from work.

Johnny, Thanks for clarification. What you said about 'man made fault' has brought up a good point. I forgot that I posted previously about an oscillating replacement 6U7G (I'm pretty sure the volume wasn't an issue then). I ended up replacing the missing grid cap on the original 6U7G and putting it back into service... I'm going to check that tonight. perhaps the new grid cap isn't making proper electrical contact....