Hi, I'm restoring an old 1948-1950ish Astor console Radio/gramophone and although the unit works, there is a very loud hum and the audio output is very quiet. I understand the filter caps are buggered, and I am unable to confirm whether the valves are all functioning correctly (one is missing the metal cap off the top (the wire is soldered directory onto the tip atm). Does anyone know where I might obtain a schematic?

Also, if the identification markings are worn off the valves, how do I figure out what they are? (there's only 5 valves in the unit - 2 in metal cases.

I've been able to obtain a schematic for this unit through the help of Paul Ledger (http://www.southcom.com.au/~pauledgr/)
It's a Astor HM radiogram and I've found out exactly what the valves are.

G'day Mawdryn,

At this time I wouldn't worry too much about the soldering on the valve that is missing the grid cap though I do agree that it is ideal to have a cap and associated clip so that the valve can be removed when needed.

The priority at the moment is the dried out filter condensers. If there is hum then the cap(s) badly need replacing. If you get the occasional cracking noise in the speaker, this is the cap shorting out temporarily and this isn't good either because it will build up heat and explode.

Replacement condensers can be obtained from Reurrection Radio in Melbourne - details are in the service directory on this site. They can be obtained elsewhere too though you will need to do a bit of browsing to find the closest place to you.

Faulty filter condensers can reduce the volume a little bit but if the problem persists after renewing them I would look at the receiver's alignment before thinking of replacing valves as tweaking the set costs nothing, providing you have the right gear of course.

Thanks for the reply Brad, I've been doing a lot of research, and your site has a lot of great information. I've learned a lot from your site alone. I have to admit though, I've been having problems locating the filter caps as the schematic specifies 525V caps, although it's only on a 280V power rail. Does that make sense, or can I substitute 450V caps which are much easier to get a hold of.

The voltage on the two outer secondary windings varies alot between models and can vary between 350V and 1000V. You would need to remove the faulty condensers and then measure the voltage on the secondary terminals - make sure you locate the correct wires as most have a centre tap which serves the return from the cathode of the rectifier valve. I usually try to get caps with the highest possible working voltage as a tougher cap will last longer, in theory anyway, though it is also a way of playing safe. To be really sure though, I wouldn't recommend using anything less than the values of the original components. Manufacturers were never generous with specifications and this invariably means that if they put a 525WV condenser in a model then there would have been a need for it.