Monday, October 8, 2012

RCA Victor 8x541 schematic and trouble shouting

Bringing life to valve radio (RCA Victor 8x541)

The antique valve radios suffer from aging. If you like this kind of devices and want to bring them to life, here is a simple procedure to trouble shout.

1. Check the power cable for any damage before plugging it.
2. Turn the radio on.
3. Did all the valves glow?! No = One or more of the valves are burnt out and need replacement.
4. Yes all valves are glowing and after that you here nothing but humming. This means the filter capacitors needs replacement.

I bought RCA Victor 8x541.
I turned it on and got nothing but strong humming and it kind of pick a station.
No cover = this means there is no loop antenna for the LC tank circuit.

I replaced the burnt light bulb with a 16Watts 12V because this is equivalent to the 4Watts 6V ... right?!

I replaced the power source big capacitor with two 47uf 160v capacitors. This terminated the humming totally.
Here is two schematics of five valves superheterodyne receivers. I looked at those schematics and found that this receiver is not much different. I simply couldn't find the free official schematic of this Victor 8x541.

Clearly this very double capacitor of 30uf 50uf 160V is doing the job of filtering the humming in the final output and all the stages before.It is #15 in the schematic above.

After that I connected the classical ~260uH coil discarding its low number of turns parts to replace the loop antenna. It worked good but not really sensitive. I decided to boost the reception by adding an antenna wire. I rapped the wire round the power cable to pick the signal from the power cable. I put 1nf capacitor between the antenna wire and the radio to do two jobs. First to filter the high voltage of the radio from the antenna and to filter the AC humming signal noise from the LC tank. I mean this 1nf capacitor will work as high pass filter. Check the schematic above for the loop antenna in #20. If you connect direct wire to the loop to make an antenna you are actually connected to -B. It should be the neutral of the AC but what if you connected the AC mains in an inverted way?!!

Be careful when dealing with those valve radios for they are mainly ALL not protecting you from electric shock for innocent antenna wire!! But with this 1nF capacitor you can largely screen the AC voltage. Nevertheless, It will give light sting of electricity instead. I noticed very strong and clear reception if you connect the same wire to an EARTH like water pipe at home. This will work a wave trap or an injection of the AM signal by picking it from the AC mains wire and sending it ground and injecting some of it to the LC tank circuit through the 1nF capacitor.
Check the video above and look at the green wire.
The double capacitor that I mentioned earlier is not mistaken for being yellow big one with three wires (black, red, blue).

I noticed not much written online on how to fix those lovely antique radios. A friend of mine just bought an older valve radio than this about 1948 one. I promise to report it here on this blog if I fixed it. Thank you for reading. 


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