Advice needed - slipping tuning drive cord

[westcliff]

The radio is definitely not working; the valve filaments light up but my lamp limiter goes dim then bright again. The mains transformer is okay as is the output valve and rectifier, so I suspect the soothing/reservoir capacitors. Can they be reformed without removing them from the radio by removing all of the valves except the rectifier?

Gus.

[Terry]

That should be fine, Gus.

Something to be wary of, though, is any form of resistive path from the HT line to ground. There is no need to remove it but the current through it will affect the lowest reading while reforming - it will still be passed even if the capacitors stop drawing current or come close to it.

Just calculate what it is for various voltages and subtract it from the readings you take.

[Refugee]

I just added a rectifier and smoothing cap to my isolating transformer and then connected 100K 2W resistors to the smoothing caps at the rectifier cathodes. I did two 60 year old amps by this method and it took three days. I did not even need to take the valves out but I was able to check the g1s for volts from leaky coupling caps while the main caps were getting close to reforming.

[Crackle]

Hi Gus
I would not recommend taking all the valves out and running the radio on full mains. The HT voltage, with no load on, may over cook the smoothing capacitor.
I would advise adding at least a 50k resistor in series with the positive output from the rectifier so the electrolytic can reform in its own time rather than forced to reform with potentially 400 volts straight from the rectifier.
Monitor the HT voltage on the smoothing cap as well as its temperature as it reforms. The voltage will slowly rise as the capacitor reforms, if and when it is within a few volts of the output from the rectifier (the other side of the 50k resistor) then it has reformed quite well and should be ready for use.
Mike

[Terry]

Gus, Mike makes a valid point there but don't forget my comments about potential dividers across HT supply.

You are more likely to find them in a TV rather than a radio but it is worth checking the schematic, just in case.

However, if you do find one, the simplest solution is to temporarily disconnect one end of it until the reforming is done.