Recapping old tube equipment...

[eccerr0r]

Re: Recapping old tube equipment...

These are old axial electrolytic caps that are being used, and since they came in this radio, I have no idea where they came from. The caps are at a 45 degree angle off the PCB so that the leads are long enough to reach the holes...

The caps are Cornell Dubilier 85C 100uF 150V axial caps... and "Made in USA" (!) It looks like Cornell Dubilier is still in business and bought out Illinois Capacitor?

[eccerr0r]

Re: Recapping old tube equipment...

These are the caps... Looks kind of hideous?



The radio works... a little humming but not too bad.

https://www.badcaps.net/filedata/fetch?id=1974818

[juliadream]

Re: Recapping old tube equipment...

That is a serious "quality" bodge job from the past. I'm guessing they are replacing a board mounted vertical can dual cap and somebody has just shoved in whatever they had to hand at the time.

If they work and aren't leaking I would tidy up how they fit (get the babes vertical at least) with the ground end to the top and the "hot" ends through the board holes.. and then glue them down with a hot melt gun or something similar.

Most of this old gear hums a little, it's often from the series heater chain leaking to the cathodes, or low level signal wiring, or the nasty habit they had of using the tube base rivets as ground returns for various sections.. or maybe in the course of the previous bodging the 100 ohm (anything between 50ohm and 1k in reality) resistor often found between the reservoir and smoothing sections of the dual caps has been lost.

It's common on tube supplies to find a 40u reservoir and a 80-100u smoothing section and a low value resistor between them to filter the worst of the mains ripple. They did that to avoid overstressing a rectifier tube's maximum current rating (inrush on starting up with a huge reservoir cap.. I've seen big directly heated rectifier tubes actually arc over inside until they get hot and start passing current properly) Do you have the original schematics? If not any other schematic with the same or similar tube lineup will have a pretty identical power supply circuit.

There are various mods you can pull to beat the worst of the hum, but sometimes it's just stubborn. One of the best mods with a series heater chain is to rectify and smooth the heater supply, and add a bit more decoupling to the 1st AF amplifier stage HT+ area...

Don't forget that in single ended tube output stages the OP transformer primary winding is as much part of the PSU as it is part of the output section.. it acts as a choke (inductance) making a LC network with the smoothing cap and has a time constant effect on the HT ripple value.. Try adding a nice chunky 100ohm 1 watt resistor between the two capacitor + ends if they currently just go to the same place

[eccerr0r]

Re: Recapping old tube equipment...

Indeed it was a radial dual section cap that used to be there.

I was able to find the schematic, and the original values were 90 and 60 microfarads. The audio output transformer and a 900 ohm resistor sit beween the two sections - so indeed that transformer is used for filtering. A silicon diode is the main power rectifier. A 47 ohm resistor (which I think was replaced with a wire link ...) is the inrush protector.

The heater chain is series like most cheap radios (it's a 6-tube AM/FM with a few semiconductor diodes) - a series chain powered straight from the mains.

I don't think there's enough space to have both caps upright as they are way too big - have to swap with something else. That inductor to the left is the AM mixer/local oscillator inductor, so it's one of the high frequency components...