Randall RD5C hums

[Pixel]

I picked up a nice Randall RD5C, everything works as it should except it hums a little bit too much. A quick examination shows that C62 and C63 in the DC heater supply are bulging and leaking. In the amp C62 and C63 are only 3300uF while in the schematic they are 4700uF.

Should I just put a pair of new 3300uF in the amp and call it a day or should I go to 4700uF? The 3300uF caps look original and I can see no evidence this amp was ever serviced before.

I will do an ESR test of the rest of the electrolytic caps once I get the PCB out, but unless they test really horrible I will leave them alone.

RD5 {Schematic}.pdf

[Jon Snell]

No need to remove the pcb to test the heater capacitors. Remove the valves and unplug CN13, that's it.
The capacitors bulge on the top end and split the cutaway on them.
4k7uF 10v are less expensive than 3k3uF 10v. That is why they were replaced in the factory.
If you remove the pcb, replace C5 & C53. That will save you the effort at a later date.

[Pixel]

I finally got the caps I needed to finish this up. I had most of what I needed already on hand but a few of them I needed smaller diameter and I wanted one of them to be even smaller still because it was quite close to a heat sink. Most of the capacitors in this unit either had esr 5-10 times higher than expected or capacitance below 80%.

All the electrolytics were JCCON, a brand I have never seen before. Since only a few capacitors tested good, I decided to replace them all with Nichicons and Panasonics. The small value electrolytics got replaced with films.

I thought I was in trouble because when I first tested after recapping the volume was very low. The guy who sold me the amp said it had an intermittent problem. I realized I was using the cable that had come with the amp, using my own cable everything was fine. Turns out his cable had one stray whisker of copper that was shorting.

The amp works great now.

[Mick Bailey]

That's a good result. A thing that's sometimes overlooked when recapping amps is that the main HT filter caps get replaced but none of the low-voltage electrolytics even get tested. There was also the capacitor plague that affected lots of equipment at one time, mainly PCs but other equipment as well. Interestingly, I rarely have to replace Nichicon or Panasonic caps, but the no-name or obscure brands are often defective or show signs of creeping failure due to suspiciously high ESR which only gets worse.