"That capacitor" bites back. - UK Vintage Radio Repair and Restoration Discussion Forum

sentinel040 · 11th Dec 2021, 6:37 pm

This infamous component and its ability to create electrical mayhem should need little introduction. However, it can still bring about the odd surprise.

Enter from stage right, an Ekco Princess portable (P63); a handful of years ago this was repaired and a number of leaky capacitors replaced, including "that one". It has since been used from time to time powered by stacks of dry batteries.

Next to enter the stage is an Amplion Convette, to replace one that died during my childhood experiments; not deliberately I hasten to add and I suspect its eventual demise with an o/c transformer primary was down to natural causes, allegedly.

The Convette was duly repaired (shorted LT rectifier and weak LT smoothers) and placed in the Princess, but the HT was rather low at circa 45V. Thinks, HT rectifier? No, the AC volts are also low. Measure HT current, at nearly 20mA, that's too high. Hmmm; why is the radio drawing too much? Why is there a positive voltage on the grid of the output valve?

Well would you believe it, the (by now almost) new grid coupling capacitor had gone leaky. The little blighter....

Had I continued to use it off batteries I may not have noticed this until something else gave up the ghost. An image of the culprit is attached and it is not a type I would have expected to pull this stunt.

I hope this helps others I some way.

Ian

paulsherwin · 11th Dec 2021, 6:57 pm

It does happen now and then, presumably because of a manufacturing fault rather than through the aging process.

line sync · 11th Dec 2021, 8:16 pm

I had to read this a couple of times before i remembered that an Amplion Convette is a mains power supply that you can put in your battery set instead of buying batteries.
Not every body knows about these.

Robin

Mitch-W · 11th Dec 2021, 10:44 pm

I had to repair a 1950s radio where "that" capacitor was replaced few years ago. The problem was that it sounded distorted. I noticed positive voltage on output valve grid which increased with valve removed. The culprit was the capacitor which was replaced, but the repairman used NOS paper caps from late 1960s. All of the replaced capacitors were becoming leaky (although they had much less leakage than caps from 1940/50s), so I did the full recap to be sure. The NOS caps looked identical to the newer style film caps, but they were in fact paper dipped in araldite.

11th Dec 2021, 6:37 pm	#1
sentinel040 Hexode Join Date: Feb 2007 Location: Near Lichfield, Staffordshire, UK, most of the time and Crystal Palace, S London, some of the time.. Posts: 330	"That capacitor" bites back. This infamous component and its ability to create electrical mayhem should need little introduction. However, it can still bring about the odd surprise. Enter from stage right, an Ekco Princess portable (P63); a handful of years ago this was repaired and a number of leaky capacitors replaced, including "that one". It has since been used from time to time powered by stacks of dry batteries. Next to enter the stage is an Amplion Convette, to replace one that died during my childhood experiments; not deliberately I hasten to add and I suspect its eventual demise with an o/c transformer primary was down to natural causes, allegedly. The Convette was duly repaired (shorted LT rectifier and weak LT smoothers) and placed in the Princess, but the HT was rather low at circa 45V. Thinks, HT rectifier? No, the AC volts are also low. Measure HT current, at nearly 20mA, that's too high. Hmmm; why is the radio drawing too much? Why is there a positive voltage on the grid of the output valve? Well would you believe it, the (by now almost) new grid coupling capacitor had gone leaky. The little blighter.... Had I continued to use it off batteries I may not have noticed this until something else gave up the ghost. An image of the culprit is attached and it is not a type I would have expected to pull this stunt. I hope this helps others I some way. Ian Attached Thumbnails

11th Dec 2021, 6:57 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 28,012	Re: "That capacitor" bites back. It does happen now and then, presumably because of a manufacturing fault rather than through the aging process.

11th Dec 2021, 8:16 pm	#3
line sync Heptode Join Date: Sep 2009 Location: Kinver, Staffordshire, UK. Posts: 635	Re: "That capacitor" bites back. I had to read this a couple of times before i remembered that an Amplion Convette is a mains power supply that you can put in your battery set instead of buying batteries. Not every body knows about these. Robin

11th Dec 2021, 10:44 pm	#4
Mitch-W Tetrode Join Date: Feb 2015 Location: Celje, Slovenia Posts: 70	Re: "That capacitor" bites back. I had to repair a 1950s radio where "that" capacitor was replaced few years ago. The problem was that it sounded distorted. I noticed positive voltage on output valve grid which increased with valve removed. The culprit was the capacitor which was replaced, but the repairman used NOS paper caps from late 1960s. All of the replaced capacitors were becoming leaky (although they had much less leakage than caps from 1940/50s), so I did the full recap to be sure. The NOS caps looked identical to the newer style film caps, but they were in fact paper dipped in araldite.