On the weekend I was helping a friend out with his T21 chassis Pye Pedigree from 1967.

After attending to the immediate fault we suddenly had no sound - or at least, very weak sound.

Measuring voltages around the 6DT6 Syntector, I found only 7 volts on the anode, with or without the valve in its socket (no series heaters here!)

220k load resistor was immediate suspect but it was OK. What I did measure was 30k to ground. The only component in that path was a 470pf 630 volt styro cap.

"No, it won't be that!"

So I scraped the solder mask away between the track and ground, expecting to remove the 30k ohms.

Nup.

So I pulled the styro off the board. And that was it, it had become a 30k resistor!

Now I've been working with styros since they first appeared in the late 50's and I have NEVER seen this happen!

Styros are known for open circuits in horizontal oscillators that fix themselves when touched with a meter probe, but never leakage.

Anyone else seen this?

Might this be the start of a new "silver mica" disease with tin and styrene? That takes 60 years to happen?

No, never Ian.

But we were all praising Mica's not so long ago. As you say.

I guess we and components are all getting older and showing different effects.

BTW, I have noticed a lot of subject matter regarding failing power transformers.

Right up until even recently I have noticed no difference in failures.

Looking at 55,000 odd invoices over 60 odd years, less than 20 mention a faulty power transformer.
Excluding "monkeyed " ones and noisy/buzzing ones replaced under warranty.
So,...... fake news.
JJ

Added comment
Apart from the failures/burn ups of power transformers caused by faulty external components. A few of those over the years.

Might this be the start of a new "silver mica" disease with tin and styrene? That takes 60 years to happen?

Could be.

When it comes to old gear, these days I don't automatically trust any original component.

Tin is subject to allotropic change from white to grey tin (different crystal structures) particularly over time at low temperatures, so maybe there is something in your suggestion Ian. The changeover temperature between the two structures is nominally about 13C. White tin is metallic and malleable; grey is nonmetallic. Above tin in the periodic table are germanium and silicon, and grey tin has physical similarities. Below is lead which is well and truly metallic.

Another possibility maybe long term chemical interaction between tin and styrene, or some other form of corrosion, particularly at high humidity. It is easy to imagine moisture penetrating the styrene wraps.

In the case of the Styro capacitors the connection leads were spot welded to the capacitor plates which I always thought was aluminium foil. Hence the explanation of them going open circuit. At the spot weld.

Their leads were also thin and prone to breaking off, a negative if left loose in a tool box.

So, all sorts of corrosion/growth could be going on with aluminium, tinned copper wire, spot weld and plastics.

Hope I live long enough to find if this is a trend with Styro's.
And perhaps even problems with Philips Mustard caps in the future.
JJ

I was thinking of the tinned leads rather than the foil. But corrosion is probably the more likely cause, the mechanisms of corrosion being varied and sometimes very subtle in action. Then you can add fatigue, corrosion-fatigue and maybe even stress-corrosion cracking where leads are iron rather than copper. A capacitor manufacturer would be hard pressed to predict failure mechanisms over the life a product, let alone our venerable collectors items.

I have a box of NOS bakelite micas (all one value unfortunately) which I checked with a megger finding a number with a low resistance. So they were chucked into the oven at about 150C to see if the low value was due to moisture. No change in resistance, but wax came out of the bakelite - lead junction of all of these duds. So I tried a high resistance cap in the oven. Again no resistance change but the wax came out of that too. This made me wonder if bakelite mica caps were treated with wax to prevent moisture ingress at the lead. If so corrosion might have been on the manufacturer's mind.

I have never really seen wax used on caps & coils, as a good sealant. The major issue with the Styroseal caps tended to be heat soldering & Haemostats tended to be a good idea for stopping heat getting to the body of the cap. Can' recall ever replacing any.

This cap was right down near the lower edge of the PCB in about as cool a spot as you could get.
No sign of anything unusual looking at it.
Max voltage across it is about 90 volts, for a blue 630V cap.

That same set also had two identical 56k resistors go O/C (well more than 20 megohms) in the anode of the sync separator. One from B+, the other to ground.

Both H and V sync were still strong! Just the picture shifted 20% to the left on the raster.

Go figure!!

A number of people were looking for this picture shift fault over a few years without success.

The problem with heat is not environmental: Its from soldering. What I mean is; there is, if you do not watch yourself & apply heat for too long, it has been known to damage the clear plastic Styrene cap & that warning came from a reliable external source in the trade.

That is why I will place a heat sink plier, or Haemostat between the solder joint & the body of the cap, to protect that cap.