Invicta CT7050 crackling EHT lead

[Mikey405]

Hi Everyone.

Tazman1966 came round today to help me fit an on-and-off switch to my Invicta CT7050 as I was finding it a bit tricky. Thanks Tas. Anyway, the set works fine now (although 405 still doesn't work yet due to a "modification" that someone has performed) but the EHT cable is constantly crackling with static. The lead has been replaced at some time and the solder joint looks reasonably smooth so there's no corona from the end. It's just that the whole lead is bristling with static and you can hear it constantly rustling away. Although I haven't measured the EHT yet, it doesn't seem high; the width and height are about right (although someone may have compensated for any high EHT by adjusting the width and height I suppose). I will check the EHT tomorrow, but in the mean time does anyone have any ideas regarding the problem? The wrong kind of EHT cable perhaps?

Thanks everyone.

From Mike.

[marcus 3500]

Hi Mike

At the back of my workshop theres an old CT7051 metal timebase lopt section if you want it and if it's the same type EHT lead i'll post it to you with your service manuals

Kind regards

Marcus........

[Mike Phelan]

Mike
How good is the cable, and do you know what it was replaced with?

From your description the problem cannot really be anything else. Hie ye to your local motor factor and get a piece of HT lead (the sort with proper wire, not graphited string). OK for about 30kV at least.
Or go here.

[ppppenguin]

Originally Posted by Mike Phelan

Hie ye to your local motor factor and get a piece of HT lead....

The inner from a length of some types of co-ax cable will also work well. Wants to be solid dielectric, not semi-air spaced, and the thickest you can find though the inner from UR67 is a bit OTT.

[Mikey405]

Hi everyone.

Thank you Marcus, Mike, Jeffrey and Dave for your suggestions and help.

I have just replaced the EHT lead with one that I had on a scrap chassis from an identical set and it works. When I started to remove the old cable, the soldered end fell apart as I was wiggling it and the copper came away even before I had melted the solder. I wonder if it was anything to do with the X-Rays from the GY501 or perhaps just the heat? The rest of the cable was very supple, but the end in the EHT rectifier / shunt stabiliser box was as hard as rock and would snap if you tried to bend it.

I also managed to unsolder all the linked parts of the system switch in the line scan box and scrounged a bowden cable from my spare parts bin, so the set is now "working" on 405-lines too. Well, it's sort-of working anyway - VHF seems to be extremely low gain at the moment so there may be a tuner or AGC problem lurking somewhere too.

If anyone would like to see the set working, I'll post a picture when it's all back together again.

Thanks everyone.

From Mike.
PS. Marcus - Although I don't need the EHT cable now, I don't suppose your chassis still has its "Set EHT" pots for 405 and 625 does it? The 405 line one was removed on mine (so both systems are sharing one pot at the moment), and the ones on my spare scrap chassis are both cracked.

[Mikey405]

Here's a picture of the old Invicta monster. I've just been watching a film called "Silent Hill" on it. It was very scary.

From Mike.