EHT fizz - UK Vintage Radio Repair and Restoration Discussion Forum

mole42uk · 11th Jan 2015, 1:55 pm

Hi,

I'm working on a Marconi 2955 VDU driver board. I have ring-tested the LOPT and I have replaced the BU806 driver transistor. I don't have much experience with EHT circuits so I'm looking for guidance.

The problem is that when first switched on, after warming up for a few seconds I get a perfect display but after a few more seconds there are four detected symptoms:

A fizzing sound starts up which gets louder, but I can't work out where it comes from
The display starts to fade until it can't be seen even with the brightness on full
The current drawn into the board starts to rise.
The measured EHT voltage slowly falls to below 2kV

I'm hoping there's a capacitor which is breaking down and really hoping it isn't the LOPT !

julie_m · 11th Jan 2015, 2:12 pm

Have you tried it with the lights off? You might be able to see some visible discharge in the dark.

maninashed · 11th Jan 2015, 3:29 pm

Hi Richard, the eht brushing is a symptom of corona discharge, carefully dust off using a clean dry paint brush, around the eht connection to the tube. Beware, because the tube acts as a capacitor and holds a charge! Dust off the eht lead and lopt, then as AJS says, turn the lights off and see if you can see any tracking to ground. It could be a capacitor failing, but it could just be overloading caused by the excessive eht drain.
Bill

Mike Phelan · 11th Jan 2015, 5:23 pm

It's generally the EHT cap on the CRT.

The way I used to clean it and the CRT was to wipe both items with a rag soaked in meths.

In damp houses I wiped the CRT with WD40.

Sometimes the plastic cap was beyond repair because of continual tracking.

Dave Moll · 11th Jan 2015, 5:50 pm

Quote:

Originally Posted by Mike Phelan

In damp houses I wiped the CRT with WD40.

Wow! WD40 actually being used for Water Displacement.

GrimJosef · 11th Jan 2015, 6:21 pm

Quote:

Originally Posted by ajs_derby

Have you tried it with the lights off? You might be able to see some visible discharge in the dark.

Good advice. But do make sure you're not likely to touch anything dangerous in the dark or to trip over anything and inadvertently put a hand where you shouldn't. In all the years I worked with kilovolt supplies the only death I ever heard about happened to a chap looking for corona discharge in the dark. He tripped and fell, hands first, into the equipment he was working on (30kV or so with an amp or two behind it, but the mains can kill too).

Cheers,

GJ

mole42uk · 12th Jan 2015, 7:00 pm

Thanks! I sorted the fizz easily - the EHT cap was held off the tube by part of the tube support strap, once I'd moved that the noise went away.

I have another problem with the board now for which I'll start a new thread.

11th Jan 2015, 1:55 pm	#1
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,614	EHT fizz Hi, I'm working on a Marconi 2955 VDU driver board. I have ring-tested the LOPT and I have replaced the BU806 driver transistor. I don't have much experience with EHT circuits so I'm looking for guidance. The problem is that when first switched on, after warming up for a few seconds I get a perfect display but after a few more seconds there are four detected symptoms: A fizzing sound starts up which gets louder, but I can't work out where it comes from The display starts to fade until it can't be seen even with the brightness on full The current drawn into the board starts to rise. The measured EHT voltage slowly falls to below 2kV I'm hoping there's a capacitor which is breaking down and really hoping it isn't the LOPT ! __________________ Richard Index: recursive loop: see recursive loop

11th Jan 2015, 2:12 pm	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: EHT fizz Have you tried it with the lights off? You might be able to see some visible discharge in the dark. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

11th Jan 2015, 3:29 pm	#3
maninashed Rest in Peace Join Date: May 2011 Location: Preston, Lancashire UK Posts: 955	Re: EHT fizz Hi Richard, the eht brushing is a symptom of corona discharge, carefully dust off using a clean dry paint brush, around the eht connection to the tube. Beware, because the tube acts as a capacitor and holds a charge! Dust off the eht lead and lopt, then as AJS says, turn the lights off and see if you can see any tracking to ground. It could be a capacitor failing, but it could just be overloading caused by the excessive eht drain. Bill

11th Jan 2015, 5:23 pm	#4
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: EHT fizz It's generally the EHT cap on the CRT. The way I used to clean it and the CRT was to wipe both items with a rag soaked in meths. In damp houses I wiped the CRT with WD40. Sometimes the plastic cap was beyond repair because of continual tracking. __________________ Mike.

12th Jan 2015, 7:00 pm	#7
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,614	Re: EHT fizz Thanks! I sorted the fizz easily - the EHT cap was held off the tube by part of the tube support strap, once I'd moved that the noise went away. I have another problem with the board now for which I'll start a new thread. __________________ Richard Index: recursive loop: see recursive loop