CurtPalme.com Home Theater Forum

thelasernut · Joined: 10 Jan 2007 Posts: 24 Location: London UK

I'm interested to know if people have attempted to make their own EHT MULTIPLIERS - in particular for Barco BG808s since it looks like I'll be needing to replace some of these parts in my own machine

Some years back I was working with EHT TRIPLERS for domestic colour televisions running around 25 kV and the insulation used was an epoxy compound which had been degassed under vacuum - we found most failures were caused by poor construction and mechanical faults such as connections where sharp points had created high electrical stress

In the development of Night Vision power supplies producing around 6 kV at bugger all current - any corona discharge would kill the EHT output so experimental electronics were insulated by emersing the assembly in a clear water-like liquid claimed to be a BLOOD SUBSTITUTE

Going back to the 1950's and domestic projection television using the Mullard / Philips Optical Box and MW6-2 CRT running on 25 kV at around 1mA - the whole EHT assembly ( consisting of a Ringing Choke and a Valve Diode / Capacitor Tripler using EY51 EHT rectifiers ) was mounted in a tin can filled with Transformer Oil. Low(ish) voltage connection were made via Glass / Metal Feed Through insulators and the EHT output lead was sealed with a screwed gland on top of the metal can. So far as I am aware the main causes of failures - apart from ringing choke transformers - was from worn out rectifiers - the Transformer Oil having given excellent insulation to the electronic "works"

So I am forced to wonder if a similar concept might be applied to the Barco Quadruplers ? - If failure is due to things like micro cracks developing in the potting medium - it might well be that using Transformer Oil for insulation (although it tends to be messy) could improve long term reliability ?

gjaky · Joined: 05 Jun 2010 Posts: 2790 Location: Budapest, Hungary

There have been ventures in repairing barco quads. The quad itself is containing a transformer and a 6 stage multiplier (sextupler). It looks like when a quad fails most of the times the capacitor that is filtering the last stage breaks down. The capacitor itself is rated close to the nominal output voltage (35kV I think). Drop in replacement is hard to find, let alone with higher voltage version. Using transformer oil is good idea, but could be problematic to adapt to the Barco "infrastructure", as you'd have to develop your own quad container etc.

thelasernut · Joined: 10 Jan 2007 Posts: 24 Location: London UK

If capacitor breakdown is failure mode then it would seem that replacement with a series pair and bleed resistors would overcome the single capacitor voltage limitation - assuming there was sufficient room for the additional components.

Although it could be left to chance - bleed resistors would ensure good voltage sharing over the series pair and could be of sufficiently high value to avoid undue heating - although these resistors would themselves need to be made up of strings of resistors to meet their working voltage limits and by the very nature of Sod's Law every joint and connection would need to be without spikes or defects to prevent localised voltage stress points

**Curt Palme** · Posted: Thu Aug 22, 2019 3:54 pm Post subject:

Without sounding like a downer, I've never seen anyone rebuild one that worked for any length of time. At least two forum members have tried to rebuild them, with zero results. I've got a few good tested ones here if anyone needs them, but if you do want to try rebuilding them, I might be able to come up with some carcasses as well. I've been throwing them out for the last 5 years since no one could rebuild them.

thelasernut · Joined: 10 Jan 2007 Posts: 24 Location: London UK

Yes Curt I take your point completely and I do want to buy spare EHT components from you when it's convenient - as yet with the BG808s still on the ceiling and a cluttered room I have not been able to check the actual part numbers - other than to know it has not been fiddled with so all boards have matching i/d's

I have had past experience with CTV triplers as I was involved in making and testing a so-called "High Technollogy Multiplyer" which consisted of two capacitor "reels" with the Diodes wound into the reels at strategic points - in this way we produced an electrical equivalent of a discreet component multiplyer but without soldered connections. We would then install these in competitors CTV's so we could make before and after measurements of Mains Conducted RFI to compare with legislated curves - RFI was significantly affected by the location and orientation of the Tripler module in the Television Chassis - and our aim was to minimise RFI and demonstrate the superiority of our multiplyer

The big problem when examining competitors Triplers was how to "see through" the Potting Compound to determine the component layout

Luckilly we had access to an old pre Health & Safety X-Ray machine from valve making days - for which our factory security Dept. kept the keys

It was in a 6 foot high cabinet with EHT & Current controls for an X-Ray Tube firing through a "Sample" to cast a shadow on a viewing screen

A Tripler placed on its turntable and rotated - revealed the construction - by how the Shadows of the Components moved on the X-Ray viewing screen

The alternative method of trying to dis-assemble potted triplers was no fun at all - even with so called Araldite softening chemicals it was hard and very destructive to the Tripler internals. However given good components and housekeeping - Clean Housings - Good Capacitors and EHT Diodes - constructing a tripler with a suitable de-gassed potting compound was not that difficult and good results were usually obtained.

We did have some initial problems with our Hi-Tech triplers - but these were at the level of the "poor soldered joint problem" - in our case capacitors had been damaged by some EHT Diodes with sharp ends to their leadout wires - which had slipped through the inspection process

tfuller · Joined: 17 Nov 2017 Posts: 22