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Setting up EHT on a BRC 3000/3500

Colour Television

Last Post by crustytv 9 years ago

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crustytv

(@crustytv)

Posts: 12486

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There are two recommended and official methods

The first method is by setting the collector peak voltage at VT504, this requires the use of the Thorn CPV diode kit. The device basically takes the collector voltage, rectifies and smooths it and then is used in conjunction with an AVO 8 (shown below). Not only can you set the 3K series chassis using this device but also the other chassis in the BRC/Thorn family, details of which are in the following table.

Series	Collector	AVO8 Range	Meter Reading
2000	VT7	250V	155V
3000/3500	VT504	1000V	490V
8000	VT401	2500V	1280V
8500	VT401	2500V	1350V
4000	VT307	1000V	550V

The second method involves measuring the current in the focus resistor chain which is connected in shunt with the EHT circuit. To do this you have to transfer the focus panel to the long earth tag on the left-hand side of the spark protection on the CRT base PCB. With an AVO 8 set on 250uA range between chassis and the focus panel test point, with the contrast at minimum and the three gun switches off, the SET EHT control is adjusted for a reading of 12uA.

Other ways

Some engineers of old admitted they just adjusted the control to obtain 58-65 on the Chopper rail fuse F603. Although under certain conditions and depending on the Line Timebase module, this was known to cause excessive EHT. Although not officially approved many service engineers were advised if using this method to set it for 61V in the interests of preserving the tripler.

Monitoring EHT

I like to monitor the EHT when repairing a set and in the interests of knowing if its too high. However with the advent of solid state receivers the use of EHT wands like the LHM 80B (see below), carries a risk of flash over and subsequently turning the little 3 legged beasties and their multi-leg counterparts into toast. Far better to monitor EHT is with an in-line device like the Brandenberg ( see below)