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Setting up EHT on a BRC 3000/3500

 
crustytv
(@crustytv)
Vrat Founder Admin
Posts: 10842

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

eht2.jpg

eht3.jpg

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)

eht1.jpg

Hope this post was of use.

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Topic starter Posted : 10/04/2016 12:00 am
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