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Mains Derived EHT

It is impossible to over-stress the dangers of mains-derived EHT systems found in pre-war and some early post-war sets. These systems are lethal, so treat them with respect. Unlike modern EHT systems, which are limited in the amount of current they can supply, these older systems using transformers straight off the mains can deliver enough current to kill an army. If you are not sure what you are doing, please ask a friendly colleague. For testing, use an EHT meter; they are still used by the TV servicing trade.

A few simple points will prolong your well-being. Keep one hand behind your back. This will prevent a shock across your body when testing. Don't laugh – a number of early TV engineers were electrocuted servicing these units. Note that the chain of 4.7megohms bleed resistors can go open circuit, leaving the capacitors fully charged after switch-off.

When starting work, switch off set. Remove mains plug. Discharge all EHT smoothing and, if fitted, reservoir capacitors by shorting them out with a 1k ohms resistor for 30 seconds and then by permanent links whilst working on the unit. Make up the links using plastic insulated rods, crocodile clips and probes. Do not come into contact with any part of the EHT circuit until you have carried out the above. Do not forget to remove all shorting links before applying power again. HVR2 and V16 valves are difficult to obtain!

So you have a faulty EHT power pack on your vintage television set. How do you start? Common faults are:

1. Breakdown of the high voltage secondary causing arcing, smoke, very low EHT or in the early stages, white interference spots on the picture with a general lack of brilliance.

2. Faulty EHT rectifier valve such as the Mullard HVR2, Mazda V22 or Marconi V16. Symptoms such as internal sparking, blue glow (soft valve) are common.

3. Failure of smoothing condenser, typically 0.1uF at 7kV. This can go short circuit and can damage the valve and if the fuses don't fail, the transformer as well.

If you have a failed transformer and cannot wait to get that raster, you may wish to try a few of these suggestions.

Silicon diodes of BY182 type will act as temporary test EHT rectifiers if used with a 47k resistor at the anode to act as a surge limiter.  Transformers removed from small neon signs produce around 2 to 4kV and make good emergency replacements. Wire in with EHT cable and take care.

The focus voltage in most old colour TV sets is around 4.5kV and can be hitched up to test. Note, the colour TV will no doubt have a live chassis. Run it from an isolating transformer for safety.

EHT triplers removed from old colour TVs and Thorn mono chassis will produce EHT when their input is connected to the line output valve anode (top cap).

Mains EHT transformers can be rewound to complete your restoration but the cost is high. Not surprising when you consider that the high voltage secondary may contain up to 7,000 turns of hair-thin wire in layers. And that's to say nothing of the wax and pitch that have to be removed to gain access.

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