XG-8 Mk3s, EHT unit

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Michael Bringer2004-02-23 22:23

I have a Dayton Wright Model XG-8 MK3s where the common EHT Unit has lost its function. The HT Transformer is OK, so any capacitors and /or rectifiers probably are gone. If anybody should be able to provide me with a circuit diagram, this should be very helpful. I am experienced in rebuilding EHT Units. Please send me a fax to +49 6131 582885. Thank you.

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Paul Young2004-02-23 22:23

I have not been able to locate a circuit diagram for the EHT unit - but I do remember that it was a conventional diode / capacitor ladder type, composed of approx. 8 to 12 repeats (can\'t remember) of the same components. My memory of these parts was that they were fairly easy to purchase ceramic (radial lead) caps rated at 1500 volts or higher and that the diodes were low current plastic body units rated above 1500 volts also. The important construction detail was that the ladder was built on a fixture that suspended all of the components in air while they were soldered. No PCB was used to avoid HV arcing on its surface. All solder joints were kept smooth to avoid corona generating sharp points. The completed assembly was then potted in a plastic box by pouring a melted ”wax” (not really just wax - it was a product formulated for HV potting) into the box while the ladder of parts was suspended in the center - well away from the walls. The input voltage to the EHT unit was 90 VAC. Most IM10\'s also had a bias adjust power resistor in series with the primary side of the EHT transformer (either 0-2.5K ohms or 0-10K ohms - 25 Watt) to allow for reducing the 90 VAC to lower values - but factory practice and recommendation was to set this resistor at zero ohms and run the system at full voltage. This produced an operating bias voltage of approx. 10.2 kV (+/- 10%).

NOTE - this is very difficult to measure! The only tool that we found effective for these tests was an electrostatic voltmeter. You cannot use a HV Probe that performs 1000/1 division. All probes of this type that I am aware of will SIGNIFICANTLY drag down the actual voltage due to their low impedance. Electrostatic meters look like they came from Dr. Frankenstein\'s lab (huge wooden boxes, big swinging needles, etc..) but they do not load the circuit. A probe can pull the EHT down to half of its real output in some cases.

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