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replacement pots for 60's EHT

Discussion in 'Troubleshooting and Repair' started by paul_leslie83, Feb 12, 2014.

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  1. paul_leslie83

    paul_leslie83

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    Feb 12, 2014
    HI, I'm trying to fix a Phillip Harris 6KV EHT unit. Its late 60s and the only fault seems to be the pots. The main coarse voltage control has a slightly burned out winding and there is an internal pot (trim/calibration?) which is burned to dust on the inside. they are both wire wound type with the only markings as 'W3' followed by the resistance followed by 'D11'. as the trim pot has been set on 400 ohms (measuring from the remaining windings) for presumably the last 50 years i was going to replace this with a power resistor. i was hoping someone could point me in the right direction with respect to power ratings for this resistor and the remaining potentiometer. the unit has an internal 1A fuse (so <230W) but no actual power ratings. using the potentiometer (which is 5k) that works out at about 12W but but thats not counting impedence of the transformers (i haven't done any serious AC calculation stuff for years). Comparing the windings on the pots, they do seem thin compared pictures of modern 5-10W wire pots, does this suggest a lower wattage pot would be a suitable replacement?
    I'm usually good at this stuff but im a bit out of my depth with this one and i dont want to spend £15-20 on parts for them just to burn out but equally id rater not have to spend extra for 50W+ pots if they are not necessary.

    I have included some pictures:
    1 a crude circuit
    2 the smaller trim 500 ohm trim pot ( the remaining windings are just to the left of the wiper)
    3 the 5k voltage control pot and trim pot for comparison

    Any help, advice or finding any faults in my logic would be appreciated.
     

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