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Old 31st Dec 2017, 9:54 am   #1
GeoffPaige
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Default Telequipment D34 the display is too bright.

Mains transformer PCB track pealed, shorted and repaired but now have bright trace with little control over brightness. Flyback can now be seen. Was working perfectly before the short. Any ideas where to start? Power supplies seem ok but haven't measured eht. X&Y channels seem ok. Any help greatly appreciated.
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Old 31st Dec 2017, 2:35 pm   #2
Tim
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Default Re: Telequipment D34 the too bright

I would start by Checking voltages on brightness control, control itself and associated resistors.
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Old 31st Dec 2017, 8:39 pm   #3
Radio Wrangler
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Default Re: Telequipment D34 the display is too bright.

It sounds like you've lost grid-cathode bias voltage for the CRT.

You should see a long string of resistors and pots spanning the negative EHT supply to chassis. Along this string will be the brightness pot and the focus pot with several fixed resistors. If the string is broken somewhere nearer chassis than the brightness pot, the current in the string will go to zero, the gathode and grid will be at the same potential, neat EHT negative, and the other anodes may be close to chassis potential.

You can fault-find two ways. Cold with an ohmmeter is safest. If you want to measure voltages, you need a very high resistance EHT probe. that is not only safe at >2kV, but also doesn't load the high resistance string. Without an high voltage probe, you could float a voltmeter connected across grid and cathode, but you have to be very careful, and it only gets you so far. The CRT is already telling you there isn't enough negative bias from grid with respect to cathode.

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Old 6th Jan 2018, 11:31 pm   #4
GeoffPaige
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Default Re: Telequipment D34 the display is too bright.

Thanks, There is a chain of 10M resistors driving an FET (gate) pin 223/32. Resistor chain looks good as I get progressive voltage drop across the chain i replaced the 10Ms just in case.
Have the required 150v across the zener, on the drain but the source isn't providing any voltage to the blanking circuit. Also -1630v rail is actually -1400v
Don't seem to have any measurable voltage across the intensity pot so wondering if the FET is faulty. Any suggestions for a replacement for the Wn537?
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Old 7th Jan 2018, 7:36 am   #5
Diabolical Artificer
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Default Re: Telequipment D34 the display is too bright.

ECC82? Sorry, no idea. Try a FET of MosFET out of your parts stash.

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Old 7th Jan 2018, 9:01 am   #6
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Default Re: Telequipment D34 the display is too bright.

Good morning Geoff,
some time ago, in that thread found WaveyDipole, that a replacement is possibly with 2N4117--19 or BS140 (here was discussed too)..
Regards, Karl
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