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Old 28th Nov 2008, 8:14 am   #1
RogerWalker
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Default EHT Rectifiers for Solartron Scope CD1400

Can anyone help me with a substitute for the STC K8/50 please? There seems to be NOTHING about this beast on the web. Is it safe/advisable to replace with multiple silicons in series? If so how many, which type and do they need parallel balancing resistors? Whilst I have your attention - does anyone have a colour code list for 'metrosil' resistors - I have a number of them that I can't identify. Thanks in advance! Roger
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Old 28th Nov 2008, 9:16 am   #2
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Default Re: EHT Rectifiers for Solartron Scope CD1400

I have used several BY133 in series for a Telequipment S51B It worked fine, for the time I owned it.
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Old 28th Nov 2008, 1:14 pm   #3
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Default Re: EHT Rectifiers for Solartron Scope CD1400

Going way way back, I vaguely remember repairing similar by cutting just one single vero board strip, cutting the track as required and mounting a string of diodes. Not sure what type now, probably what I could get at the time so probably 1N4007's. I covered the complete assembly with heat shrink and it worked very well.
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Old 28th Nov 2008, 2:27 pm   #4
Mike Phelan
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Default Re: EHT Rectifiers for Solartron Scope CD1400

In the past, I've used BY182s in series potted in resin when the EHT rectifiers on Thorn 8000 TVs were unobtainable, and also made a long string of BY127s and caps to give a few kV for a room ioniser.

Your idea should be fine, Roger; personally, I would not bother about resistors, but others might have different opinions on that.
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Old 28th Nov 2008, 2:39 pm   #5
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Default Re: EHT Rectifiers for Solartron Scope CD1400

Hello Roger
Please see attached data sheet on the K5/80 rectifier. It has a PIV of 3400 volts and is rated at 5mA up to a temperature of 40C. From the data sheet I would estimate the RMS input voltage at around 1250 volts AC. The DC output voltage measured across the smoothing capacitor should be between 1700 to 1800 volts. As mentioned in the other posts you can use three or four series connected 1N4007 silicon diodes to replace the old selenium K5/80 rectifier. But always connect an old carbon composition resistor in series with the silicon diodes to limit the switch on surge, which will not damage the new silicon diodes, but may over stress the EHT winding on the mains transformer. Try a value around 4.7 to 10k to start with, and supply the scope with a variac if possible and monitor the EHT voltage at the output of the new silicon rectifier. If the voltage is too high or low adjust the value of the series resistor to obtain as near as possible the correct EHT voltage. The resistor compensates for the high forward voltage drop of the original K5/50. Sometimes on old, equipment I fit a thermistor in series with mains transformer primary to control the switch on surge.
With reference to metrosil resistors, If you are referring to the types as fitted to TVís you will find a list of types and colour codes in the old Mullard Valve data books, in these books they are listed as VDR's, Voltage Dependant Resistors
Regards Stan.
Attached Files
File Type: pdf K8-50datasheet[1].pdf (42.1 KB, 319 views)
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Old 28th Nov 2008, 3:34 pm   #6
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Default Re: EHT Rectifiers for Solartron Scope CD1400

What a fantastic resource this forum is! Thanks to all!

Roger
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Old 29th Nov 2008, 12:37 am   #7
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Default Re: EHT Rectifiers for Solartron Scope CD1400

ISTR I used 4 diodes with 220k resistors between them.
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Old 1st Dec 2008, 11:22 am   #8
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Default Re: EHT Rectifiers for Solartron Scope CD1400

Just to let you know the outcome - when I fitted the substitutes, I found that the defective K8/50 rectifiers had also taken out the EHT secondary on the mains transformer - so no joy unless I can locate a replacement transformer. Thanks to everyone who responded anyway.

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