Hi. I am repairing a CRO and the power supply has what looks like a high voltage diode which is open circuit. It has a designation of RG70 on it which I don't recognise. I can find no reference to it with a web search. Physically it's a little bit longer than a IN4001 and a shade narrower in diameter. The RG and the 70 are printed along the body of the device above one-another and the cathode is marked using chevrons at the appropriate end. Would anyone who recognises this part number let me know what it is so that I can find a replacement?
Thanks in advance
roman

 

Sorry, but I don't recognize it. I searched as I am sure you did. I saw several instances of RG70 as a lead and halogen free component indicator. For example, here you can see a listing for a 1N4148 with an RG70 designation. Same deal here for a zener.

 

You got to the same places I did. This diode is being used as a half wave rectifier. The only thing that’s unusual about this power supply is that it runs at a high frequency with two primary windings making up part of a transistor-based LC oscillator. I’m presuming that the designer did this to reduce the size of the transformer that was required. The lower voltage parts of the secondary winding use 1N4001s to rectify voltage from earlier taps off and what is confusing me is why the circuit doesn’t just use a higher reverse voltage member of the 1N400x family? What is special about this RG70? I’m going to just put a 1N4007 in there and see what it does. Thank you for your time.

 

If it feeds a reservoir capacitor then the voltage rating of that capacitor will give you a guide to the volrage rating of the diode. The diode should be rated for comfortably more than twice the capacitor rating.

 

High voltage diodes are frequently stacks of diode chips in series. If you have concluded the diode is open by trying to measure forward voltage with a DMM, it might be saying open because the forward voltage exceeds what the meter can measure. If that is the case (i.e. DMM saying open) I suggest using something like a DC power supply of at least 9 or 10 volts (battery OK) with a resistor of perhaps 10k in series with the diode and measuring the voltage.

It is rare for a normal diode to fail open-circuit. I don't know what failure mode to expect for stacked-die diodes.

Are you sure the other diodes are 1N400x and not UF400x? Usually diodes for such circuits are fast-recovery types.

 

Thanks for the comments.

The high voltage section is smooth by a couple of CR networks where the caps are 2kV 0.1u. Guess it doesn't have to be that smooth!

I tried to test the diode up to 80V DC forward biased on a current limited power supply and didn't get any appreciable current draw (milli-amps) so I am presuming it really has gone high resistance.
I have some 800V FR106's diodes coming and will replace all of the diodes with these. I measured the open circuit voltage from the transformer and it's about 250V so FR106's should do the job. The worst it can do is go bang!

 

Are you sure that your meter can accurately measure whatever the frequency coming from the transformer? Most will only work at quite low frequencies. Also the meter will be calibrated assuming the input is a sine wave and the displayed reading will the RMS equivalent voltage.

 

The voltage was measured on an oscilloscope.